Class-X

Physics

Chapter-13

Magnetic effect of electric current

Force acting on a Current carrying conductor

An electric current flowing through a conductor produces a magnetic field.

This field will exert a force on a magnet placed in the proximity of the

conductor.

Andre Marie Ampere (1775–1836) suggested that the magnet also exert an

equal and opposite force on the current carrying conductor.

We will observe that the rod will displace i.e. the rod will experience a force,

when it is placed in magnetic field, in a perpendicular direction to its length.

The direction of the exerted force will be reversed if the direction of current

through the conductor is reversed.

If we change the direction of field by inter changing the two poles of the

magnet, again the direction of exert force will change.

Therefore, the direction of exerted force depends on

(a) direction of current

(b) direction of magnetic field lines.

Fleming’s Left-Hand Rule

Fleming’s Left-Hand Thumb Rule says- ‘Stretch the thumb, forefinger and

middle finger of your left hand such that they are mutually perpendicular

to each other (as shown in the figure). If the middle finger points in the

direction of the current in the conductor, the forefinger points in the

direction of the magnetic field and the thumb points in the direction of the

force acting on the conductor.’

OR

Electric Motor

Electric motor is a device that converts electrical energy to mechanical

energy.

Principle of electric motor:

A motor works on the principle that 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 of an Electric Motor

a) Insulated Copper wire

b) Magnet Poles: A Horse shoe magnet. This creates a magnetic field

c) Split Rings: Two disjoint C

commutator (which can reverse the direction of current)

d) Axle: The split rings are

e) Brushes: The outside of the split rings are connected to conducting

brushes X and Y.

f) Source Battery: To source current

1.When electric current is passed into the rectangular coil, this current

produces a magnetic field around the coil.

2.The magnetic field of horse shoe

magnetic field of the coil and causes the coil to rotate continuously.

3.If the coil ABCD is in horizontal position current from battery enters the

coil through brush B1, and commutator half ring C

direction ABCD and leaves via ring C

A motor works on the principle that when a rectangular coil is placed in a

magnetic field and current is passed through it. A force acts on the coil which

Construction of an Electric Motor

Copper wire: A rectangular coil of wire ABCD

: A Horse shoe magnet. This creates a magnetic field

: Two disjoint C-shaped rings P and Q. It acts as a

commutator (which can reverse the direction of current)

: The split rings are placed on the axle which can rotate freely.

: The outside of the split rings are connected to conducting

: To source current

1.When electric current is passed into the rectangular coil, this current

netic field around the coil.

2.The magnetic field of horse shoe-type magnet then interacts with the

magnetic field of the coil and causes the coil to rotate continuously.

3.If the coil ABCD is in horizontal position current from battery enters the

, and commutator half ring C1. The current flows in the

direction ABCD and leaves via ring C2 and brush B2.

A motor works on the principle that when a rectangular coil is placed in a

magnetic field and current is passed through it. A force acts on the coil which

: A rectangular coil of wire ABCD

: A Horse shoe magnet. This creates a magnetic field

shaped rings P and Q. It acts as a

commutator (which can reverse the direction of current)

placed on the axle which can rotate freely.

: The outside of the split rings are connected to conducting

1.When electric current is passed into the rectangular coil, this current

type magnet then interacts with the

magnetic field of the coil and causes the coil to rotate continuously.

3.If the coil ABCD is in horizontal position current from battery enters the

The current flows in the

4.The direction of current in the arm AB of the coil ABCD is from A to B and

the direction of current in the arm CD of the coil ABCD is from C to D. The

direction of magnetic field is from N to S. Now applying Fleming’s left- hand

rule to arms AB and CD of the coil we find that the force on side AB of the

coil is in the downward direction. Whereas the force on the side CD of the

coil is in the upward direction. As a result, coil ABCD rotates in anti-clockwise

direction.

5.While rotating when the coil reaches vertical position then the brushes B1

and B2 will touch the gap between the commutator rings and current to the

coil is cut off. But the coil does not stop rotating because it has already

gained momentum.

6.At half rotation, C2 makes contact with brush B1 and C1 with brush B2.

Therefore, the current in the coil gets reversed and flows along the path

DCBA. The reversal of current also reverses the direction of force acting on

the two arms AB and CD. Thus, the arm AB of the coil that was earlier pushed

down is now pushed up and the arm CD of the coil previously pushed up is

now pushed down. Therefore, the coil and the axle rotate half a turn more in

same direction.

7.The reversing of current in the coil is repeated after every half rotation

due to which the coil continues to rotate as long as current from the battery

is passed through it. The rotating shaft of electric motor can drive a large

number of machines which are connected to it.

Applications:

It is used as an important component in Electric fans, refrigerators, mixers,

washing machines, MP3 players etc.

Significance of Split rings in an electric motor:

A device that reverses the direction of flow current through a circuit is called

a commutator. The split rings in the electric motor acts as a commutator.

Significance of brushes in an electric motor:

We can notjoin the battery wires directly to the two commutator half rings

to pass current into the coil because if we do so, then the connecting wires

will get twisted when the coil rotates. So, to pass in electric current to the

coil, we use two stationary conducting brushes. Current in the coil enters

from the source battery through first brush and flows back to the battery

through second brush.

The main function of brushes is to make contact with the rotating rings of

the commutator and through them to supply current to the coil.

Electromagnetic Induction

Michael Faraday gave the law of

that when a conductor is set to move inside a magnetic field or a magnetic

field is set to be changing around a conductor, electric current is induced in

the conductor.

Faraday’s experiment

Electromagnetic Induction Can be explained by two experiments

(a) First Experiment “Self

In this experiment, when the north pole of bar magnet is brought closer to

the coil or away from the coil, we see momentary deflection in the needle of

galvanometer on either side of null point. First right and then left.

Similarly, if we keep the magnet stationery and coil is made to move towards

or away from the north pole of magnet. Again, we will observe deflection in

the needle of galvanometer.

If both bar magnet and coil are kept stationary, there will be no deflection in

galvanometer. This experiment can also be done with the south pole of

magnet, we will observe the deflection in galvanometer, but it would be in

opposite direction to the previous case.

Conclusion:

It concludes that motion of magnet with respect to coil or vice

changes the magnetic field. Due to this change in magnetic field lines,

hrough first brush and flows back to the battery

The main function of brushes is to make contact with the rotating rings of

the commutator and through them to supply current to the coil.

Electromagnetic Induction

the law of Electromagnetic Induction

that when a conductor is set to move inside a magnetic field or a magnetic

field is set to be changing around a conductor, electric current is induced in

Induction Can be explained by two experiments

“Self-Induction”

In this experiment, when the north pole of bar magnet is brought closer to

the coil or away from the coil, we see momentary deflection in the needle of

ither side of null point. First right and then left.

Similarly, if we keep the magnet stationery and coil is made to move towards

or away from the north pole of magnet. Again, we will observe deflection in

the needle of galvanometer.

coil are kept stationary, there will be no deflection in

galvanometer. This experiment can also be done with the south pole of

magnet, we will observe the deflection in galvanometer, but it would be in

opposite direction to the previous case.

It concludes that motion of magnet with respect to coil or vice

changes the magnetic field. Due to this change in magnetic field lines,

hrough first brush and flows back to the battery

The main function of brushes is to make contact with the rotating rings of

the commutator and through them to supply current to the coil.

Electromagnetic Induction. According to

that when a conductor is set to move inside a magnetic field or a magnetic

field is set to be changing around a conductor, electric current is induced in

Induction Can be explained by two experiments

In this experiment, when the north pole of bar magnet is brought closer to

the coil or away from the coil, we see momentary deflection in the needle of

ither side of null point. First right and then left.

Similarly, if we keep the magnet stationery and coil is made to move towards

or away from the north pole of magnet. Again, we will observe deflection in

coil are kept stationary, there will be no deflection in

galvanometer. This experiment can also be done with the south pole of

magnet, we will observe the deflection in galvanometer, but it would be in

It concludes that motion of magnet with respect to coil or vice-versa,

changes the magnetic field. Due to this change in magnetic field lines,

potential difference is induced in the same coil, which set up an induced

current in the circuit.

(b) Second Experiment: Mutual Induction

In this experiment plug in the key that connects coil

battery and observe the deflection in galvanometer which is connected with

coil-2 (secondary coil) . Now plug out the key that disconnect the coil

battery and observe the deflection in galvanometer in coil

reverse direction.

Hence, we conclude that potential difference is induced in secondary coil

(coil-2), whenever there is a change in current, in primary coil (coil

and off of key).

This is because, whenever there is change in current in primary coil

Magnetic field associated with it also changes

Now, magnetic field lines around the secondary coil (coil

induces the electric current in it (obser

Galvanometer in secondary circuit)

This process, by which changing of strength of current in primary coil,

induces a current in secondary coil is called Electromagnetic Induction”

The induced current is found to be

coil is at right angles to the magnetic field.

FARADAY'S LAW

Faraday's Law, discovered in 1831 by Michael Faraday, states that the

induced electromotive force in a closed circuit is equal to the time rate of

change of the magnetic flux through the circuit. Under Faraday's Law,

potential difference is induced in the same coil, which set up an induced

periment: Mutual Induction

In this experiment plug in the key that connects coil-1 (primary coil ) with

battery and observe the deflection in galvanometer which is connected with

2 (secondary coil) . Now plug out the key that disconnect the coil

battery and observe the deflection in galvanometer in coil-

Hence, we conclude that potential difference is induced in secondary coil

2), whenever there is a change in current, in primary coil (coil

This is because, whenever there is change in current in primary coil

Magnetic field associated with it also changes

Now, magnetic field lines around the secondary coil (coil-2) will change and

induces the electric current in it (observed by the deflection of needle of

Galvanometer in secondary circuit)

This process, by which changing of strength of current in primary coil,

induces a current in secondary coil is called Electromagnetic Induction”

The induced current is found to be highest when the direction of motion of

coil is at right angles to the magnetic field.

Faraday's Law, discovered in 1831 by Michael Faraday, states that the

induced electromotive force in a closed circuit is equal to the time rate of

f the magnetic flux through the circuit. Under Faraday's Law,

potential difference is induced in the same coil, which set up an induced

1 (primary coil ) with

battery and observe the deflection in galvanometer which is connected with

2 (secondary coil) . Now plug out the key that disconnect the coil-1 from

-2, which will be in

Hence, we conclude that potential difference is induced in secondary coil

2), whenever there is a change in current, in primary coil (coil-1) (by on

This is because, whenever there is change in current in primary coil

2) will change and

ved by the deflection of needle of

This process, by which changing of strength of current in primary coil,

induces a current in secondary coil is called Electromagnetic Induction”

highest when the direction of motion of

Faraday's Law, discovered in 1831 by Michael Faraday, states that the

induced electromotive force in a closed circuit is equal to the time rate of

f the magnetic flux through the circuit. Under Faraday's Law,

electric current is induced only if either the magnetic field is changing or the

conductor is moving.

Electromagnetic induction

The phenomenon of electromagnetic induction is the production of i

EMF and thereby current in a coil, due to the varying magnetic field with

time. If a coil is placed near to a current

field changes due to a change in I or due to the relative motion between the

coil and conductor. The direction of the induced current is given by Fleming’s

right-hand rule.

Fleming’s right-hand rule

According to Fleming’s right

finger of the right hand are stretched to be perpendicular to each other as

indicated below, and if the thumb indicates the direction of the movement of

conductor, fore-finger indicating direction of the magnetic field, then the

middle finger indicates direction of the induced current.

Galvanometer

It is an instrument that can detec

pointer is at zero (the centre of scale) then there will be no flow of current.

If the pointer deflects on either side right or left, this will show the direction

of current. Represented by

electric current is induced only if either the magnetic field is changing or the

Electromagnetic induction

The phenomenon of electromagnetic induction is the production of i

EMF and thereby current in a coil, due to the varying magnetic field with

If a coil is placed near to a current-carrying conductor, the magnetic

field changes due to a change in I or due to the relative motion between the

he direction of the induced current is given by Fleming’s

hand rule

According to Fleming’s right-hand rule, the thumb, forefinger and middle

finger of the right hand are stretched to be perpendicular to each other as

ted below, and if the thumb indicates the direction of the movement of

finger indicating direction of the magnetic field, then the

middle finger indicates direction of the induced current.

It is an instrument that can detect the presence of a current in a circuit. If

pointer is at zero (the centre of scale) then there will be no flow of current.

If the pointer deflects on either side right or left, this will show the direction

of current. Represented by

electric current is induced only if either the magnetic field is changing or the

The phenomenon of electromagnetic induction is the production of induced

EMF and thereby current in a coil, due to the varying magnetic field with

carrying conductor, the magnetic

field changes due to a change in I or due to the relative motion between the

he direction of the induced current is given by Fleming’s

hand rule, the thumb, forefinger and middle

finger of the right hand are stretched to be perpendicular to each other as

ted below, and if the thumb indicates the direction of the movement of

finger indicating direction of the magnetic field, then the

t the presence of a current in a circuit. If

pointer is at zero (the centre of scale) then there will be no flow of current.

If the pointer deflects on either side right or left, this will show the direction

Types of electric current

Advantages of Alternate Current (AC) over Direct Current (DC)

Electric power can be transmitted to longer distances without much loss of

energy. Therefore, cost of transmission is low.

In India the frequency of AC is 50Hz. It means after every 1/100 second it

changes its direction.

INTEXT QUESTIONS

Page no.224

Ans 1):

The compass needle is a small magnet. When the compass needle is brought

close to a bar magnet, the magnetic field lines of the compass needle

interact with the magnetic field lines of bar magnet which causes the

compass needle to deflect.

Page no.228

Ans 1)

Magnetic field lines of a bar magnet emerge from the North Pole and

terminate at the South Pole as shown in the figure below.

Ans 2).

The properties of magnetic field lines are as follows:

i. Magnetic field lines originate from North pole and end at South pole

and inside the magnet, the direction of field lines is from its south

pole to its north pole. Thus, the magnetic field lines are closed

curves.

ii. Magnetic field lines come closer to one another near the poles of

magnet but widely separated at other places.

iii. The magnetic field lines do not intersect (or cross) one another.

Ans 3).

If two magnetic field lines intersect then at the point of intersection the

compass needle shows two different direction which is not possible hence

they do not intersect with each other.

Page no.229

Ans 1).

Let the current pass through the circular loop lying on the plane of the table

in clockwise direction, then the direction of the magnetic field will be as if

they are emerging from the table outside the loop and merging to the table

inside the loop.

Ans 2).

The magnetic field lines inside a current-carrying long straight solenoid are

parallel.

Ans 3). Choose the correct option.

The magnetic field inside a long straight solenoid-carrying current

a) is zero.

b) decreases as we move towards its end.

c)) increases as we move towards its end.

d) is the same at all points.

Ans:

d. is the same at all points

The magnetic field inside a long straight current carrying solenoid is uniform

therefore it is same at all points.

NCERT INTEXT QUESTIONS

Page no.231

Ans 1).

(c) and (d)

When a proton enters the region of magnetic field, it experiences magnetic

force. Due to which the path of the proton becomes circular. As a result, the

velocity and the momentum change.

Ans 2).

A current carrying conductor when placed in a magnetic field experiences

force. The magnitude of this force will increase with the increase in the

amount of current, length of conductor and the strength of the magnetic

field. Hence, the strength of the magnetic force exerted on the rod AB and its

displacement will increase if

i) The current in rod AB is increased

ii) Stronger horse shoe magnet is used

iii) When the length of the rod AB increases

Ans 3).

The direction of the magnetic field can be determined using the Fleming’s

Left- hand rule. According to the rule, if we arrange our thumb, forefinger

and the middle finger of the left hand right perpendicular to each other, then

the thumb points towards the direction of the magnetic force, the middle

finger the direction of current and the forefinger the direction of magnetic

field. Since the direction of positively charged particle is towards west, the

direction of the current will also be towards the west. The direction of the

magnetic force is towards the north hence the direction of magnetic field will

be upward according to Fleming’s Left-hand rule.

Page no.233

Ans 1).

Fleming’s Left-Hand Thumb Rule says- ‘Stretch the thumb, forefinger and

middle finger of your left hand such that they are mutually perpendicular

to each other (as shown in the figure). If the middle finger points in the

direction of the current in the conductor, the forefinger points in the

direction of the magnetic field and the thumb points in the direction of the

force acting on the conductor.’

Ans 2).

The working principle of electric motor is based on the magnetic effect of

current. A current carrying conductor when placed in a magnetic field

experiences force and rotates. The direction of the rotation of the conductor

can be determined by Fleming’s Left-hand rule.

Ans 3).

Split ring plays the role of commutator in an electric motor. The commutator

reverses the direction of the current flowing through the coil after each half

rotation of the coil. Due to this reversal of current, the coil continues to

rotate in the same direction.

Page No.236

Ans 1.

Following are the different ways to induce current in a coil:

If the coil is moved rapidly between the two poles of horse shoe

magnet, electric current is induced in the coil.

When a magnet is moved relative to the coil, an electric current is

induced in the coil.

Page No.237

Ans 1.

Principal: An electric generator works on the principle of electromagnetic

induction. When a coil is rotated between the magnet or when the magnet is

rotated in and out of the coil the current is induced in the coil and the

direction of current is given by Fleming's right-hand rule.

Ans 2.

A voltaic cell, a dry cell, battery, DC generator, etc. are some sources of

direct current.

Ans 3

Hydro-electric power plant, Thermal power plant, Nuclear power plant

generators are some of the sources that produce alternating current.

Ans 4.

c. half revolution

When a rectangular coil is rotated in magnetic field, the direction of the

induced current changes once in half revolution. As result, the direction of

the current in the coil remains the same.

NCERT EXERCISES

Ans 1.

(iv) The field consists of concentric circles centred on the wire

Ans 2.

(iii) Producing induced current in a coil due to relative motion between a

magnet and the coil

Ans 3.

(i) Generator.

Ans 4.

(iv) AC generator has slip rings while the DC generator has a commutator

Ans 5.

(iii) Increases heavily.

Question 6

State whether the following statements are True or False.

(i) An electric motor converts mechanical energy into electrical energy.

False: electric motor converts electrical energy into mechanical energy.

(ii) An electric generator works on the principle of electromagnetic induction.

True

(iii) The field at the centre of a long circular coil carrying current will be

parallel straight lines. True

(iv) A wire with a green insulation is usually the live wire of an electric supply.

False: The wire with green insulation is the earth wire not the live wire.

Ans 7.

(i) Current carrying conductor

(ii) Electromagnets

(iii) Permanent magnets

Ans 8.

A solenoid behaves like a magnet when electric current passes through it.

Such a solenoid is called electromagnet. The magnetic field produced by a

current carrying solenoid (electromagnet) is very much similar to that of a

bar magnet.

Like a bar magnet, one end of the solenoid has N-polarity while the other

end has S-polarity.

To determine the north and south poles, we bring N-pole of the bar magnet

near one end of the solenoid. If there is an attraction, then that end of the

solenoid has south polarity and the other has north polarity. If there is a

repulsion, then that end of the solenoid has north polarity and the other end

has south polarity because similar poles repel each other.

Ans 9.

When the current carrying conductor is kept perpendicular to the direction

of the magnetic field, the force experienced by the conductor is largest.

Ans 10.

Here the electron beam is moving from our back wall to the front wall, so the

direction of current will be in the opposite direction, from front wall towards

back wall or towards us. The direction of deflection (or force) is towards our

right side.

We now know two things:

direction of current is from front towards us, and

direction of force is towards our right side.

Let us now hold the forefinger, middle finger and thumb of our left hand at

right angles to one another. We now adjust the hand in such a way that our

centre finger points towards us (in the direction of current) and thumb

points towards right side (in the direction of force). Now, if we look at our

forefinger, it will be pointing vertically downwards. Since the direction of

forefinger gives the direction of magnetic field, therefore, the magnetic field

is in the vertically downward direction.

Ans 11.

Electric Motor

Electric motor is a device that converts electrical energy to mechanical energy. Principle of electric motor:

A motor works on the principle that 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 of an Electric Motor

g) Insulated Copper wire: A rectangular coil of wire ABCD

h) Magnet Poles: A Horse shoe magnet. This creates a magnetic field

i) Split Rings: Two disjoint C-shaped rings P and Q. It acts as a

commutator (which can reverse the direction of current)

j) Axle: The split rings are placed on the axle which can rotate freely.

k) Brushes: The outside of the split rings are connected to conducting

brushes X and Y.

l) Source Battery: To source current

1.When electric current is passed into the rectangular coil, this current

produces a magnetic field around the coil.

2.The magnetic field of horse shoe

magnetic field of the coil and causes the coil to rotate continuou

3.If the coil ABCD is in horizontal position current from battery enters the

coil through brush B1, and commutator half ring C

direction ABCD and leaves via ring C

4.The direction of current in the arm AB of

the direction of current in the arm CD of the coil ABCD is from C to D. The

direction of magnetic field is from N to S. Now applying Fleming’s left

rule to arms AB and CD of the coil we find that the force on side AB

coil is in the downward direction. Whereas the force on the side CD of the

coil is in the upward direction. As a result, coil ABCD rotates in anti

direction.

5.While rotating when the coil reaches vertical position then the brushes

and B2 will touch the gap between the commutator rings and current to the

coil is cut off. But the coil does not stop rotating because it has already

gained momentum.

6.At half rotation, C2 makes contact with brush B

Therefore, the current in the coil gets reversed and flows along the path

DCBA. The reversal of current also reverses the direction of force acting on

the two arms AB and CD. Thus, the arm AB of the coil that was earlier pushed

1.When electric current is passed into the rectangular coil, this current

produces a magnetic field around the coil.

2.The magnetic field of horse shoe-type magnet then interacts with the

magnetic field of the coil and causes the coil to rotate continuou

3.If the coil ABCD is in horizontal position current from battery enters the

, and commutator half ring C1. The current flows in the

direction ABCD and leaves via ring C2 and brush B2.

4.The direction of current in the arm AB of the coil ABCD is from A to B and

the direction of current in the arm CD of the coil ABCD is from C to D. The

direction of magnetic field is from N to S. Now applying Fleming’s left

rule to arms AB and CD of the coil we find that the force on side AB

coil is in the downward direction. Whereas the force on the side CD of the

coil is in the upward direction. As a result, coil ABCD rotates in anti

5.While rotating when the coil reaches vertical position then the brushes

will touch the gap between the commutator rings and current to the

coil is cut off. But the coil does not stop rotating because it has already

makes contact with brush B1 and C

rrent in the coil gets reversed and flows along the path

DCBA. The reversal of current also reverses the direction of force acting on

the two arms AB and CD. Thus, the arm AB of the coil that was earlier pushed

1.When electric current is passed into the rectangular coil, this current

type magnet then interacts with the

magnetic field of the coil and causes the coil to rotate continuously.

3.If the coil ABCD is in horizontal position current from battery enters the

The current flows in the

the coil ABCD is from A to B and

the direction of current in the arm CD of the coil ABCD is from C to D. The

direction of magnetic field is from N to S. Now applying Fleming’s left- hand

rule to arms AB and CD of the coil we find that the force on side AB of the

coil is in the downward direction. Whereas the force on the side CD of the

coil is in the upward direction. As a result, coil ABCD rotates in anti-clockwise

5.While rotating when the coil reaches vertical position then the brushes B1

will touch the gap between the commutator rings and current to the

coil is cut off. But the coil does not stop rotating because it has already

and C1 with brush B2.

rrent in the coil gets reversed and flows along the path

DCBA. The reversal of current also reverses the direction of force acting on

the two arms AB and CD. Thus, the arm AB of the coil that was earlier pushed

down is now pushed up and the arm CD of the coil previously pushed up is

now pushed down. Therefore, the coil and the axle rotate half a turn more in

same direction.

7.The reversing of current in the coil is repeated after every half rotation

due to which the coil continues to rotate as long as current from the battery

is passed through it. The rotating shaft of electric motor can drive a large

number of machines which are connected to it.

Significance of Split rings in an electric motor:

A device that reverses the direction of flow current through a circuit is called

a commutator. The split rings in the electric motor acts as a commutator.

Ans 12.

Electric motor is used in the appliances like electric fans, washing machine,

mixers, grinders, blenders, computers, MP3 players, etc.

Ans 13.

(i) As a bar magnet is pushed into the coil, a momentary deflection is

observed in the galvanometer indicating the production of a momentary

current in the coil.

(ii) When the bar magnet is withdrawn from the coil, the deflection of

galvanometer is in opposite direction showing the production of an opposite

current.

(iii) When the bar magnet is held stationary inside the coil, there is no

deflection in galvanometer indicating that no current is produced in the coil.

Ans 14.

Yes, some current will be induced in the coil B. When the current in coil A is

changed, some current is induced in the coil B. Due to change in current in

coil A, the magnetic field lines linked with coil A and with coil B get changed.

This sets up induced current in coil B.

Ans 15.

(i) Right hand thumb rule : If the current carrying conductor is held in the

right hand such that the thumb points in the direction of the current, then

the direction of the curl of the fingers will give the direction of the magnetic

field.

(ii) Fleming’s left hand rule : Stretch the forefinger, the central finger and the

thumb of the left hand mutually perpendicular to each other. If the

forefinger points in the direction of the magnetic field, the middle finger in

the direction of current, then the thumb points in the direction of force in

the conductor.

(iii) Fleming’s right hand rule : Stretch the thumb, forefinger and the central

finger of the right hand mutually perpendicular to each other. If the

forefinger points in the direction of magnetic field, thumb in the direction of

motion of the conductor, then the middle finger points in the direction of

current induced in the conductor.

Question 16

Explain the underlying principle and working of an electric generator by

drawing a labelled diagram. What is the function of brushes ?

Ans 16.

ELECTRIC GENERATOR

An electric generator is a circuit that converts mechanical energy into

electrical energy.

Principal: An electric generator works on the principle of electromagnetic

induction. When a coil is rotated between the magnet or when the magnet is

rotated in and out of the coil the current is induced in the coil and the

direction of current is given by Fleming's right-hand rule.

Parts of an AC Electric Generator

Armature: It is arectangular coil of wire ABCD having large number of turns

of insulated copper wire wound over a soft iron core.

Field Magnet: It is a powerful magnet that provides a uniform magnetic field

perpendicular to the axis of rotation of coil between the North and South

poles.

Slip Rings: In an AC generator, we use slip rings-full rings with which the ends

of coil are in contact.

Axle: The slip rings are placed on the axle which is made to rotate freely from

an external source.

Brushes: There are two stationary metallic carbon brushes which are in

contact with external device and rings.

Galvanometer: The outer ends of the brushes are connected to the

galvanometer to measure the current.

Types of electric generator

Electric generators are of two types:

1. Alternating Current generators (AC generator)

2. Direct Current generators (DC generator)

1.AC generator:

It generates the current which changes its direction after equal intervals of

time, i.e. Alternating Current.

Construction:

An AC generator consists of a rectangular armature coil (ABCD) placed in a

region of strong magnetic field (between the two poles of a permanent

magnet). It experiences a torque due to the forces acting on it and produces

a current. The two slip rings (R

load resistor through brushes (B

There are two stationary metallic carbon brushes which are in

contact with external device and rings.

The outer ends of the brushes are connected to the

galvanometer to measure the current.

f electric generator

Electric generators are of two types:

Alternating Current generators (AC generator)

Direct Current generators (DC generator)

It generates the current which changes its direction after equal intervals of

rnating Current.

An AC generator consists of a rectangular armature coil (ABCD) placed in a

region of strong magnetic field (between the two poles of a permanent

magnet). It experiences a torque due to the forces acting on it and produces

current. The two slip rings (R1 and R2 ) maintain contact of the coil with the

load resistor through brushes (B1and B2) without resistor being moved.

There are two stationary metallic carbon brushes which are in

The outer ends of the brushes are connected to the

It generates the current which changes its direction after equal intervals of

An AC generator consists of a rectangular armature coil (ABCD) placed in a

region of strong magnetic field (between the two poles of a permanent

magnet). It experiences a torque due to the forces acting on it and produces

) maintain contact of the coil with the

) without resistor being moved.

Working

1.The axle is rotated such that it moves in the clockwise directions that is AB

moves up and CD moves down.

2.According to Fleming's Right-Hand rule, the induced current is setup in the

coil along B1-> AB -> BC -> CD -> B2. This means that the external current

flows from B2 to B1.

3.After half a rotation, arm CD starts moves up and AB moves down.

4.According to Fleming's Right-Hand rule, the induced current is setup in the

coil along B2-> AB -> BC -> CD -> B1. This means that the external current

flows from B1 to B2.

5.Thus, after every half rotation of the coil, the current changes

direction. This is called an AC current.

DC Generator

DC generator is used to generate DC current. The slip rings of an AC

generator, is replaced by a commutator, then it will become a DC generator.

Working: When the two half rings of commutator are connected to the two

ends of the generator coil, then one carbon brush is at all times in contact

with the coil arm moving down in the magnetic field while the other carbon

brush always remains in contact with the coil arm moving up in the magnetic

field. Due to this, the current in outer circuit always flows in one direction.

So, it is direct current.

Functions of Brushes: Brushes in contact with rings provide the current for

external use.

Ans 17.

If due to defective or damage wiring, the live and neutral wires come in

contact either directly or via conducting wire, the resistance of the circuit

becomes almost zero and an extremely large current flows through the

circuit. This is called short circuiting. This heats up the wire dangerously and

may lead to fire.

Ans 18.

Earth wire is a safety measure that provides a low resistance conducting path

to the current. Sometimes due to excess heat or wear and tear, the live wire

comes in direct contact with the metallic cover of the appliances, which can

give an electric shock on touching them. To prevent from the shock the

metallic part is connected to the earth through a three-pin plug due to which

the current flows to the earth at the instant there is a short circuit.

It is necessary to earth metallic appliances because it ensures that if there is

any current leakage in the metallic cover, the potential of the appliance

becomes equal to that of the earth. The potential of the earth is zero. As a

result, the person handling the appliance will not get an electric shock.