A Numerical from kinematics

when a obj. crosses a 5m mark below the top of a building….a 2nd object is released at tat time which is 25m below the top…n den they reach the ground simultaneously…wat is the height of the building??????( dey are both free fallin bodies)Q. from mechanics part 1 DC PANDEY

Numerical Problems based on Newton's Laws

1.A machine gun has a mass of 20 kg. it fires 30g bullets at the rate of 400 bullets per second with a speed of 400 m/s. what force must be applied to the gun to keep it in position?
2.A ball of mass 20g hits a wall at an angle of 45 degree with a velocity of 15 m/s. if the ball rebounds at 90 degree to the direction of incidents, calculate the impulse received by the ball.

Is the equation for displacement in the n th second dimensionally correct?

Is the equation for displacement in the n th second dimensionally correct?

S = u + a (n- 1/2)

Answer:

Yes

See the explanation given by Vimal Raj

If we notice the derivation of this formula
let t1=x and t2=y (for my easiness)
S(y)=uy + 1/2 ay^2----------(1)
S(x)=ux + 1/2 ax^2-----------(2)
sub (2) from (1)
S(y)-S(x)=uy + 1/2 ay^2 - ux - 1/2 ax^2
S(y)-S(x)=u(y-x)+a/2(y^2-x​^2)
here we know y-x =1 s ,then
Sn = u (1s) + a/2(y+x)(y-x)
Sn =u(1s) + a/2(y+x)(1s)

Dimension of Sn must be equal to u(1s)

u=LT^1
THEN Sn=[LT^1][T]=[L]

(Also Visit https://www.facebook.com/groups/askphysics/ for obtaining help from the Facebook group members)

An MCQ from Projectile motion

Devender posted this question: A plane is flying horizontally at 98m/s and releases an object, which reaches the ground in 10s. the angle made by it while hitting the ground is

(a) 55 degree
(b) 45 degree
(c) 60 degree
(d) 75 degree

What Type of questions are asked in Viva Voce?

The methodology of questioning varies from examiner to examiner. The normal way of questioning is to start from the experiment you are alloted.
Example:

What experiment are you doing?
Ans: To determine the value of unknown resistance using metre bridge (Don't say just metre bridge or unknown resistance alone; you should tell the complete aim of the experiment.)

What is the principle on which the experiment is based?

Ans: The metre bridge experiment is based on the Wheatstone's Bridge Principle

(You can go on to state the principle; or it will be the next question)

Then the examiner may go on to ask some tricky questions related to Wheatstone's Bridge.

The questions are generally based on what you answer and usually different sections are asked when you do not give a satisfactory answer.

Have good presence of mind and think before you answer.

You can find some sample questions here arranged experimentwise

A VERY INTERESTING CONVERSATION

A VERY INTERESTING CONVERSATION
Received by email
An Atheist Professor of Philosophy was speaking to his Class on the Problem Science has
with GOD, the ALMIGHTY. He asked one of his New Christian Students to stand and . . .

Professor : You are Christian, aren't you, son ?
Student : Yes, sir.
Professor : So, you Believe in GOD ?
Student : Absolutely, sir.
Professor : Is GOD Good ?
Student : Sure.
Professor : Is GOD ALL - POWERFUL ?
Student : Yes.
Professor : My Brother died of Cancer even though he Prayed to GOD to Heal him.
Most of us would attempt to help others who are ill.
But GOD didn't. How is this GOD good then? Hmm?

(Student was silent )

Time Management - A must follow skill to succeed in Life

We can define time management as the different techniques through which an individual can utilize his time and other associated resources. Moreover, time management brings many benefits for the students such as gaining on time,get motivated and learn to take initiatives, manage to minimize evasion of any sort and much more. You can not get time to review your learning. By time management, you can manage to reduce nervousness.
You are probably here because you want to make the most of your time. You recognize that time is a unique and precious resource that you need in order to do your work and accomplish your goals.
Personal time management skills are essential skills for effective people. People who use these techniques routinely are the highest achievers in all walks of life, from business to sport to public service. If you use these skills well, then you will be able to function exceptionally well, even under intense pressure.
At the heart of time management is an important shift in focus:

Concentrate on results, not on being busy

15 Time Management Tips

Unit 01 - ElectroStatics Class XII CBSE Question Bank

ElectroStatics

(At present all the questions are posted together; soon we will be categorizing according to marks)

You can either browse these questions online or download them by clicking on the link "save as pdf". You can also send the link to your own email or your friends email address by clicking on the link "send to friend".

Â

1. What is Frictional electricity? (1)


2. When can we say that a body is charged?(1)


3. What is electric charge? Is it a scalar or a vector quantity? (1)


4. What is electrostatics? (1)


5. Describe an experiment to show that there are two kinds of charges. (2)


6. Describe an experiment to show that like charges repel and unlike charges attract. (2)


7. Describe the electronic theory of frictional electricity. (2)

Question Bank on Electromagnetic Waves for Class 12 CBSE

Summary

Properties of Electromagnetic Waves

Electromagnetic waves are composed of oscillating electric and magnetic fields at right angles to each other and both are perpendicular to the direction of propagation of the wave. Electromagnetic waves differ in wavelength (or frequency).

In  an electronegative wave the electric field E(vector) and the Magnetic field B(vector) oscillate perpendicular to each other and both are perpendicular to direction of propagation of wave.

The source that produce them and methods of their detection are different, but they have the following common properties :
1. Electromagnetic waves are propagated by oscillating electric and magnetic fields oscillating at right angles to each other.
2. Electromagnetic waves travel with a constant velocity of  3 x 10⁸ ms^-1 in vacuum.
3. Electromagnetic waves are not deflected by electric or magnetic field.
4. Electromagnetic waves can show interference or diffraction.
5. Electromagnetic waves are transverse waves.
6. Electromagnetic waves may be polarized.
7. Electromagnetic waves need no medium of propagation. The energy from the sun is received by the earth through electromagnetic waves.
8. The wavelength (λ) and the frequency (v) of electromagnetic wave is related as
c = v λ = ω/k
The S.I. unit of frequency is Hertz.
1 Hertz = 1 c / s
The S.I. unit of wavelength is metre.
We however, often express wavelength in Angstrom unit [ Ã… ]
1 Ã… = 10^-10 m
Also, 1 nanometer = l nm = 10^-9 m

(In the chapter electromagnetic waves, only 1 mark, 2 marks or 3 marks questions will be asked in Board Exams. There is no chance for a 5 marks question. So, we've included all in a single section.)

1. Discuss the inconsistency in Ampere's Circuital Law.


2. What modification was made to Ampere's Circuital Law by Maxwell?


3. What is displacement current?


4. Conduction current and displacement current can be separately discontinuous. But their sum is continuous. Explain.


5. Can displacement current (or a changing electric flux induce a magnetic field? Explain


6. State the important consequences of displacement current.


7. State the important properties of displacement current.


8. Write Maxwell's equations of electromagnetism and state the law underlying each equation.


9. Explain what led Maxwell to predict the existence of electromagnetic waves.


10. How are electromagnetic waves represented mathematically?


11. What is the dimension of (a)μ₀ ε₀(b)√(μ₀ ε₀)


12. Discuss how an accelerating charge becomes a source of an electromagnetic wave?


13. Describe Hertz's experiment for producing and detecting electromagnetic waves.


14. Explain how Hertz demonstrated the various properties of electromagnetic waves?


15. List the different landmarks in the history of electromagnetic waves.


16. Prove the transverse nature of electromagnetic waves mathematically.


17. Write the properties of electromagnetic waves.


18. What is electromagnetic spectrum.


19. Name the main parts of the electromagnetic spectrum and mention their frequency range and source of production. Also write their important properties and uses.

HOTS QUESTIONS FROM ELECTROMAGNETIC WAVES

1. If you find closed loops of magnetic field in a region of space, does it necessarily mean that actual charges are flowing across the area bounded by the loops? Explain.


2. Why is it that induced electric fields due to changing magnetic flux are more readily observable than the induced magnetic fields due to changing electric fields?


3. A variable frequency AC source is connected to a capacitor. Will the displacement current increase or decrease with increase in frequency?


4. Why is the quantity ε₀ dφ_E/dt called displacement current?


5. What oscillates in am Electromagnetic Wave?


6. Why sound waves are not considered em waves?


7. What is the phase relationship between the oscillating electric and magnetic fields in an electromagnetic wave?


8. X- rays and γ rays have a region of overlapping frequencies and have similar characteristics. But why they are treated as different? (OR) How are γ rays different from X rays?


9. Why is the ozone layer of atmosphere crucial to the existence of life on earth?


10. Why does "RADAR" use microwaves?


11. In a microwave oven, the food kept in a closed plastic container get s cooked without melting or burning the plastic container. Explain how?


12. Can an electromagnetic wave be deflected by an electric or magnetic field? Justify your answer.


13. Name the constituent radiation of electromagnetic spectrum which
    (a) is used in satellite communication
    (b)Is used for studying crystal structure
    (c)is emitted during decay of radio active nuclei
    (d) is absorbed by ozone layer Write two more uses of each


14. What is the dimension of E/B?


15. Show that the dimensions of are that of electric current


16. he sunlight reaching the earth has a maximum electric field of 810 V/m. What is the maximum magnetic field associated with it?


17. Calculate the relative permitivity of a medium of relative permeability 1.0 if the velocity of light through the medium is 2 x 10⁸ m/s.


18. Which are the different layers of earth's atmosphere?

(There are no limits to the Hots Questions to be framed from ANY chapter. What is important is to understand the concepts thoroughly and apply your brain)

Numerical Problems from ELECTROMAGNETIC WAVES for CBSE Class XII

1. A parallel plate capacitor has circular plates, each of radius 5.0 cm. It is being charged so that electric field in the gap between its plates rises steadily at the rate of 10¹²V m^-1s^-1. What is the displacement current?


2. The voltage between the plates of a parallel plate capacitor of capacitance 1.0 μF is changing at the rate of 5 V s^-1. What is the displacement current in the capacitor?


3. A parallel plate capacitor has two metal plates of size 30 cm X 15 cm and separated by 2.0 mm. The capacitor is being charged so that the charging current has a steady value of 100 mA. Calculate the rate of change of potential difference between the capacitor plates. What is the displacement current between the capacitor plates?


4. The frequencies of radio waves in AM broadcast band range from 0.55 x 10⁶ to 1.6 x 10⁶ Hz. What are the longest and shortest wavelengths in this band?


5. A plane electromagnetic wave of frequency 25 MHz travels in free space along X - direction. At a particular point in space and time, E = 6.3 j V/m. What is B at this point?

There are many other similar Numerical Problems which may be asked in CBSE Exam

Some Questions and Solutions can be downloaded from here

HOW TO PREPARE YOUR ROUTINE TO SCORE GOOD MARKS IN PHYSICS AND SUCCEED IN MEDICAL ENGINEERING ENTRANCE TESTS, IIT (JEE) - PART I

A General Briefing on Studying Physics (for Indian Students)

Quite often we hear “There is no substitute for hard work”

Everyone you come across advise you to work hard.

But,

Mere hard work  will not pay.

 

I know several students who work hard and spent many hours a day in front of books but score pathetic marks / grades.

Why is it so?

What is the problem?

What went wrong?

When we come to Physics, (and in General, Science) reading alone will not help you score big. Whether you read again and again or write the concepts several times, you will not be able to score commendable marks unless you understand the concepts thoroughly and apply the concepts to different situations independently.

Just take the case of “Principle of Conservation of Momentum”

There is no end to the number of questions which can be framed from this single concept.

So, The basic point is understanding concepts  thoroughly.

But How?

Thinking on what you read, hear or see is very important in understanding Physics. When you think, many questions will arise in your mind. Finding the answers to these questions form the basic of understanding the concept.

When you have a question, ask it to your teacher or discuss with your friends.

Remember, A question asked or an idea shared by you may be the beginning of the next scientific revolution

Which Text book?

The NCERT Text book is the one which Indian students should take as the basic one. Not only the +2 level exams and entrance exams but even the Civil Services exams mainly base the core syllabus on the NCERT texts.

The main complaint from the students and some ‘others’' is that the NCERT  text book is incomplete and donot convey the full idea at several places.

This is intentional. The incompleteness is penned in wisely to enable the students to ask questions and explore beyond the text book.

Telling everything is not the scientific way of learning. There is sufficient scope left in the text book for the students to explore the world they live in and learn themselves.

As the Chinese proverb says,

“Don’t give fish to a hungry man, instead teach him how to fish” (Then he will not be hungry anymore)

Learning to learn is more important than learning itself.

No single text book can tell you everything. No single teacher can tell you everything. A lot is learnt by self through thinking, contemplation, experiment and interrogation.

The NCERT Text Book concentrates on the prescribed syllabus as the core content and give hints, probes and thrusts at places for the readers to go beyond the textbooks and the curriculum.

For scoring good marks in Physics, it is very important master “the  art of problem solving” . Start with solving all the numerical and conceptual problems of the NCERT text book and then go on to solve as much related problems as you can.

Make it a habit to solve 20 – 25 numerical problems related to the topics you learn in Physics daily.

… To be continued.

(Expect soon articles on “managing time”  , preparing for Exams,  “How to Revise” and related items)

If you have questions, please leave a comment.

You don't have to Work All the Time

Robert H. Grubbs the Nobel Prize winner in Chemistry 2005 says

"You don't have to WORK all the time,

But

You have to THINK  all the time"

Reading or studying without thinking is futile. As I said earlier, In learning science, Questioning every answer is more important than answering every question.

Don’t just be satisfied by the mere facts and figures given in texts or said by your teachers.

Ask questions yourself first, clarify the concepts and find answers to the questions arising in your mind and discuss your answers with your friends and teachers.

Let them question your answers and you try defending your answers.

This way everyone will be tempted to think and question the answers.

Physics Nobel Prize 2010

Professor Andre Geim and Professor Konstantin Novoselov have been awarded Nobel Prize in Physics (2010) for their pioneering work with the world’s thinnest material.

Sometimes the old gives rise to the new in wonderfully unexpected ways. Such was the case with graphene: an entirely new form of carbon, the world's first 2-dimensional material and the subject of the 2010 Nobel Prize in Physics. This novel wonder material, which offers possibilities ranging from faster computers to new insights into quantum physics, was produced from plain, familiar old graphite, the stuff that fills your pencils. Pencils work because graphite is made from layer upon layer of carbon atoms arranged in sheets a single atom thick; every time you move the pencil across the paper, clumps of these sheets shear off and are left on the paper. Graphene, which consists of just one of these sheets, can, it turned out, also be sheared off a lump of graphite.

Andre Geim and Konstantin Novoselov, this year's Nobel Laureates, actually isolated Graphene in 2004 in one of their 'Friday evening experiments' where they habitually play with new ideas. They ended up using another familiar material, ordinary sticky tape, to 'exfoliate' a graphite crystal and found that, after several rounds, they were able to peel off the elusive graphene monolayers. Virtually transparent and of atomic thickness, graphene can only be seen under very specific conditions, and coincidentally Geim and Novoselov chose exactly the right substrate to place their flakes on, allowing them to view them in an ordinary microscope. A new research field was born.

Graphene's remarkable strength and extreme conductivity, it is a hundred times stronger than steel and more conductive than copper, result from its hexagonal lattice of carbon atoms permeated by a sea of delocalized electrons. Aside from the insights into fundamental quantum physics they offer, graphene's properties have set the world's material scientists dreaming of, and exploring, a wealth of possible applications. Among the most realistic is its potential use in touch screens where the transparency, strength and conductivity it offers appear to provide a highly desirable combination. Perhaps most immediately enticing is the vision of further miniaturizing computer chips by using graphene's atomic scale to overcome the size constraints now being encountered with silicon-based components.

Previous results of the Geim lab's playful approach to physics have included levitating live frogs, in a demonstration of the importance of diamagnetism, and the biomimetic nanomaterial known as gecko tape. As Geim himself says, "getting some play during working hours for which you are paid is the best job I can recommend for anyone around!"
[ "The Nobel Prize in Physics 2010 - Speed Read". Nobelprize.org. 14 Oct 2010 http://nobelprize.org/nobel_prizes/physics/laureates/2010/speedread.html]
Want to know more on the discovery which led to Nobel Prize?
Click Here to Read and Download more about Graphene and its uses

INTERESTING PHYSICS ARTICLES

Find below a collection of interesting Articles I came across while Browsing the web. Links are provided to the original website for complete reference

 

LATEST PHYSICS NEWS http://feeds.feedburner.com/physicsnews

Can The Invisible Man See?

 

H.G.Wells classic novel "The invisible man" may have inspired a lot of ideas in the new generation, but if we think deeper, the question should ariseCan an Invisible man see??. If one knows the details about optics and the laws of refraction, one can safely say that the invisible man will be blind and helpless.

In the novel "The Invisible Man", H.G.Wells demonstrated with wit and logic that an invisible man acquires an almost unlimited amount of power. . He is able to enter any place unnoticed and steal anything with impunity. Elusive, thanks to invisibility; he successfully fights a whole crowd of armed people. Threatening to smite all those who are visible, the invisible man subjugated the population an entire town. Himself elusive and invulnerable, he strikes down all their opponents despite their every precaution. The invisible man is thus able to issue to the terrified population of his home town an order of the following content: "Port Burdock is no longer under the Queen, tell your Colonel of Police, and the rest of them; it is under me… this is a day one of year one of the new epoch- the Epoch of the Invisible man. I am invisible Man the First. To begin with, the rule will be easy. The first day there will be one execution for the sake of example- a man named Kemp. Death starts for him today. He may lock himself away, hide himself away, get guards about him, put on Armour if he likes- Death, the unseen Death, is coming. Let him take precautions- it will impress my people… death starts. Help him not, my people, lest death fall upon you also."

Read more at http://www.buzzle.com/editorials/7-30-2004-57280.asp

 

Electromagnetic Waves: Origin and Theory

 

Electromagnetism is defined as the combinations of alternating electric and magnetic fields created by accelerated charges that propagate out from these charges at the speed of light in the form of waves- electromagnetic waves or radiation. Earths environment is widely affected by various types of radiation- power waves, radio waves, microwaves, infrared, visible, ultraviolet, X-rays and gamma rays. A brief look into the origin and theory of Electromagnetic waves.

Read more at http://www.buzzle.com/editorials/3-26-2004-52192.asp

Physics of Roller Coasters

 

A theme park or an amusement park is usually visited by many for the simple reason that it has roller coaster. People especially children like the thrills involved in the ride of the roller coaster. Let us have a look at the physics of Roller Coaster…

http://www.buzzle.com/articles/physics-of-roller-coasters.html

 

Does Gravity exist everywhere?

It has long since the dream of many scientists to isolate gravity producing matter. But it is realized that gravity cannot be isolated but is an inherent property of every mass in the universe. But larger the mass, stronger the gravitational field and vice versa. A brief look into this aspect of gravity.

http://www.buzzle.com/editorials/5-22-2004-54512.asp

The Nobel Prize Articles http://nobelprize.org/nobel_prizes/physics/articles/

LATEST PHYSICS NEWS http://feeds.feedburner.com/physicsnews

Science and Religion

  "Science without religion is lame, religion without science is blind."

-Albert Einstein

Statements about the world made by science and religion rely on different methodologies. Religions rely on revelation while science relies on observable and repeatable experiences.
Both science and religion represent distinct ways of approaching experience and these differences are sources of debate.Science is closely tied to mathematics-a very abstract experience, while religion is more closely tied to the ordinary experiences of life. As interpretations of experiences in life, science is descriptive and religion is perspective. For science and mathematics to concentrate on what the world ought to be like in the way that religion also can be inappropriate be descriptive can also lead to inappropriately assigning properties to the natural world. A notable example is the changes in scientific and religious thinking and may lead to improperly assigning properties to the natural world. The reverse situation where religion attempts in scientific and religious thinking brought about by Galileo and proponents of his views.
Modern scientists are less concerned with establishing universal truth which is seen, and dismissed, as the pursuit of philosophy, and more inclined towards the creation of functional models of physical systems. Christian Theology - excluding those fundamentalist churches whose aim is to reassert doctrinal truths - has likewise softened many of its universal claims, due to increased exposure to both scientific insights and the contrasting theological claims of other faiths. Scientific and theological perspectives. scientific ideas. The scientific advances made by Muslim scholars during the early period play a significant role. Even many 19th century Christian communities welcomed scientists who claimed that science was not at all concerned with discovering the ultimate nature of reality.
“Religions die when they are proved to be true. Science is the record of dead religions”
A religious person is devout in the sense that he has no doubt of the significance of those superpersonal objects and goals which neither require nor are capable of rational foundation. They exist with the same necessity and matter-of- factness as he himself. In this sense religion is the age-old endeavor of mankind to become clearly and completely conscious of these values and goals and constantly to strengthen and extend their effect. If one conceives of religion and science according to these definitions then a conflict between them appears impossible. For science can only ascertain what is, but not what should be, and outside of its domain value judgements of all kinds remain necessary. Religion, on the other hand, deals only with evaluations of human thought and action: it cannot justifiably speak of facts and relationships between facts. According to this interpretation the well-known conflicts between religion and science in the past must all be ascribed to a misapprehension of the situation which has been described.

            Like two sides of a coin, there may be conflicts also between science and religion, both science and religion complement each other like branches of  the same tree.

Physics Resources

Astrophysics Science Project: Integrating Research and Education (ASPIRE) offers lessons for learning about the atmosphere, force, gas particles, cosmic rays, kinetic energy, lunar phases, machines, momentum, the night sky, refraction demos (prisms and...  (National Science Foundation)
The Physics Front provides selected resources for teachers of high school physics. Hundreds of lessons, labs, and activities can be found in four categories: conceptual physics, algebra-based physics...  (National Science Foundation)

Physical Sciences Resource Center offers hundreds of annotated lessons, simulations, and resources for teaching topics in the physical sciences, including astronomy, classical mechanics, electricity and magnetism...  (National Science Foundation)

The Elegant Universe: It's String Theory is a 3-hour miniseries exploring one of the most ambitious theories ever proposed: the "theory of everything." String theory proposes that the fundamental ingredients of nature are...  (WGBH Educational Foundation, supported by Multiple Agencies)
Physics to Go is a collection of reviewed resources for teaching and learning about astronomy, electricity and magnetism, fluids, light, modern physics, motion and energy, quantum physics, and waves...  (American Physical Society, supported by National Science Foundation)

NSTA: Teaching Objects provides web-based science learning activities for teachers. Activities include simulations, questions, and students' common misconceptions (with practical ideas for addressing them)...  (Multiple Agencies)
Matter provides video lesson, video clips, and interactive resources for learning about air, atoms, atomic basis of the properties of matter, atomic nucleus, chemical change, chemical...  (Teachers' Domain, supported by Multiple Agencies)
Motions and Forces provides video clips and interactive resources for learning about electricity and magnetism, forces between objects, gravity, objects in motion, tension and compression, and velocity...  (Teachers' Domain, supported by Multiple Agencies)

Searching for the Building Blocks of Matter looks at Fermilab's search for the smallest building blocks of matter. It also describes the accelerator and detectors needed for the discovery of these building blocks (quarks...  (Multiple Agencies)
Windows to the Universe explores the Earth, planets of our solar system, and the universe. It includes images, animations, and data sets, and information about books, movies, scientists, and myths...  (National Aeronautics and Space Administration)
Origins lets us look over the shoulders of scientists and glimpse the often-unseen moments of investigation. Take "virtual field trips" to eight observatories -- Arecibo, where...  (National Science Foundation)
CERN, the World's Largest Particle Accelerator features scientists and machines that explore the universe's tiniest particles. Follow a proton through the accelerator. Meet scientists at CERN. Hear why...  (National Science Foundation)

Project Links provides 7 web-based modules for teaching advanced math methods, probability and statistics, differential equations, discrete mathematics, linear systems, and calculus...  (Rensselaer Polytechnic Institute, supported by National Science Foundation)
National Science Digital Library provides access to resources in science, technology, engineering, and mathematics education and research. From video clips teaching cell division to simulations demonstrating plate...  (National Science Foundation)

The Particle Adventure introduces, through an interactive adventure tour, the theory of fundamental particles and forces. It also looks at why physicists want to go beyond the Standard Model theory. In...  (Multiple Agencies)

From Stargazers to Starships is a self-contained book-on-the-web course on basic astronomy, Newtonian mechanics, the sun (and associated physics), and spaceflight and spacecraft...  (National Aeronautics and Space Administration)
Citizen Kurchatov: Stalin's Bombmaker tells the story of a complex, world-class physicist who became the driving force the Soviet Union's race to develop the atomic and hydrogen bomb...  (Oregon Public Broadcasting, supported by National Endowment for the Humanities)
A Science Odyssey highlights some the most spectacular discoveries in science and technology during the 20th century. The site includes an Educator's Guide with activities, discussion questions, and...  (WGBH, supported by National Science Foundation)
Decades of Discovery describes 100 important discoveries in energy sciences, nuclear and plasma physics, advanced computing research, and biological and environmental research. Topics include the world's...  (Department of Energy)

MicroWorlds looks at materials sciences research at Berkeley Lab's Advanced Light Source (ALS). ALS, a facility that produces light one billion times brighter than the sun, offers opportunities...  (Department of Energy)

Physics Education Technology (PhET) produces fun, interactive simulations of physical phenomena. More than 80 simulations let students experiment with circuits, string tension, kinetic and potential energy, radios waves...  (The Kavli Operating Institute, supported by National Science Foundation)

NSF Special Reports presents web-based reports on language and linguistics, Einstein and physics, weather patterns, the chemistry of water, the 2004 tsunami, arctic climate research, Admiral Byrd's...  (National Science Foundation)

Activity-Based Physics presents "thinking problems" for physics topics: vectors, kinematics, momentum, circular motion, universal gravitation, sound, energy, temperature, and circuits. Problems include...  (University of Maryland Physics Education Research Group, supported by National Science Foundation)

Physics offers a bird's-eye view of the great challenges in physics today. Read about self-organization, network theory, superconductivity, superfluidity, supersolids, quantum mechanics...  (National Science Foundation)

ComPADRE: Resources for Physics and Astronomy Education provides reviewed collections of resources and interactive learning environments for teaching physics and astronomy. Topics include motion, forces, energy, heat, wave energy...  (National Science Digital Library, supported by National Science Foundation)

Teacher Workshops: Science provides materials from 2007 summer teacher workshops sponsored by the U.S. Department of Education. Find slides and handouts on teaching analogical reasoning, motion and forces...  (Department of Education)

Pedagogy in Action documents more than 25 pedagogic techniques for teaching sciences to undergraduates: case-based learning, game-based learning, making and testing conjectures, peer review...  (SERC, supported by National Science Foundation)

Physics provides more than 40 online interactives that demonstrate concepts in mechanics, fluid mechanics and dynamics, electromagnetism, and quantum physics. Watch simulations of a pendulum...  (Concord Consortium, supported by National Science Foundation)

Einstein's Big Idea tells the story behind the world's most famous equation, E = mc2. Learn about its discovery and legacy. Meet scientists whose experiments laid the groundwork. Read about the unknown...  (Public Broadcasting Service, supported by National Science Foundation)

Diagnoser is a web-based program that lets teachers assign questions for students to answer on the web. As students answer questions, they receive feedback. Teachers can see reports on...  (UCLA, supported by National Science Foundation)

Science of Spectroscopy features a wiki-based effort to tell the story of spectroscopy -- the use of light to study matter. Find answers to questions about the basic theory of light, energy, and the...  (National Aeronautics and Space Administration)

World Year of Physics 2005 celebrates the centennial of the publication of four strange research papers -- written by a 26-year-old patent clerk -- that revolutionized how we think about light, matter, energy...  (National Science Foundation)

QuarkNet is the website for a professional development program in which teachers work on particle physics experiments over the summer and join a cadre of scientists and teachers introducing...  (Multiple Agencies)

ABC's of Nuclear Science introduces the object that contains almost all of the mass in the universe, the atomic nucleus. Antimatter, beta rays, fission and fusion, the structure of the atomic nucleus, how...  (Department of Energy)

Inquiring Minds: Fermilab features an introduction to elementary particles and forces in our universe, physics questions answered by Fermilab scientists, an interactive timeline illustrating the history of...  (Fermi National Accelerator Laboratory, supported by Department of Energy)

Gravity Probe B is a "relativity gyroscope" experiment designed to test two unverified predictions of Albert Einstein's general theory of relativity (1916): that the presence of a mass in space, such...  (National Aeronautics and Space Administration)

Alsos Digital Library for Nuclear Issues offers annotated references to articles, books, films, and websites for the study of nuclear issues. Topics include fission and fusion, nuclear power and waste, plutonium and tritium...  (Washington and Lee University, supported by National Science Foundation)

Earth's Magnetic Field is the focus of the POETRY website, which explores solar storms and how they affect us, space weather, and the Northern Lights. A 64-page workbook of hands-on activities examines...  (National Aeronautics and Space Administration)

Magnetic Field Activities for the High School Classroom helps students understand the vector nature of fields, the ubiquity of fields in the environment, and the 3-dimensionality of fields. Activities include mapping the magnetic field of...  (National Aeronautics and Space Administration)

Fundamental Theory provides lessons, video clips, and interactive resources for learning about cosmology and gravity, quantum mechanics, string theory, the special theory of relativity, and Einstein's...  (Teachers' Domain, supported by Multiple Agencies)

Fusion: Physics of a Fundamental Energy Source provides introductory educational materials on fusion, the process that powers the sun and other stars; and plasmas, known as the "fourth state of matter."...  (Department of Energy)

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REVISION QUESTIONs FROM MAGNETISM & MAGNETIC EFFECTS OF CURRENT

Practice the numerical problems from Textbook and the typical  numerical problems given here.

Ask if you have any doubt online at www.askphysics.com

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MUST PRACTISE THESE QUESTIONS

1. 
   
   State Biot - Savart Law and apply it to determine the magnetic field at a point on the axis of a circular loop carrying current.


2. 
   
   State Ampere's Circuital law and apply it to find the magnetic field at a point due to (a) A straight conductor carrying current (b) A current carrying solenoid (c) A current carrying toroid


3. 
   
   Describe the principle construction and working of a cyclotron.


4. 
   
   Explain the elements of earth's magnetism.


5. 
   
   Define the terms magnetisation and magnetic intensity.


6. 
   
   Compare the properties of dia para and ferromagnetic substances.


7. 
   
   Describe the principle, construction and working of a moving coil galvanometer.


8. 
   
   Describe the conversion of a galvanometer in to (a)Voltmeter (b) Ammeter


9. 
   
   Derive an expression for cyclotron frequency


10. 
    
    What is hysterisis? Draw the hysterisis curve for a magnetic substance and explain the terms retentivity and coercivity. How do these factors help in selecting suitable materials for (a) Permanent magnet (b) Electromagnet


11. 
    
    Derive an expression for the intensity of magnetic field (a)on the axial position (b) equatorial position


12. 
    
    Derive an expression for the torque on a current carrying coil placed in a uniform magnetic field


13. 
    
    Discuss the equivalence of a current carrying solenoid and a bar magnet


14. 
    
    Discuss the action of a current carrying coil as an equivalent magnetic dipole.


15. 
    
    How are magnetic lines of force different from electric lines of force?


16. 
    
    Define magnetic susceptibility


17. 
    
    Compare the properties of an electromagnet and a permanent magnet


18. 
    
    Explain the domain theory of magnetism


19. 
    
    Derive an expression for the potential energy of a magnetic dipole in a uniform magnetic field. What is Bohr magneton? Derive an expression for it and calculate its value.


20. 
    
    Define the terms magnetic meridian and geographic meridian.


21. 
    
    What is a radial magnetic field? Draw diagram to illustrate how is it realised in a moving coil galvanometer. What is the advantage of a radial magnetic field in MCG?


22. 
    
    Derive an expression for the force between two straight parallel current carrying conductors of infinite length and hence define one ampere.


23. 
    
    Derive an expression for the force on a current carrying conductor in a uniform magnetic field.


24. 
    
    Describe the motion of a charged particle that enters into a magnetic field at right angle. Obtain expression for (i) time period (ii) frequency and (iii) angular frequency. Describe the motion of the charged particle when it enters into the field at a certain angle θ


25. 
    
    Find an expression for the force on a moving charge in a magnetic field. State Fleming's left hand rule. Find the force on a moving charge when (i) moving parallel or anti parallel to the field (ii) moving at right angle to the field and (iii) at rest. Also give the definition of unit magnetic field.


26. 
    
    Compare Biot Savarts law and Ampere's circuital law


27. 
    
    What is Lorentz force? Give some important characteristics of this force. How can it be used to differentiate the motion of a charged particle in a magnetic field and electric field.


28. 
    
    Show that the kinetic energy of a charged particle moving in a uniform magnetic field remains constant.


29. 
    
    Derive an expression for the maximum kinetic energy acquired by a charged particle accelerate din a cyclotron.


30. 
    
    Why electrons and neutrons cannot be accelerated in a cyclotron?


31. 
    
    Write the dimensions of (a) magnetic induction (b) magnetic permeability.


32. 
    
    
    

    TYPICAL NUMERICALS

    
    

1. 
   

   An electron and a proton travelling with the same speed enter into a region of uniform magnetic field at right angle, For which of these the radius of the circular path will be smaller?

   
   


2. 
   

   A wire of length 0.04 m carrying a current of 12 A is placed inside a solenoid making an angle off 30 degree with its axis, where the field due to the solenoid is 0.25 T. Find the force on the wire. [0.06 N]

   
   


3. 
   

   A galvanometer coil has a resistance of 100 Ω and it shows full scale deflection for a current of 1 mA. How can it be transformed into an ammeter of range 0-1A?

   
   


4. 
   

   An electron moving with kinetic energy 25 KeV moves perpendicular to a uniform magnetic field 0.2 mT. Calculate the time period of rotation of electron in the magnetic field.

   
   


5. 
   

   An electron is revolving around the nucleus of an atom in an orbit of radius 0.53 A. Calculate the equivalent magnetic moment if the frequency  of revolution of electron is 6.6 x 10⁹ MHz

   
   

Magnetism and magnetic Effects of Current Question Bank for Class 12 CBSE

1. State Biot - Savart Law and apply it to determine the magnetic field at a point on the axis of a circular loop carrying current.


2. State Ampere's Circuital law and apply it to find the magnetic field at a point due to (a) A straight conductor carrying current (b) A current carrying solenoid (c) A current carrying toroid


3. Describe the principle construction and working of a cyclotron.


4. Explain the elements of earth's magnetism.


5. Define the terms magnetisation and magnetic intensity.


6. Compare the properties of dia para and ferromagnetic substances.


7. Describe the principle, construction and working of a moving coil galvanometer.


8. Describe the conversion of a galvanometer in to (a)Voltmeter (b) Ammeter


9. Derive an expression for cyclotron frequency


10. What is hysterisis? Draw the hysterisis curve for a magnetic substance and explain the terms retentivity and coercivity. How do these factors help in selecting suitable materials for (a) Permanent magnet (b) Electromagnet


11. Derive an expression for the intensity of magnetic field (a)on the axial position (b) equatorial position


12. Derive an expression for the torque on a current carrying coil placed in a uniform magnetic field


13. Discuss the equivalence of a current carrying solenoid and a bar magnet


14. Discuss the action of a current carrying coil as an equivalent magnetic dipole.


15. How are magnetic lines of force different from electric lines of force?


16. Define magnetic susceptibility


17. Compare teh properties of an electromagnet and a permanent magnet


18. Explain the domain theory of magnetism


19. Derive an expression for the potential energy of a magnetic dipole in a uniform magnetic field. What is Bohr magneton? Derive an expression for it and calculate its value.


20. Define the terms magnetic meridian and geographic meridian.


21. What is a radial magnetic field? Draw diagram to illustrate how is it realised in a moving coil galvanometer. What is the advantage of a radial magnetic field in MCG?


22. Derive an expression for the force between two straight parallel current carrying conductors of infinite length and hence define one ampere.


23. Derive an expression for the force on a current carrying conductor in a uniform magnetic field.


24. Describe the motion of a charged particle that enters into a magnetic field at right angle. Obtain expression for (i) time period (ii) frequency and (iii) angualr frequency
    
    . Describe the motion of the charged particle when it enters into the field at a certain angle θ


25. Find an expression for the force on a moving charge in a magnetci field. State Fleming's left hand rule. Find the force on a moving charge when (i) moving parallel or anti parallel to the field (ii) moving at right angle to the field and (iii) at rest. Also give the definition of unit magnetic field.


26. Compare Biot Savarts law and Ampere's circuital law


27. What is Lorentz force? Give some important characteristics of this force. How can it be used to differentiate the motion of a charged particle in a magnetic field and electric field.


28. Show that the kinetic energy of a charged particle moving in a uniform magnetic field remains constant.


29. Derive an expression for the maximum kinetic energy acquired by a charged particle accelerate din a cyclotron.


30. Why electrons and neutrons cannot be accelerated in a cyclotron?

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The Man Who Wins Is The Man Who Thinks He Can

"Think on what you Learn :: In Physics Questioning every Answer is more important than answering Every Question"

Learning without thinking deeply about it and not relating to already known facts is not permanent.
Ask Questions about new concepts learned and don't rest until you get a satisfactory answer.
Don't be shy in asking questions.

"Only those questions are foolish which are not asked"

You will find new dimensions and directions when you contemplate on what you learn or read.

Wish you all a happy learning experience atCBSE Physics

Science Physics Project Links

Help for all those who prepare for Science projects.
To start with, here are some links.
http://cpphysics.homestead.com/student.html
http://clackhi.nclack.k12.or.us/physics/projects.html
http://www.all-science-fair-projects.com/category56.html
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/projects/proj.html
http://www.iolani.honolulu.hi.us/Departments/Science/Physics/Heyler/pages/projects.htm

Members are requested to post links and articles and your own project ideas as comments to this post. Remember, whatever you post will soon be reflected in twitter at http://twitter.com/cbsephysics

Conceptual Questions - Capacitors

Q. 1. The two charged conductors are touched mutually and then separated. What will be the charge on them?
Ans. The charge on them will be divided in the ratio of their capacitances. We know that q = CV.
When the charged conductors touch, they acquire the same potential. Hence, q proportional to C.
Q. 2. The plates of a charged capacitor are connected to a voltmeter. If the plates of the capacitor are
separated further, what will be the effect on the reading of the voltmeter?
As the capacitor plates are separated, C decreases. Since charge on the plates remains the same, value of V increases. Hence, the reading of the voltmeter will increase.
Q. 3. Any conducting object connected to earth is said to be grounded. Explain.
Ans. The earth is an electron source or sink and is arbitrarily said to be at zero potential. A conducting
body connected to earth is also at zero potential or “ground potential”. Alternatively, the
capacitance of earth is so large that removal of electrons from it or supply of electrons to it
makes no difference either in the charge or potential of earth.
Q. 4. How does a spark discharge occur between two charged objects?
Ans. The air between the two charged objects is subjected to an electric field. If the potential gradient
in the intervening air column becomes high enough, the air is ionised and conducting path is formed for free electrons which move across to discharge the surfaces. Stored electric potential energy is dissipated as heat, light and sound.
Q. 5. If a solid dielectric is placed between the plates of a capacitor, its capacitance increases. Is thereany other advantage of solid dielectric?
Ans. There are other two advantages of a solid dielectric. First, it helps in keeping the plates close together without touching. Secondly, we can now charge the capacitor to a high potential (V = q/ C).
Q.6 Given a solid metal sphere and a hollow metal sphere. Which will hold more charge? Bothspheres are of same radius.
Ans. Both the spheres will hold the same charge. It is because charge remains on the outer surface of a charged conductor (whether solid or hollow) and the spheres have equal surface areas.
Q. 7. Two capacitors of capacitances 1 μF and 0.01 μF are charged to the same potential. Which willgive more intense electric shock if touched?
Ans. q = CV. Since V is constant, q∝C. It means that capacitor having large capacitance will store more
charge. Hence, when 1μF capacitor is touched, the discharging current will be high and you will
get more intense electric shock than in case of 0.01μF capacitor.
Q. 8. Two spheres of different capacitances are charged to different potentials. They are then joined by
a wire. Will total energy increase, decrease or remain the same?
Ans. The two spheres are at different potentials. Therefore, when they are connected by a wire, there
will be redistribution of charge (i.e., flow of charge through wire) till the two spheres attain the
same potential. Due to the flow of charge through the connecting wire, some energy will be lost as
heat. Hence, the total energy after connecting the spheres will decrease.
Q. 9. Can there be potential difference between two adjacent conductors which carry the same positive
charge?
Ans. Yes. We know that V = q/C. The capacitance depends upon the dimensions of the conductor. If
the two conductors are of different shapes and sizes, they will be charged to different potentials
when given the same charge.
Q. 10. What are the differences between conductors and dielectrics?
Ans. (i) Conductors have a large number of free electrons while dielectrics have practically no free electrons.
(ii) When a conductor is placed in an external electric field, there is no electric field inside the conductor. However, when a dielectric is placed in an electric field, its molecules are polarised.
The effect of this polarisation is to weaken the applied electric field within the dielectric.
(iii) The dielectric constant of conductors is infinity while that of dielectrics is finite.
(iv) The dielectric strength of conductors is zero while that of dielectrics is finite.
(v) There is no limit to the current that a conductor can carry, provided that it can be kept coolenough. However, there is a limit to the electric flux that a dielectric will carry without breaking down.

Human Eye and Colourful World :: HOTS Questions for Class X

Human Eye and Colourful World
HOTS Questions
1. What is the least distance of distinct vision of a normal human eye ?
2. Name the muscle responsible for bringing change in the focal length of the eye lens ?
3. Name one defect of vision which cannot be corrected by any type of spectacle lenses ?
4. State one effect produced by the scattering of light by the atmosphere ?
5. What is the nature of image formed on the retina of the eye ?
6. What type of lens is used for correcting hypermetropia ?
7. Who was the first person to obtain the spectrum of sunlight ?
8. What is the function of optic nerve in human eye ?
9. What is range of vision ?
10. Why do different colours deviate through different angles on passing through a prism?
11. As light rays pass from air into glass prism, are they refracted towards or away from the normal ?
12. Which color has largest wavelength ?
13. Which defect of vision can be rectified using a concave lens ?
14. What phenomenon causes twinkling of star on a clear night ?
15. What is meant by scattering of light ?
16. Why does the sky appear black instead of blue to an astronaut?
17. What is the basic cause of atmospheric refraction?
18. Why does clear sky look blue?
19. Can visible light be scattered by atoms/molecules in earth’s atmosphere?
20. What is a spectrum?
21. Name the defect of vision in person
a. Whose near point is more than 25cm away?.
b. Whose far point is less than infinity.

More Questions for Practice
1. A person can see only objects beyond 1m. From his eyes. Name the defect of the eye.
2. Out of light of blue and red colours which one is scattered most?
3. What is the function of crystalline lens in the human eye?
4. Which phenomenon is responsible for increasing the apparent length of the day by 4 minute?
5. What is the far point of a person suffering from Myopia.
6. What name is given to front transparent part of human eye?
7. Where do we see :(1) Concave and (2) Convex lens in bifocal lenses.
8. What is the nature of image formed by our eye?
9. Name the liquid which is present between eye lens and cornea.
10. Where does most of the refraction of light in an eye occurs?
11. Which kind of lens is an eye lens?
12. What is the cause of dispersion of light?
13. Under very dim light, we are able to see the objects but can not distinguish between colours why ?
14. What is the range of vision for normal human eye ?
15. How is the amount of light entering the eye controlled ?
16. What is the colour of danger signal? Why?
17. What is rainbow? How is rainbow formed?
18. State two causes of myopic vision.
19. How an uncorrected myopic eye sees far off objects.
20. What is presbyopia? Name the type of lens which can be used to correct presbyopia.
21. Explain why planet do not twinkle at night?
22. Explain about the colour of the sun at sunrise and sunset.
23. Define the term (1) Near point (2) Far point
24. Why is a normal eye not able to see clearly the object closer than 25 cm.
25. Draw a ray diagram to show the refraction of light through a glass prism on the diagram mark.
(a) Incident ray (b) Emergent ray and (c) Angle of deviation
26. How is the dispersed white light recomposed ?
27 The near point of a hypermetropic eye is at 75 cm from the eye. What is the power of the lens required to enable him to read clearly a book held at 25 cm from the eye.

LIGHT- REFLECTION AND REFRACTION - HOTS for class X

LIGHT- REFLECTION AND REFRACTION
HOTS Questions
1. Where is the image formed in a convex mirror, when the object is anywhere in
front of it ?
2. A person uses concave mirror for shaving, where should he position his face in
front of it ?
3. A ray of light is incident on a concave mirror along its principal axis. What will
be the angle of reflection?
4. What will happen to ray of light when it travels from rarer medium to a denser
medium ?
5. What does negative sign in the value of magnification of a mirror indicate?
6. Name the point inside the lens through which a ray of light goes undeviated?
7. Which of the two has a great power? A lens of short focal length or a lens of
large focal length?
8. Name the lens which always gives an erect and diminished image?
9. Which mirror is used as rear view mirror in vehicles and why ?
10. Define one dioptre?
11. The size of an object is 2cm.The magnification produced by a mirror is +1. What
is the size of the image?
12. When a ray of light passes from a denser medium to a rarer medium which angle
is greater: angle of incidence or angle of refraction?
13. An image formed in a spherical mirror has magnification -2.Is the image real or
virtual?
14. The power of a lens is -2D. Is the lens convex or concave?
15. Focal length of a convex mirror is 10cm.Find the radius of curvature of the
mirror?
16. An object is placed at a distance of 50cm from a convex mirror. State two
characteristics of the image formed.
17.Write two uses of concave mirror.
18. An object 1cm high produces a real image 1.5 cm high, when placed at a distance
of 15 cm from concave mirror. Calculate the position of the image.
19. Find the power of a concave lens of focal length 2m.
20. Which phenomenon occurs when light falls on(a) highly polished surface (b) a
transparent medium ?
21. What will happen to a ray of light when it falls normally on a surface ?
22. What is absolute refractive index ?
23. If refractive index of glass is 1.65, What is the speed of light in glass. ?
24. The magnification “ m “ for a mirror is +1 what does this signify ?

More Questions for Practice
1. What is angle of incidence?
2. A ray of light passing through centre of curvature of a concave mirror retraces its path on reflection, Why?
3. An object is placed at the focus of a concave mirror, Where is the image formed?
4. What is meant by refraction of light?
5. Define principal focus of a concave mirror?
6. State Snell’s law of refraction?
7. Will the lateral displacement increase/decrease if glass block is made more thicker?
8. Why convex lens is called conversing lens?
9. Printed letters appears diminished, when viewed through a lens. What is the nature of lens.?
10. At What angle a ray of light should strike the surface of glass, So that it does not suffer any refraction?
11. Does the value of speed of light change with medium?
12. What is the cause of refraction of light?
13. Which lens is used as a magnifying glass?
14. What is an optically denser medium of light?
15. What is the difference between reflection and refraction?
16. If a ray of light traveling in air is incident on the water surface obliquely, Draw a ray diagram and show the change in its path in water?
17. Define refractive index in terms of a speed of light in two media. What is the unit of refractive index?
18. A ray of light strikes the mirror at 15o , What is the angle of reflection?
19. What is refractive index of air? Why the refractive index of other medium is taken with respected to air?
20. Distinguish between real and virtual images?
21. For what position of an object, a virtual image is formed by a convex lens? Give ray diagram?
22. Find the position and nature of image formed in a concave mirror for the following position of an object. (a) At infinity (b) Beyond C.
23. An object is placed at a distance of 10cm from convex mirror of focal length 15cm; find the position and nature of image?
24. A thin lens has a focal length of -25cm. What is the power of the lens? Is it convex or concave?
25. Calculate the distance at which an object should be placed in front of convex lens of focal length 10cm to obtain an image double its size?
26. Why a mirror does has one principal focus while a lens has two principal foci?
27. Focal length of the lens in a photographic camera is 5cm.What is the power
and nature of the lens?
28. Define linear magnification. Does it have any unit?
29. Why a concave mirror has a real principal focus, while convex mirror has a virtual principal focus?
30. Which of the following lenses would you prefer to use while reading the small letters found in dictionary.
a. A convex lens of focal length 30 cm.
b. A concave lens of focal length 30 cm.
c. A concave lens of focal length 5 cm.
d. A convex lens of focal length 5 cm.

REVISION MATERIALS FOR CLASS X PHYSICS FOR CBSE

MAGNETIC EFFECTS OF ELECTRIC CURRENT

FOREVIEW
This chapter is concerned chiefly with magnetic field and the entire syllabus is covered under the following heads.

• Magnetic field and Magnetic field lines


• Magnetic field around a straight conductor carrying current


• Magnetic field due to a current carrying circular coil


• Magnetic field due to a current in a Solenoid


• Electromagnet and permanent magnet


• Force on a current carrying conductor in a magnetic field


• Electric motor


• Electromagnetic Induction.


• Direct and Alternating Current


• Electric Generator


• Overloading and Short Circuiting.

EXPOSITION OF THE SUBJECT MATTER
Magnetic Compass: It is a compact of magnetic needle which is pivoted at the centre of a small brass box with glass top. It is used to (a) To find the magnetic north-south direction.(b) To find the direction of magnetic field at a place and (c) To test the polarity of a magnet.
Magnetic field: It is the space around a magnet in which the force of attraction or repulsion due to the magnet can be detected. It has both magnitude and direction.
Sources of magnetic fields : (i) Natural and artificial magnets (ii) Electro magnets (iii)A conductor, a coil and a solenoid carrying current. (iv) Earth.
Magnetic field lines: It is the curved paths along which the iron filings arrange
themselves due to the force acting on them in the magnetic field of the bar magnet.
Magnetic Flux: It is the number of magnetic lines of force passing through the given area.

Properties of Magnetic field lines:
(i) They start from the north pole of a magnet and end at its south pole (outside the magnet).
(ii) They are always normal to the surface of the magnet.
(iii) They are closed and continuous curve.
(iv) Two lines of force do not intersect one another. If they intersect at a point, it would mean that compass needle will point towards two directions at that point which are not possible.
(v) They come closer to one another near the poles of a magnet but they are widely separated at other places.

OERSTED EXPERIMENT:
Oersted observed that when a magnetic needle is brought near the current carrying conductor, he observed that it undergoes deflection and also observed that when the direction of current is reversed, direction of deflection is also reversed.
Observation: -
• The North Pole of the needle is deflected towards east when current flows from North to South.(Fig a)
• The North Pole of the needle is deflected towards the west when the current flows from South to North. ( Fig b)
• There is no deflection in the needle if no current is passed.

The direction of deflection is given by Ampere's swimming rule.
Ampere's swimming rule: -
Imagine a man swimming along the conductor, the direction of current is from feet to head, looking at the needle, and then the north pole of the needle is deflected towards his left hand.
Magnetic field around a straight conductor carrying current:-
The magnetic field around a current carrying straight conductor consists of concentric circles of magnetic lines of force lying in a plane, which is right angle to the current carrying conductor. The conductor acts as the centre of magnetic lines of force. These lines are crowded near the conductor and become farther apart as the distance from the conductor increases. This indicates magnetic field near the conductor is stronger and becomes weaker as the distance from the conductor increases.
The magnitude of magnetic field produced by a straight current carrying wire at a given point is:
(i) Directly proportional to the current passing in the wire, and (ii) Inversely proportional to the distance of that point from the wire.
Magnetic field (µo I) / (2 π r)
Where o = Permeability of free space (constant)
I = Current flowing through the wire
r = radius of the circular wire
The direction of magnetic field is given by right -Hand Thumb Rule.
* Diagram – Refer NCERT Text Book

Right hand Thumb Rule:-
If a straight conductor is held in right hand, such that thumb point along the direction of the current, then the tips of the finger show the direction of magnetic field or magnetic lines of force. This is known as the Right -Hand Thumb Rule. This rule is also called Maxwell's Corkscrew Rule.
According to this rule , if we imagine a right handed screw placed along the current carrying conductor, be rotated such that the screw moves in the direction of flow of current, then the rotation of the thumb gives the direction of magnetic lines of force.

Magnetic field due to a current carrying circular coil :
In order to find the magnetic field due to a coil, it is held in a vertical plane and is made to pass through a smooth cardboard in such a way that the centre (O) of the coil lies at the cardboard. A current is passed through the coil and iron fillings are sprinkled on the cardboard. These iron filings arrange themselves in a pattern similar to one shown in the figure. (REF TEXT)

Conclusion:
The magnetic field lines near the coil are nearly circular and concentric.
The field lines are in the same direction in the space enclosed by the coil.
Near the centre of the coil, the field lines are nearly straight and parallel.
The direction of magnetic field at the centre is perpendicular to the plane of the coil.

The magnitude of magnetic field at the centre of the coil is
(a) Directly proportional to the current ( I ) flowing through it.
(b) Inversely proportional to the radius ( r )of the coil
(c) Directly proportional to the total number of turns (N) in the coil.
Magnetic field, B = ( 쁭o I ) / ( 2r )

The direction of magnetic field is given by right hand thumb rule.
Solenoid : It is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
Magnetic field due to a current in a Solenoid: The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet and the polarities of its ends depend upon the direction of current flowing through it.
Determination of polarities of a current carrying solenoid: Place it in a brass hook and suspend it with a long thread so that it moves freely .Bring north pole of bar magnet near one of its ends. In case the solenoid moves towards the bar magnet that end of the solenoid is a south pole and in case the solenoid moves away from the bar magnet that end of the solenoid is its north pole. The polarity of the other end of the solenoid can similarly be determined.
 The polarity of a solenoid can also be determined with the help of a Clock Rule.
The anti clockwise current in a face of the solenoid gives north polarity and clockwise current gives south polarity.
The lines of magnetic force pass through the solenoid and return to the other end as shown in figure. If a current carrying solenoid is suspended freely, it comes to rest pointing north and south acts like a suspended magnetic needle .One end of the solenoid acts like a N-pole and the other end a S-pole. Since the current in each circular turn of the solenoid flows in the same direction, the magnetic field produced by each turn of the solenoid adds up, giving a strong resultant magnetic field inside the solenoid. The strength of magnetic field produced by a current carrying solenoid depends on:
 The number of turns per unit length in the solenoid i.e B  n
 The strength of current in the solenoid i.e B  I
 The nature of core material used in making solenoid i.e B  
 Magnetic field B =μo n I

ELECTROMAGNET AND PERMANENT MAGNET
Electromagnet
An electric current can be used for making temporary magnets known as electromagnets. It works on the magnetic effect of current. It consists of a long coil of insulated copper wire wound on a soft iron core. To make an electromagnet all that we have to do is to take a rod NS of soft iron and wind a coil C of insulated copper wire round it. When the two ends of the copper coil are connected to a battery, an electromagnet is formed. It should be noted that the solenoid containing soft iron core in it acts as a magnet only as long as the current is flowing through the solenoid. If we switch off the current in the solenoid, it will not behave as a magnet. All the magnetism of the soft iron core disappears as soon as the current in the coil is switched off. A very important point to be noted is that it is the iron piece inside the coil which becomes a strong electromagnet on passing the current.

Factors affecting the strength of Electromagnet:
The strength of an electromagnet depends on:
• The number of turns in the coil. If we increase the number of turns in the coil, the strength of the electromagnet increases.
• The current flowing in the coil. If the current in the coil is increased, the strength of electromagnet increases.
• The length of air gap between the poles. If we reduce the length of air gap between the poles of an electromagnet, then its strength increases.
Uses of Electromagnets:
• Electromagnets are used in electrical devices such as an electric bell, an electric fan, telegraph, an electric train, an electric motor, generator, etc.
• For lifting and transporting large masses of iron in the form of girders.
• In medical practice for removing pieces of iron from wounds.

Permanent Magnets
Permanent magnets are usually made of alloys such as: Carbon steel, Chromium steel, Cobalt steel, Tungsten steel, and Alnico (Alnico is an alloy of Aluminium, Nickel, Cobalt and Iron). Permanent magnets of these alloys are much stronger than those made of ordinary steel.

Uses of Permanent Magnets:
• Electric meters (galvanometers, ammeters, voltmeters, speedometers, etc.)
• Microphones, Loudspeakers
• Electric clocks
S.No Permanent bar magnet Electromagnet
1 It is a permanent magnet It is a temporary magnet. Its magnetism is only for that duration till the current flows through it.
2 It produces a weak magnetic field It produces a strong magnetic field.
3 Its strength cannot be changed Its strength can be changed.
4 The north – south polarity of a permanent magnet is fixed. The north- south polarity of an electromagnet can be changed by changing the direction of current in the coil.

Force acting on a current carrying conductor in a magnetic field :
When a current carrying conductor is placed in a magnetic field, a mechanical force is exerted on the conductor which can make the conductor move.
The direction of force acting on a current carrying wire placed in a magnetic field is:
• Perpendicular to the direction of current
• Perpendicular to the direction of the magnetic field.

It should be noted that the maximum force is exerted on a current carrying conductor only when it is perpendicular to the direction of the magnetic field.
The direction of force on a current carrying conductor placed in a magnetic field can be reversed by reversing the direction of current flowing in the conductor.
The direction of force acting on the current-carrying conductor can be found using Fleming’s left-hand rule.
According to Fleming’s left-hand rule:
Hold the forefinger, the centre finger and the thumb of your left hand at right angles to one another. Adjust your hand in such a way that the forefinger points in the direction of the magnetic field and the centre finger points in the direction of current, the direction in which thumb points, gives the direction of force acting on the conductor.
Magnitude of Force: F=I L B
Where
B= magnitude of magnetic field
I = current flowing in the wire
L= length of the current-carrying wire placed in the magnetic

ELECTRIC MOTOR:
It converts electrical energy into mechanical energy.

Principle of a motor:
When a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously.

Construction:
Main components of electric motor are given below:
1. Armature: It consists of a large number of turns of insulated copper wire wound over a soft iron core.
2. Field Magnet: It produces magnetic field.
3. Split-Ring or Commutator: These are two halves of the same metallic ring. The ends of the armature coil are connected to these halves which also rotate with the armature.
4. Brushes or Sliding Contacts: These are two flexible metal plates or carbon rods which are so fixed that they constantly touch the revolving Commutator.
5. Battery: It is connected across the brushes. This battery supplies the current to the coil
 Diagram –Refer NCERT TEXT BOOK

Working:
(a) Let us suppose that the battery sends current to the armature in the direction South (S) to North (N). Applying Fleming’s Left-Hand Rule, we find that the arm BA experiences a force which is acting outwards and perpendicular to it and arm CD experiences a force which is acting inwards and perpendicular to it. These two forces form a couple, makes the armature rotates in the anti-clockwise direction.
(b) After the armature has completed half a revolution the direction of current in the arm BA and CD is reversed. Now arm CD experiences an outward force and arm BA experiences an inward force. The armature thus continues to rotate about its axis in the same anti-clockwise direction.

The speed rotation of the motor can be increased by
1. Increasing the current through the armature.
2. Increasing the number of turns in the coil of armature.
3. increasing the area of the coil
4. Increasing the strength of the magnetic field.

Uses of Electric Motors:
• They are used in electric fans for cooling and ventilation.
• They are used for pumping water.
• They are used in electric locomotives, electric cars, electric cranes and electric lifts.
• Small motors are used in various toys.
• Used in Washing machine.
QUESTIONS
1. What is the purpose of Fleming's left-hand rule?
2. A motor converts energy from one form to other .name the two forms in Sequence.
3. A generator converts energy from one form to other .name the two forms in Sequence.
4. A stream of positively charged particles are moving towards west is deflected towards north by a magnetic field. What is the direction of magnetic field?

ELECTROMAGNETIC INDUCTION
It is the phenomenon of producing induced current in a Moving conductor or coil in a magnetic field. The current produced by moving a straight wire in a magnetic field is called induced current.
It was discovered by Faraday. The direction of the induced current is given by Fleming’s right hand rule.
The potential difference corresponding to induced current is induced potential difference (Pd) or induced electromotive force (emf). The magnitude of the induced potential difference is directly proportional to the rate of change of magnetic flux.
In figure (a) : (i) When a bar magnet is pushed into the coil: The magnetic flux linked with the coil changes i.e increases. As a result of this , an induced current flows in the coil and the galvanometer shows a deflection .
(ii) When a bar magnet is taken from the coil : The magnetic flux linked with the coil changes i.e decreases. As a result of this , an induced current flows in the coil but in a direction opposite to that in case (i) . Obviously , the galvanometer shows a deflection in the opposite direction. .

Conclusion:
 Whenever there is a relative motion between a coil and the magnet, induced current flows through the coil.
 Large induced current is produced in the coil if the relative motion between the magnet and the coil is large.
(iii) When the bar magnet is held stationary inside the coil: When the bar magnet is held stationary inside the coil, there will be a magnetic flux in the coil but it will remain constant. Since the magnetic flux does not change, there is no induced current in the coil and the galvanometer shows no deflection.

Conclusion :
 Induced current is produced in a coil when varying current flows through a neighboring coil.
The direction of induced current produced in a straight conductor or coil moving in a magnetic field is given by Fleming’s Right Hand Rule.
According to Fleming’s Right Hand and Rule:
Hold the thumb, the forefinger and the centre of your right hand at right angles to one another. Adjust your hand in such a way that forefinger points in the direction of magnetic field, and thumb points in the direction of motion of the conductor, the direction in which centre finger points, gives the direction of induced current in the conductor.
Direct current: The current which has a constant magnitude and same direction is called direct current (D.C).
The frequency of the D.C current is zero.
Sources : Dry cell , dry cell battery , car battery and DC generator

Alternating current :
The current which changes in magnitude and direction at regular interval of time is called alternating current.
Frequency of AC is the number of cycles per second made by the current.
The frequency of the alternating current in India is 50 HZ.
Sources : House generator and Bicycle dynamo.
Advantages of AC over DC :
1. A.C can be transmitted over long distances without much loss of energy.
2. A.C can be produced easily and cheaply than D.C.
3. A. C voltage can be transformed to any desired value with help of a transformer.
4. A.C can be converted into D.C when required.

Disadvantages of A.C over D.C:
1. A.C is more dangerous than DC
2. A.C cannot be used for electroplating, electrotyping and other electrolytic processes.

ALTERNATING CURRENT (AC) GENERATOR:
It converts mechanical energy into electrical energy.
Principle:
It works on the principle of electro magnetic induction i.e When a coil is rotated in uniform magnetic field , electric current is induced in the Coil.

Construction:
Main components of a.c generator are given below :
1. Armature (abcd ): It consists of a large number of turns of insulated copper wire wound over a soft iron core.
2. Field Magnet: It produces magnetic field. The armature coil rotates between the pole pieces of the field magnet.
3. Slip Rings: The two ends of the armature coil are connected to two hollow metal rings. These rings rotate along with armature coil.
4. Brushes or Sliding Contacts: B1 and B2 are flexible metal plates or carbon rods. These are called brushes. The brushes remain fixed while slip rings rotate along with the armature.
 Diagram –Refer NCERT TEXT BOOK
Working:
As the armature coil is rotated about an axis , the magnetic flux linked with armature changes. Therefore, an induced current is produced in the armature coil.
(a) Let us suppose that the armature coil ABCD is rotating anti-clockwise so that the arm BA moves inwards and CD moves outwards. Applying Fleming’s Right Hand Rule, we find that the induced current in the armature coil and in the circuit is due to which galvanometer (G) shows deflection towards right.
(b) After the armature coil has turned through 180°, it occupies the position as shown in the fig.. With the armature coil rotating in the same direction, CD moves inwards and BA moves outwards. Thus, again applying Fleming’s Right Hand rule, we find the induced current in the external circuit flows in the opposite direction due to which the direction of deflection in galvanometer is towards left.

Domestic Electric Circuits:
It is a well known fact that the house connections to all the devices are made in parallel, each having independent switch and fuse . Thus ,whenever some fault occurs in circuit of one particular device in one room , devices in other rooms do not suffer.
Live wire (positive) – Red colour
Neutral wire – black colour
Earth wire – Green colour
Earthing : Connecting the metal case of electrical appliance to the earth by means of metal wire is called Earthing.
If the appliance is earthed , its body potential remains zero due to contact with the earth. No electric shock is felt when such an appliance is operated.

Overloading and Short- Circuiting :
Usually there are two separate circuits in a house, the lighting circuit with a 5A fuse and the power circuit with a 15A fuse.
All electrical appliances like bulbs, fans and sockets, etc., are connected tin parallel across the live wire and neutral wire.
An electric current more than the tolerable value will overheat the wire and can cause a fire.
The current may exceed the limit due to two reasons :
(i) Over-loading. (ii) Short – Circuiting.
(i)Over-loading : The too many electrical appliances of high power rating are switched on at the same time; they draw an extremely large current from the circuit. This is known as overloading the circuit.
Prevention : To avoid over –loading , circuit is divided in different sections having its own fuse in series. Also simultaneous use of high powered appliances must be avoided.
(ii) Short – Circuiting: The touching of the live wire and neutral wire directly is known as short – Circuiting.
When the two wires touch each other, the resistance of the circuit so formed is very, very small. Since the resistance is too small, the current flowing through the wire is very large and heats the wires to a dangerously high temperature and it may lead to fire accident.
This occurs when (a) the insulation of wires is damaged and (ii) there is a fault in the electric appliance.
Prevention: To avoid short-circuiting, good quality wire must be used. Wire used must be coated with PVC.
Electric Fuse : It is a device which is used in series to limit the current in an electric circuit so that it easily melts due to overheating when excessive current passes through it, and hence the circuit gets disconnected.
It is made of a wire of an alloy of lead (75 %) and tin (25%), which melts at around 200o C.
Few points regarding a fuse are as follows.
1. It is always connected in live wire and not in neutral wire.
2. It is always connected in the beginning of the circuit.
3. Fuses of various current capacities are available. Thicker fuse wire will always have higher current capacity.
QUESTIONS
1. What do you understand by live, neutral and earth wires? Do all the three normally carry electricity?
2. What is the function of the earth wire in electric lines? Why is the metallic body of an electric appliance connected to the earth wire?
3. What is short circuit? How does a fuse help in case there is short circuit?
4. What is the frequency of AC supply in India?
5. Name some sources of direct current.
6. Which sources produce alternating current?

REVIEW
1. Magnetic field: It is the space around a magnet in which the force of attraction or Repulsion due to the magnet can be detected. It has both magnitude and direction.
2. Magnetic field lines: It is the curved paths along which the iron filings arrange
Themselves due to the force acting on them in the magnetic field of the bar magnet.
3. Properties of Magnetic field lines: (i) It starts from the north pole of a magnet and end at its south pole.
(ii) It is a closed and continuous curve.
(iii) They do not intersect one another.
(iv) They come closer to one another near the poles of a magnet but they are widely separated at other places.
4. Oersted experiment: This experiment demonstrated that around every conductor carrying an electric current, there is a magnetic field. The direction of deflection of the needle due to magnetic field of a current carrying conductor is given by Ampere's Swimming Rule.

5. Magnitude of Magnetic field due to a current carrying straight conductor:
( µo I ) / (2 r)
Where B = Magnetic field
µo = Permeability of vacuum
I = Current flowing in conductor.
R = Distance from the conductor.
The magnetic lines of force round a straight conductor carrying current are concentric circles whose centers lie on the wire.
The direction of magnetic field produced by is given by a straight conductor carrying current Right –Hand Thumb Rule.
The SI unit of magnetic field is Tesla (T).

6. Magnetic field due to a current carrying circular coil
A circular coil consists of twenty or more turns of insulated copper wire closely wound together. The magnetic field produced at the center of a circular wire of radius r and carrying a current I is given by:
Magnetic field, B = (쁭o I )/ (2r )
Where o = Permeability of free space (constant)
I = Current flowing through the wire
r = radius of the circular wire

7. Solenoid: It is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet. The strength of magnetic field produced by a current carrying solenoid depends on: The number of turns in the solenoid
 The strength of current in the solenoid
 The nature of core material used in making solenoid

8. Electromagnets and Permanent Magnets
(a). Electromagnets: It works on the magnetic effect of the current. It consists of a long coil of insulated copper wire wound on a soft iron core. The core of the electromagnet must be of soft iron because soft iron looses all of its magnetism when coil is switched off.
(b). Permanent Magnets: a permanent magnet is made from steel. As steel has more retentivity than iron, it does not lose its magnetism easily.
Apart from steel alloys like Alnico (Aluminium, Nickel-Cobalt, alloy of iron) and Nipermag (an alloy of Iron, Nickel, Aluminium and Titanium) are used to make very strong permanent magnets.

9. Force on current carrying conductor placed in magnetic field
When a current carrying conductor is placed in a magnetic, a mechanical force is exerted on the conductor which can make the conductor move.
The direction of the force is given Flemings left hand rule.
The magnitude of force acting on a current carrying conductor placed in a magnetic field
F = B × I ×L
Where
B= magnitude of magnetic field
I = current flowing in the wire
L= length of the current-carrying wire placed in the magnetic field

10. Electric motor
A motor is a device which converts electrical energy into mechanical energy.
A motor works on the principle that whenever a current carrying conductor is placed in a magnetic field, it experiences a force given by Fleming’s left hand rule.

11. Electromagnetic induction: It is the phenomenon of producing induced current in a moving conductor or coil in a magnetic field. It was discovered by Faraday. The direction of the induced current is given by Fleming’s right hand rule.
The potential difference corresponding to induced current is induced potential difference (pd) or induced electromotive force (emf).

12. Electric generator: It converts mechanical energy into electrical energy. It works on principle of electromagnetic induction.

13. Direct current and alternating current: The current which does not change in direction and magnitude is called direct current (D.C).
The frequency of the D.C current is zero.
The current which changes its magnitude and direction after a certain fixed interval of time is called alternating current.
The frequency of the alternating current in India is 50 HZ.

14. Domestic electric circuits: Usually there are two separate circuits in a house, the lighting circuit with a 5A fuse and the power circuit with a 15A fuse.
All electrical appliances like bulbs, fans and sockets, etc., are connected tin parallel across the live wire and neutral wire.
To avoid the risk of electric shocks the metal body of an electrical appliance is “earthed”.

QUESTION BANK
One mark Questions:
1. How can you show that the magnetic field produced by a given electric current in the wire decreases as the distance from the wire decreases?
2. What is the advantage of the third wire of earth connection in domestic appliances?
3. What constitutes the field of a magnet?
4. What is short-circuiting in an electric supply?
5. What will be the frequency of an alternating current if its direction changes after every 0.01s?
6. An alternating electric current has a frequency of 50 Hz. How many times does it change its direction in 1s?
7. How is the strength of the magnetic field at a point near a wire related to the strength of the electric current flowing in the wire?
8. How can it be shown that a magnetic field exists around a wire through which a direct current is passing?
9. On what effect of an electric current does an electromagnet work?
10. What is the direction of magnetic field at the centre of a circular coil carrying current in anticlockwise direction?

Two Mark Questions
1. With the help of a neat-diagram, describe how you can generate induced current in a circuit.
2. What is meant by the term “Magnetic field Lines”? List two properties of magnetic field lines.
3. Write the rule which determines the direction of magnetic field developed around a straight conductor when current is passed through the conductor.
4. State the rule to determine the direction of magnetic field produced around a current carrying conductor.
5. On which factors does the force experienced by a current carrying conductor placed in a uniform magnetic field depend?
6. State Fleming’s right-hand Rule.
7. Why is series arrangement not used for domestic circuits?
8. Differentiate between electric force and magnetic forces.
9. How does AC differ from DC? What are the advantages and disadvantages of AC over DC?
10. Draw the magnetic field due to a current carrying circular coil. State the clock rule to find the polarities of the faces of the coil.

Three Mark Questions
1. Draw the pattern of field lined due to a solenoid carrying electric current. Mark the north and the south poles in the diagram.
2. Draw the pattern of lines of force due to a magnetic field through and around a current carrying loop of wire. How would the strength of the magnetic field produced at the centre of the circular loop be affected if (i) the strength of the current passing through this loop is doubled? (ii) the radius of the loop is reduced to half of the original radius?
3. Draw the pattern lines of force due to a magnetic field associated with a current carrying conductor. State how the magnetic field produced changes (i) with an increase in current in the conductor and (ii) the distance from the conductor.
4. Draw the pattern of field lines due to a bar magnet. Mention any two properties of the magnetic field lines.
5. How does the strength of the magnetic field at the centre of a circular coil of wire depend on: (i) the radius of the coil? (ii) the number of turns of the wire? (iii) the strength of the current flowing in the coil?
6. The flow of a current in a circular loop of a wire creates a magnetic field at its centre. How can existence of the field be detected? State the rule which helps to predict the direction of this magnetic field.
7. What are the factors on which the strength of magnetic field produced by current-carrying solenoid depends?
8. A coil of copper wire is connected to a galvanometer. What would happen if a bar magnet is: (i) pushed into the coil with north pole entering first (ii) pulled out of the coil (iii) held stationary inside the coil?
9. Explain what is short-circuiting and overloading in an electric supply.
10. What are magnetic field lines? How is the direction of a magnetic field at a point determined? Mention two important properties of the magnetic field lines.

Five Mark Questions:
1. (a) Suggest an activity to show the pattern of magnetic field lines, when you are provided with a bar magnet, a cardboard piece and iron filings.(b)Draw a rough sketch of the field lines which you will observe.
2. (a) What is an electromagnet? What does it consists of? (b) Name one material in each case used to make a (i) permanent magnet (ii) temporary magnet. (c) Describe an activity to show how can you make an electromagnet in your school lab?
3. State Fleming’s left-hand rule. With a labeled diagram, describe the working of an electric motor. What is the function of split-ring Commutator in a motor?
4. State Fleming’s right-hand rule. With a labeled diagram, describe the working of an AC electric generator.
5. Draw the lines of force of the magnetic field through and around (a)single loop of wire carrying current, (b) a solenoid carrying electric current.
6. Why is pure iron not used for making permanent magnets? Na