EXPLANATIONS

Objective Questions

So, both electron and proton has current in same direction.

Therefore, the forces acting on it are given by Fleming’s

1. (d) Properties of magnetic field lines are as following

left hand rule and theyare pointing intothe plane ofthe paper.

(a) Magnetic field lines are closed and continuous curves.

12. (b) The force on the left side of loop is attractive because the

(b) Magnetic field lines never intersect with each other.

direction of current in wire and loop is same, while on right

side of loop, force is repulsive.

Hence, option (d) is correct.

The force on perpendicular sides is zero. Hence, the

2. (a) In a uniform magnetic field, the magnetic field lines of

attractive force is greater than repulsive force, so, the loop

force are parallel and equidistant from each other.

will move towards the wire.

The correct representation of uniform magnetic field is

13. (d) The direction of the induced electric current in a

shown below.

conductor, when placed in a varying magnetic field can be

assessed by Fleming’s right-hand rule.

14. (d) When key is inserted and removed, then the magnetic

field lines pass through second coil increase and decrease in

3. (d) The magnetic field lines due to a straight current

two cases respectively.

carrying wire are concentric circles with centre on the wire.

Therefore, the direction of current in two cases is in

4. (b) According to right hand thumb rule, when conductor is

opposite directions and the galvanometer shows momentary

held in right hand, keeping thumb from East to West. The

deflections in opposite directions.

curve of the finger will be from North to South at a point

15. (a) The correct match for the given term is

lying directly below the wire.

(A)-(ii) The direction of force on a current carrying is given

5. (d) The magnetic field lines around a straight current

by Fleming’s left hand rule.

carrying conductor are concentric circles and the direction

(B)-(iv) The direction of induced current is given by

of magnetic field is given by right hand thumb rule.

Fleming’s right hand rule.

6. (b) The N-pole of the resultant magnet is on the face close to

(C)-(iii) The direction of magnetic field produced by a

A, because the magnetic field lines enter in loop from B and

straight current carrying conductor is given by Maxwell’s

come out from A.

right hand thumb rule.

Also, as a matter of fact magnetic lines come out of the

(D)-(i) The direction of magnetic field lines at a point in a

N-pole of magnet. Therefore, face close to A represents

magnet is the direction of magnetic force on a North pole.

N-pole. The currents in A and B are same.

16. (a) The magnitude of magnetic field is

7. (d) Proton is a charged particle. When it moves in a

magnetic field, a magnetic force is applied due to its

(i) directly proportional to the current I passing through

the wire.

velocity and hence the momentum changes.

(ii) inversely proportional to the distance r from the wire.

8. (d) The positively charged particle is moving towards West,

i.e. the direction of current is towards West (current flows in

The magnetic field is stronger at a point which is nearer to the

the direction of the motion of positive charge).

conductor and goes on decreasing on moving away from the

conductor.

The particle is deflected towards North, so the direction of

∴Both A and R are true and R is the correct explanation of A.

force is towards North.

17. (b) When a current carrying conductor is placed in a

Thus, from Fleming’s left hand rule, the direction of

magnetic field, it experiences a force except when it is

magnetic field is in upward direction.

placed parallel to the magnetic field. The force acting

9. (c) The strength of magnetic field inside a long current

on a current carrying conductor depends on magnetic field

carrying straight solenoid is same at all points.

produced by the current carrying conductor and external

Therefore, correct option is (c).

magnetic field.

10. The correct match for the given items is

∴Both A and R are true but R is not correct explanation of A.

A - (ii) The SI unit of magnetic field is Tesla (T).

18. (b) The Earth’s magnetic field is towards North and velocity

B-(iv)Themagneticfieldinsidesolenoidhasauniformvalue.

of electron is downwards. By applying, Fleming’s left hand

C-(i)Acompassneedleis asmallbarmagnetthatcanrotate.

rule, the direction of force is towards West. Also, electron

has a negative charge.

D- (iii), Solenoid is a temporary magnet.

∴Both A and R are true but R is not correct explanation of A.

11. (a) In the given figure, the proton and electron are moving

in opposite direction to each other and in perpendicular to

19. (c) The magnetic field is independent of length and area of

the direction of magnetic field. Now, we know that the

solenoid, it only depends on the number of turns and current

direction of current is taken opposite to the direction of

flowing throughthe solenoid.It isuniform inside the solenoid.

motion of electron.

∴A is true but R is false.

20. (d) When a closed circuit is placed in a varying magnetic

Subjective Questions

field, an electric current is produced in it which is known as

1. The properties of magnetic field lines are

induced current.

(i) They originate from North pole of a magnet and end at

The direction of induced current is given by Fleming’s right

its South pole.

hand rule.

(ii) They form closed and continuous curves.

∴ A is false but R is true.

(iii) They never intersect each other.

21.

(i)

(iv) They are uniform inside the magnet.

converts electrical energy into mechanical energy.

If two magnetic field lines intersect each other, then at the

point of intersection, there will be two directions of

(ii)

(b) When a soft iron bar is placed inside the solenoid, it

magnetic field lines, which is not possible. Hence, the

will magnetise the iron bar as long as there is current in

magnetic field lines never intersect with each other.

the circuit. Hence, the strength of the magnetic field

inside the solenoid will also increase.

2. Magnetic field lines due to parallel current carrying

(iii)

(a) The magnetic field lines produced inside the solenoid

conductors are shown in figure.

are similar to that produced by a bar magnet. The field

Magnetic field at X will be zero as both conductors are equal

lines inside the solenoid are parallel to each other. The

in magnitude and are opposite in direction.

magnetic field lines due to a current carrying solenoid

and bar magnet are shown below

Field lines produced by a

Field lines produced by

3. The strength of the magnetic field is inversely proportional

current carrying solenoid

a bar magnet

to the distance i.e. B ∝

¹.

(iv)

(d) The given magnetic field versus current graph is

linear. It is clear that magnetic field increases with

As, the point where magnetic field is to be determined is

moved away from the straight wire carrying constant

increase in current. Hence, the magnetic field produced

current, the magnetic field strength decreases.

by the solenoid is directly proportional to the current.

(v)

(a) From graph, when current =

. A, the magnetic field

4. According to the right hand thumb rule, magnetic field at P

is directed into the plane of paper and at Q, it is out of the

is 13 mT.

plane of paper. The strength of the magnetic field at Qwill

Upto current of 1.0 mA, the magnetic field increases

linearly with current.

be larger as strength of the field ∝

r (distance)

For larger values of current (>1A), the graph does not

depict any information.

Here, r > r

22.

(i)

(b) The direction of rotation of the coil, when viewed

∴ B <B1

i.e. B₂ has larger field.

5. According to right hand thumb rule,

from the front is anti-clockwise.

(ii)

Power line

can be corrected as, when there is no current in the coil,

it will stop rotating because force is produced only when

there is flowing current in coil.

(iii)

magnet to rotate the armature because permanent

(i) The direction of magnetic field at a point above the

magnets are weak and do not produce strong magnetic

power line is from South to North.

field in the region.

(ii) The direction of magnetic field at a point below the

(iv)

(a) Electric motor is a device which converts electrical

power line is from North to South.

energy into mechanical energy.

6. The magnetic field lines are concentric circles at every point

(v)

(d) By applying, Fleming’s left hand rule, the direction of

on a current carrying circular loop.

force experienced by wire due to magnetic field is

South-East direction.

The direction of magnetic field is determined by right hand

thumb rule. At the centre of the circular loop, the magnetic

field lines are straight and points towards North.

The direction of magnetic field lines at the point outside the

particle enters East to West or West to East as shown

surface of the loop is shown below.

below.

Circular loop

carrying current

Magnetic

12. Here, the force acting on the α-particle is directed

field lines

(anti-clockwise)

perpendicular to the plane of paper in inward direction by

(clockwise)

Fleming’s left hand rule. If the direction of magnetic field

Key

gets reversed then the force will also act in opposite

( )

direction of α-particle i.e. the force experienced by

Battery

α-particle is now in outward direction to the plane of

(i) When the coil is kept in the North-South plane and the

paper.

current is flowing in the anti-clockwise through the loop,

13.

(i) No, it will not experience any force. As, magnetic field

then the magnetic field is in the East to West direction.

exerts force on a moving charged particle only.

(ii) When the coil is in vertically East-West plane and current

(ii) No, it will not experience any force because magnetic

through the coil is in anti-clockwise direction, then the

field exerts a force in perpendicular direction to

magnetic field is in the South to North direction.

motion of the particle.

(iii) When a circular coil carrying current is placed horizontally

(iii) Yes, it will experience a force in a direction

and the direction of the current is clockwise, then the

perpendicular to the direction of its own motion and

direction of the field for the observer positioned below the

the direction of magnetic field can be determined by

coil is in the downward direction.

Fleming’s left hand rule.

8. A solenoid behaves like a magnet when electric current passes

14. Force on electron is maximum in case (a), because here

through it.

the direction of motion of electron is perpendicular to the

One end of a solenoid behaves as a North pole and the other

direction of magnetic field B.

end behaves as a South pole. We can use a bar magnet to

Similarly, the force on electron is minimum in case (c)

determine the North and South poles of a current carrying

because, in this case the direction of motion of electron is

solenoid by using the property, i.e. like poles repel and unlike

along the direction of magnetic field B, as electron is

poles attract each other.

moving along B.

The end of solenoid which attracts North pole of a bar magnet

Hence, the direction of maximum force acting on electron

is magnetic South pole of the solenoid. The end of solenoid

is perpendicular to the plane of paper and directed into it.

which repels the North pole of a bar magnet is the magnetic

15. When the gaps of the split ring commutator are aligned

North pole of the solenoid.

with the carbon brushes, then contacts are broken and the

(i) The magnetic field lines are crowded (convergent) near the

current is temporarily cut-off.

poles of solenoid. Hence, the magnetic field is strong and

However, the coil keeps on rotating in the same direction

divergent, where the magnetic field is weak.

due to its inertia until the split ring commutator and the

(ii) A freely suspended current carrying solenoid always points

carbon brushes are in contact again.

in the North-South direction even in the absence of any

16. The schematic diagram of electric motor is shown below,

other magnet. Because the earth itself behaves as a magnet

or solenoid to point always in a particular direction.

Rotating of coil

(Clockwise)

Rectangular coil

(iii) The field lines around a current carrying solenoid are

similar to field lines of a bar magnet. So, inside the solenoid

field lines are parallel to each other and the strength of

magnetic field is same i.e. uniform at all points inside a

solenoid.

10. The force experienced by a current carrying conductor placed

in a magnetic field is the largest, when conductor is kept

perpendicular to the direction of the magnetic field.

Split rings

R₂

Permanent

(R1 and R2)

Fleming’s left hand rule determines the direction of force on a

magnet

current carrying conductor. It states that, if the forefinger

Brushes

(X and Y )

(magnetic field), middle finger (current and thumb are

stretched mutually, then the direction of force acting on

Shaft

Axle

conductor is given by thumb.

11. If a charged particle moves parallel or anti-parallel to the

magnetic field, no magnetic force will act on it and remains

Split rings act as commutator and its function is to reverse

undeflected. So, in the given condition either the charged

the direction of current flowing through the coil.

17.

(i) The speed of rotation of the motor can be increased by

(iii) If the polarity of the battery is reversed, the current will

^● increasing the strength of the current in the coil.

be going from top to bottom in the wire and the magnetic

field lines will now be in the clockwise direction on the

^● increasing the number of turns in the coil.

plane, which is perpendicular to the wire carrying

^● increasing the area of the coil.

current.

^● increasing the strength of magnetic field.

(iv) Maxwell's right hand thumb rule is used to find the

(ii) Electric motor is used in electric fans, refrigerators,

direction of the magnetic field for a straight current

mixers, washing machine, computers, MP3 player, etc.

carrying conductor.

18.

(i) No induced current will be produced in the loop as the

This law states that, if you hold the current carrying straight

constant current flowing in the straight wire produces a

wire in the grip of your right hand in such a way that the

constant magnetic field. Hence, no induced current is

stretched thumb points in the direction of current, then the

produced in the loop.

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

(ii) Since, current in the straight wire is changing, hence

the magnetic field.

induced current will be produced in it. According to

Fleming’s right hand rule, the current flowing in the

loop will be in clockwise direction.

19. The observation that can be noted from the galvanometer

reading are

Magnetic field

(i) There are momentary galvanometer deflections that die

Current

out shortly.

(ii) The deflections are in opposite directions.

23. A solenoid is defined as a coil consisting of a large number of

circular turns of insulated copper wire. These turns are

20. (i) (a) Galvanometer gets deflection in one direction.

wrapped closely to form a cylinder.

(b) Galvanometer gets deflection in opposite direction of

the first one.

(ii) Phenomenon involved is electromagnetic induction.

21. When a magnetic compass needle pointing North and

South in the absence of a nearly magnet or a current loop,

it is acting upon by the earth’s magnetic field only. But in

the presence of a magnet or a current loop (which also has

a magnetic field) the earth’s magnetic field near the

(i) Magnetic field lines of force due to a

compass is modified and the needle is deflected from

current carrying solenoid

North and South directions.

The salient features of magnetic field lines are

(i) A magnetic field line is directed from North-pole to

South-pole outside the magnet.

(ii) A magnetic field line is a closed and continuous curve.

(iii) The magnetic field lines never intersect each other,

because if two lines meet, it means that force is acting

(ii) Field lines around a bar magnet

in two directions at that point which is not possible.

Distinguishing features are as follow

(iv) Closer the field lines, stronger is the magnetic field

and vice-versa is also true.

^● Magnetic field outside the solenoid is negligible as

compared to the bar magnet.

(v) Magnetic field lines are parallel and equidistant, those

represent a uniform magnetic field.

^● Magnetic field of solenoid can be varied as per our

requirement just by changing current or core of

22.

(i) According to right hand thumb rule, the magnetic field

solenoid but in bar magnet it is fixed.

produced by PQat point R is into the plane of the paper

and at point S is out of the plane of paper.

24. (i) Fleming’s left hand rule states that, if the forefinger,

(ii) Here, r

thumb and middle finger of left hand are stretched

The magnetic field will be larger at point S as compared

mutually perpendicular to each other, such that the

to that at point R.

forefinger points along the direction of external magnetic

field, middle finger indicates the direction of current,

This is because the magnetic field produced by a

then the thumb points towards the direction of force

straight current-carrying conductor is inversely

proportional to the distance from the wire. So, the

acting on the conductor.

magnetic field will be larger at the point which is

(ii) Electric motor is based on the principle that, when a

nearer to the conductor.

rectangular coil is placed in a magnetic field and current

As, point S is nearer to the conductor as compared to

is passed through it, two equal and opposite forces act on

point R. So, field at S > field at R.

the coil which rotate it continuously.

(iii)

(a) Armature It is a coil wound over a soft iron core.

(v)

Electric generators, transformers etc., are the applications

It rotates in magnetic field, when a current flows

of electromagnetic induction.

through it.

27.

(i)

Bar magnet is placed on white paper which attracts the

sprinkled iron fillings.

(b) Brushes They provide a sliding contact and

facilitate current through armature while it

(ii)

The bar magnet exerts its influence in the region

rotates in field.

surrounding it. Therefore, the iron fillings experience a

force and this force makes iron fillings to arrange in a

pattern.

through armature as it rotates in field.

(iii)

The region surrounding a magnet, in which the force of the

25.

(i)

An electric current produced in a closed circuit by a

magnet can be detected, is said to have a magnetic field.

changing magnetic field is called an induced current.

(iv)

The iron fillings arrange themselves in a particular

This phenomenon is called electromagnetic induction.

alignment that are called magnetic field lines. These field

An electric generator works on the basis of

lines represent the region in which force of the magnet

electromagnetic induction.

can be detected.

(ii)

Three different ways to produce induced current in a

(v)

The relative strength of the magnetic field is shown by the

coil of wire are as following

degree of closeness of the field lines. The field is stronger,

(a) If a magnetic field is changed around a coil, then

as the force acting on the pole of the magnet is greater,

an induced current is set up in it. It can be done

where the field lines are crowded.

by taking a bar magnet and bringing it closer to

28.

(i)

As the current in first coil changes, the magnetic field

the coil or taking it away from the coil.

associated with it also changes. Hence, the current is

(b) If a coil is moved in a magnetic field, then

induced in coil-2.

an induced current is set up in the coil.

(ii)

By Faraday’s law of electromagnetic induction, the

induced current flows in a coil only when there is a change

through another coil kept close to the coil.

in the magnetic field within the coil.

26.

(i)

Since, the magnet is at rest, so no current is induced in

(iii)

A current carrying coil has a magnetic field just like bar

the coil.

magnet. In experiment, if coil-1 is disconnected from

battery, then the needle momentarily moves, but to the

(ii)

Since, the bar magnet is at rest and the coil is moving

opposite side. It means the current flows in the opposite

away from the magnet, needle of the galvanometer coil

direction in coil-2.

swing towards right.

(iv)

The induced current is found to be highest when the

(iii)

When the magnet will move towards the coil, the

direction of motion of the coil is at right angles to the

meter needle swings to the left.

magnetic field.

(iv)

The phenomenon involved in this is electromagnetic

(v)

The phenomenon associated with this is electromagnetic

induction (EMI). In this phenomenon, current is

induction.

induced in the circuit by changing the magnetic field.