Tuesday, July 26, 2011

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

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: -
â€¢ The North Pole of the needle is deflected towards east when current flows from North to South.(Fig a)
â€¢ The North Pole of the needle is deflected towards the west when the current flows from South to North. ( Fig b)
â€¢ 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 (Âµo I) / (2 Ï€ r)
Where ï­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 â€“ 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 = ( μ­o 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.
ï¶ 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:
ï¶ The number of turns per unit length in the solenoid i.e B ï¡ n
ï¶ The strength of current in the solenoid i.e B ï¡ I
ï¶ The nature of core material used in making solenoid i.e B ï¡ ï­
ï¶ 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:
â€¢ The number of turns in the coil. If we increase the number of turns in the coil, the strength of the electromagnet increases.
â€¢ The current flowing in the coil. If the current in the coil is increased, the strength of electromagnet increases.
â€¢ 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:
â€¢ Electromagnets are used in electrical devices such as an electric bell, an electric fan, telegraph, an electric train, an electric motor, generator, etc.
â€¢ For lifting and transporting large masses of iron in the form of girders.
â€¢ 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:
â€¢ Electric meters (galvanometers, ammeters, voltmeters, speedometers, etc.)
â€¢ Microphones, Loudspeakers
â€¢ 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 â€“ 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:
â€¢ Perpendicular to the direction of current
â€¢ 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 Flemingâ€™s left-hand rule.
According to Flemingâ€™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
ï¶ Diagram â€“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 Flemingâ€™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:
â€¢ They are used in electric fans for cooling and ventilation.
â€¢ They are used for pumping water.
â€¢ They are used in electric locomotives, electric cars, electric cranes and electric lifts.
â€¢ Small motors are used in various toys.
â€¢ 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 Flemingâ€™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:
ï‚§ Whenever there is a relative motion between a coil and the magnet, induced current flows through the coil.
ï‚§ 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 :
ï¶ 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 Flemingâ€™s Right Hand Rule.
According to Flemingâ€™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.
ïƒ˜ Diagram â€“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 Flemingâ€™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 180Â°, 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 Flemingâ€™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) â€“ Red colour
Neutral wire â€“ black colour
Earth wire â€“ 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.

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 : 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 â€“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 â€“ Circuiting: The touching of the live wire and neutral wire directly is known as short â€“ 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
Âµo = 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 â€“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 = (μ­o I )/ (2r )
Where ï­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
ï¶ The strength of current in the solenoid
ï¶ 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 Ã— I Ã—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 Flemingâ€™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 Flemingâ€™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 â€œearthedâ€.

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 â€œMagnetic field Linesâ€? 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 Flemingâ€™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 Flemingâ€™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 Flemingâ€™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