Friday, September 21, 2012

Gear Ratios / Mechanics

Hi, I was just wondering if one had a system of 2 different gear ratios to one output shaft.

For example: Two identical motors A and B rev at 1000 rpm and have a torque of 10Nm. These motors are geared in the ratios of 2:1 and 0.5:1 respectively to the same output shaft, what would be the output torque and rpm, what would the characteristics of this setup, and does this type of system have any advantages over an ordinary system ?

Thanks in advance

(Posted by Raniero Falzon )


Sunday, September 16, 2012

Working of Inductors

What is an inductor?

  • An inductor is merely, a wire wound to form a coil. It is a passive two

terminal device or electrical component which has the capability to store energy in magnetic field.

Varying the current flowing in the inductor with time produces or induces a voltage in the coil in accordance with the faraday’s law of electromagnetic induction which states that an EMF proportional to the rate of change of magnetic flux is induced in the coil or the closed circuit . This voltage according to the Lenz’s Law opposes the change in current which created it.

Inductors are the basic electrical components used in electric circuits which require the value of the current and voltage flowing through it to be changed with time. Inductors possess properties to delay and reshape the alternating current.

How an inductor works?
Let us consider a circuit with a battery , a switch ,  a light bulb , a coil of wire wound around a piece of Iron . The coil of wire used here is an inductor with an Iron core.  Now if we take this circuit without the inductor, the bulb lights up normally but if the inductor is taken into consideration the behavior of the circuit is totally different.

The light bulb acts as a resistor .The wire used here has a low resistance and one can expect that the current shall follow the low resistance path and bulb will be very dimly lit up. But what happens is that when we close the switch the bulb burns brightly and gets dimmer. When the switch is open the bulb lights up brightly and turns off quickly.
This happens due to the presence of inductor.

When current flows into the coil, a magnetic field is set up. While this field is developing, the coil stops the flow of current and allows the current to flow once the field is setup. When we open the switch, magnetic field present keeps the current flowing in the circuit and this current goes off as the magnetic field in this coil collapses.
This shows the fact that the inductor stores energy in the magnetic field and the inductors resist any change in the value of current flowing through it.


  • HenryStandard unit of inductance is Henry.  Named after a scientist Joseph Henry who discovered electromagnetic induction with Faraday.

Capacity of an inductor depends upon the following four factors

  • Number of coils – More the number of coils more is the inductance.

  • The Material around which the coil is wrapped (core)

  • Cross Sectional Area of the coil - Larger the area, more the inductance.

  • Length of the coil –smaller the coil, more the inductance as smaller coil means narrower coils.The formula for calculating the capacity of an inductor is given by
    H= 4π X number of turns X area of coil X mu / coil lengthwhere mu is permeability of core. Permeability of air is 1 while that of steel is 2000. 

  • Applications Of Inductors

  • Used extensively in analog circuits and processing of the signal.

  • Along with capacitors in many tuning circuits.

  • Used in transformers to have coupled magnetic flux from two or more inductors.

  • In switch mode power supplies, inductors are used to store energy.

  •   In electrical transmission systems to reduce the voltage from lightning strikes and to limit switching as well as fault currents.


A Problem from Optics - Reflection of light

Convex and concave mirrors have same radii of curvature R. The distance between the mirrors is 2R. At what point on the common optical axis of the mirrors should a point source of light A be placed for the rays to converge at the point A after being reflected first on the convex and then on the concave mirror?

Thursday, September 13, 2012

Quick Revision for Class X Physics SA1



  1. Positive and negative charges: The charge acquired by a glass rod when rubbed with silk is called positive charge and the charge acquired by an ebonite rod when rubbed with wool is called negative charge.

  2. Coulomb: It is the S.I. unit of charge. One coulomb is defined as that amount of charge which repels an equal and similar charge with a force of 9 x 109 N when placed in vacuum at a distance of 1 meter from it. Charge on an electron = -1.6 x 10-19 coulomb.

  3. Static and current electricities: Static electricity deals with the electric charges at rest while the current electricity deals with the electric charges in motion.

  4. Conductor: A substance which allows passage of electric charges through it easily is called a ‘conductor’. A conductor offers very low resistance to the flow of current. For example copper, silver, aluminium etc.

  5. Insulator: A substance that has infinitely high resistance does not allow electric current to flow through it. It is called an ‘insulator’. For example rubber, glass, plastic, ebonite etc.

  6. Electric current: The flow of electric charges across a cross-section of a conductor constitutes an electric current. It is defined as the rate of flow of the electric charge through any section of a conductor.                                                                    

       Electric current = Charge/Time     or        I = Q/t

Electric current is a scalar quantity.

  1. Ampere: It is the S.I. unit of current. If one coulomb of charge flows through any section of a conductor in one second, then current through it is said to be one ampere.                                                                                                                         1 ampere = 1 coulomb/1 second    or      1 A = 1C/1s = 1Cs-1                                                                                                                                                                                                              1 milliampere =    1 mA = 10-3 A                                                                                                                                                                                 1 microampere = 1µA = 10-6 A

  2. Electric circuit: The closed path along which electric current flows is called an ‘electric circuit’.

  3. Conventional current: Conventionally, the direction of motion of positive charges is taken as the direction of current. The direction of conventional current is opposite to that of the negatively charged electrons.

  4. Electric field: It is the region around a charged body within which its influence can be experienced.

Sure Shot questions in Physics for CBSE Class 9 Summative Assessment (SA1)

All CBSE Schools are conducting the SA1 (First Summative Assessment) in the month of September  (Kendriya Vidyalayas have already started).  At this point, we found it would be useful to the students to have a set of sure shot questions. Practising these will essentially help you score better marks in the forthcoming exams.

 Portions for SA1

MOTION, FORCE AND WORK (Motion, Force and Newton's Laws of motion, Gravitation)

Long Answer Type Questions

  1. Derive equations of uniformly accelerated motion using graphical representation of motion.

  2. Derive F=ma

  3. State the law of conservation of linear momentum. Illustrate with an example

  4. State and explain Archimedes' principle.

  5. Describe an experiment to verify Archimedes' principle.

  6. Distinguish density and relative density.

  7. What are the effects produced by force?

  8. What is friction? How is it caused? How can it be reduced?

  9. Define impulse of a force.

  10. Why does a cricket fielder pulls his hands backwards while taking a catch?

  11. Define inertia and explain its types with suitable examples.

  12. Describe two instances each where pressure is increased by decreasing the area and pressure is decreased by increasing hte area.

  13. State Newton's universal law of gravitation

  14. Define G.

  15. Distinguish g and G

  16. Why motion and rest are said to be relative terms?

  17. Define acceleration due to gravity at a place and discuss its variation with height, depth and latitude.

  18. Write the differences between mass and weight.

  19. Can an object be accelerated if it is moving with constant speed? Justify your answer with an example

  20. Why is a person hurt more when he falls on a concrete floor than when he falls on a heap of sand from the same height?

  21. The weight of an object on the surface of moon is 1.67N and its mass on its surface is 1 kg. Calculate its weight and mass on the surface of earth, (g on earth = 10 m/s2).

  22. When a horse suddenly starts running, a careless rider falls backwards. Explain why?

  23. State the action and reaction in the swimming action of a swimmer.

  24. A stone is thrown vertically upwards with a velocity of 40 m/s and is caught back. Taking g=10 m/s2, calculate the maximum height reached by the stone.What is the net displacement and the total distance covered by the stone?

  25. Read more at:

    Read more at:


Thursday, September 6, 2012

Download ISC Physics Specimen Papers and Sample Papers for 2013

ISC Specimen Question Papers 2013

Practising sample papers is considered as a good practice to score better in exams. Start with the specimen paper supplied by the board first. We'll be providing more sample papers soon.

Download the ISC Physics Specimen Paper 2013 here

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Monday, September 3, 2012

Summative Assessment 1 (SA1) Sample Papers for class 9 Physics

The Summative Assessment 1 is about to begin. As per request from student from various parts of India and abroad, we are publishing some solved sample papers in Physics, which we think, would help you in scoring better marks in the forthcoming SA1 exams.

india calcutta bookstore

(Based on Motion)


Class 9 Physics Sample Paper 1  Solved

Class 9 Physics Sample Paper 2  Solved

Can the frictional electricity produced during landing of a plane be used for constructive purposes?

As we know that in aeroplane during landing the charged produced in it due to the friction with ground are earthed through its wheels; can this charge be stored with the help of a capacitor and used for useful purpose?


I feel that this is a metter for research.

If you feel otherwise and know some method to tap this potential, please post.