## Electromagnetic Induction Important Questions for CBSE Class 12 Physics Electromagnetic Induction Laws

### 1 Mark Questions

1.The electric current flowing in a wire in the direction from B to A Find out the direction of the induced current in the metallic loop kept the wire as shown in the figure.

Ans.According to Lenz’s law, the direction of induced current will oppose the cause of its production. So, the current in loop will induce in such a way that it will support the current flowing in the wire i.e., in the same direction. So, the direction of current in the loop will be clockwise

2.Two spherical bobs, one metallic and the other of glass, of the same size are allowed to fall freely from the same height above the ground. Which of the two wduld reach earlier and why? [Delhi 2014]
Ans.The glass bob will reach earlier on ground as acceleration due to gravity is independent of mass of the falling bodies. Being insulator, no induced current is developed in it due to the earth’s magnetic field.

3.A flexible wire of irregular shape, abed as shown in the figure, turns into a circular shape when placed in a region field which is directed normal to the plane of the loop away from the reader. Predict the direction of the induced current in the wire.[Foreign 2014]

Ans.The wire is expanding to from a circle, which means that force is acting outwards on each part of the wire because of the magnetic field (acting in the downwards direction). The direction of the induced current should be such that it will produce magnetic field in upward direction (towards the reader). Hence, the force on the wire will be towards inward direction, i.e. induced current is flowing in anti-clockwise direction in’the loop from cbadc.

4.In the given figure, a bar magnet is quickly moved towards a conducting loop having a capacitor. Predict the polarity of the plates A and B of the capacitor[All India 2014 C;2010;Hots]

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5.State Lenz’s law. A metallic rod held horizontally along east-west direction, is allowed to fall under gravity. Will there be an emf induced at its ends? Justify your answer. [Delhi 2013]
Ans. Lenz’s law states that the induced emf of induced current is a circuit which always opposes the cause that produces it.Yes, emf will be induced in the rod as there is change in magnetic flux.When a metallic rod held horizontally along East-West direction, is allowed to fall freely under gravity i.e. fall from North to South, the intensity of magnetic lines of the earth’s magnetic field changes through it, i.e. the magnetic flux changes and hence the induced emf in it

6.State the Faraday’s law of electromagnetic induction. [Foreign 2009,2012]
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7.The closed loop PQRS of wire is moved into a uniform magnetic field at right angles to the plaue of the paper as shown in the figure. Predict the direction of the induced current in the loop.

Ans.Since, magnetic flux increasing when the loop moves into uniform magnetic field. So the induced current should oppose this increase. Thus, flow will be from QPSRQ. i.e. anti-clockwise

8.Predict the direction of induced current in metal rings 1 and 2 when current I in the wire is steadily decreasing?

Ans.Current in the wire is steadily decreasing, so the induced current in rings 1 and 2 will flow in such a way that it opposes the decrease of current

So, it will flow in same direction. Now, from the figure, it is clear that the direction of induced current in (i) ring 1 is clockwise.
(ii) ring 2 is anti-clockwise

9.A bar magnet is moved in the direction indicated by the arrow between two coils PQ and CD. Predict the directions of induced current in each coil.

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### 2 Marks Questions

10.Predict the polarity of the capacitor in the situation described by adjoining as shown in figure. Explain the reason too.

Ans.According to figure shown in the question, induced current is in anti-clockwise, when seen from left hand side and its direction is in clockwise when seen from right hand side. Thus, direction of induced     current  is in clockwise sense.This implies that plate A of the capacitor is at the higher potential than plate B, i.e. B is as negative plate while  A  is  as  positive plate.

11.A metallic rod of length L is rotated with angular frequency of co with one end hinged at the centre and the other end at the circumference of a circular metallic ring of radius L, about an axis passing through the centre and perpendicular to the plane of the ring. A constant and uniform magnetic field B parallel to the axis is present everywhere. Deduce the expression for the emf between the centre and the metallic ring. [Delhi 2012]
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12.A current is induced in coil C1 due to the motion of current carrying coil C2.

(i)Write any two ways by which a large deflection can be obtained in the galvanometer G
(ii)Suggest an alternative device to demonstrate the induced current in place of a galvanometer. [Delhi 2011]
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(a)by moving quickly, the coil C2 towards C1, or by moving quickly the coil C2 away from C,.
(b)by switching off and on the key.
(ii) Alternating device in place of galvanometer can be LED or bulb.

13.A magnet is quickly moved in the direction indicated by an arrow between two coils Q and C2 as shown in the figure.

What will be the direction of induced current in each coil as seen from the magnet? Justify your answer.   [Foreign 2011]
Ans.As N-pole of the magnet is moving away from the coil C1 therefore the coil will behave as South-pole and opposes the motion of the magnet. This implies the current in the coil is anti-clockwise. Also in coil C2, it behaves as S-pole in order to repel the coming magnet. This implies current in the coil is anti-ciockwise.

14.Two identical loops, one of copper and the other of aluminium are rotated with the same angular speed in the same magnetic field. Compare
(i)the induced emf and
(ii)the current produced in the two coils. Justify your answer.[All India 2010]
Ans.(i)The induced emf in both the loops will be same as areas of the loop and time periods are same as they are identical and rotated with same angular speed.
(ii)The current induces in Cu coil is more than Al coil as Cu coil have got lesser resistance and

15.(i)When primary coil P is moved towards secondary coil S (as shown in the figure below), the galvanometer shows momentary deflection. What can be done to have larger deflection in the galvanometer with the same battery?

Ans.The coil P should be moved quickly towards or away from the coil 5.The laws involved here are Faraday’s law of electromagnetic induction.

16.A coil O is connected to low voltage bulb B and placed near another coil P as shown in the figure. Give reasons to explain the following observations.
(i)The bulb Blights.
(ii)Bulb gets dimmer. If the coil Q is moved towards left.

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17.State the law that gives the polarity of the induced emf.     [All India 2009]
Ans. Lenz’s law gives the polarity of induced emf. The induced emf (or current) always opposes the cause that produces it.
Lenz’s law The induced emf of induced current is a circuit which always opposes the cause that produces it.

When North pole of the bar magnet approaches towards the loop, the induced current in coil is anti-clockwise (forming North pole of current loop) when viewed from magnetic side. The face of coil being North pole opposes the arrival of North pole of magnet. Hence, opposes the cause that produces it.   Also, certain amount of work has to be done by external agency to bring the North pole near the coil in against of force of repulsion applied by induced current loop. The work done by external agency appears in the form of electrical energy. So, Lenz’s law is a consequence of the principle of law of conservation of energy

18.State Lenz’s law. Give one example to illustrate this law. The Lenz’s law is a consequence of the principle of conservation of energy. Justify this statement.   [All India 2009 ]
Ans.Lenz’s law The induced emf of induced current is a circuit which always opposes the cause that produces it.

When North pole of the bar magnet approaches towards the loop, the induced current in coil is anti-clockwise (forming North pole of current loop) when viewed from magnetic side. The face of coil being North pole opposes the arrival of North pole of magnet. Hence, opposes the cause that produces it.     Also, certain amount of work has to be done by external agency to bring the North pole near the coil in against of force of repulsion applied by induced current loop. The work done by external agency appears in the form of electrical energy. So, Lenz’s law is a consequence of the principle of law of conservation of energy

3 Marks Questions
19.(i) A rod of length / is moved horizontally with a uniform velocity v in a direction perpendicular to its length through a region in which a uniform magnetic field is acting vertically downward. Derive the expression for the emf induced across the ends of the rod.
(ii) How does one understand this motional emf by invoking the Lorentz force acting on the free charge carriers of the conductor? Explain. [All India 2014]
Ans.(i)Consider a straight conductor moving with velocity v and U shaped conductor placed in perpendicular magnetic field as shown in the figure.

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Component of velocity v parallel to magnetic field, will make the particle move along the field. Perpendicular component of velocity v, will cause the particle to move along a circular path in the plane perpendicular to the magnetic field. Hence, the particle will travel the helix path.

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23.A conducting rod PQ, of length /, connected to a resistor R, is moved at a uniform speed v, normal to a uniform magnetic field B, as shown in the figure

(i)Deduce the expression for the emf induced in the conductor.
(ii)Find the force required to move the rod in the magnetic field.
(iii)Mark the direction of induced current in the conductor.[Foreign 2008]
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24.Figure shows a rectangular conducting loop PQRS in which arm RS of length / is movable. The loop is kept in a uniform magnetic field B directed downward perpendicular to the plane of the loop. The arm RS is moved with a uniform speed v.

Deduce an expression for
(a)the emf induced across the arm RS
(b)the external force required to move the arm and
(c)the power dissipated as heat.[All India 2009]
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### 5 Marks Questions

25.(i) Describe a simple experiment (or activity) to show that the polarity of emf induced in a coil is always such that it tends to produce a current which opposes the change of magnetic flux that produces it. (ii) The current flowing through an inductor of self-inductance L is continuously increasing. Plot a graph showing the variation of
(a)magnetic flux versusthe current
(b)induced emf versus dl/dt
(c)magnetic potential energy stored versusthe current.[Delhi 2014]
Ans.(i)The coil P should be moved quickly towards or away from the coil 5.The laws involved here are Faraday’s law of electromagnetic induction.

26.A metallic rod of length land resistance Ris rotated with a frequency v, with one end hinged at the centre and the other end at the circumference of a circular metallic ring of radius /, about an axis passing through the centre and perpendicular to the plane of the ring. A constant and uniform magnetic field ft parallel to the axis is present everywhere.
(a) Derive the expression for the induced emf and the current in the rod.
(b)Due to the presence of the . current in the rod and of the magnetic field, find the expression for the magnitude and direction of the force acting  on this rod.
(c)Hence, obtain the expression for the power required to rotate the rod.[All India 2014 C]
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