CBSE Class 12 Physics Electromagnetic Induction And Alternating Current Worksheet Set B

CASE STUDY BASED QUESTION

1. Lenz's law states that the direction of induced current in a circuit is such that it opposes the change which produces it. Thus, if the magnetic flux linked with a closed circuit increases, the induced current flows in such a direction that magnetic flux is created in the opposite direction of the original magnetic flux. If the magnetic flux linked with the closed circuit decreases, the induced current flows in such a direction so as to create magnetic flux in the direction of the original flux.

Question. Which of the following statements is correct?
(a) The induced e.rn.f is not in the direction opposing the change in magnetic flux so as to oppose the cause which produces it.
(b) The relative motion between the coil and magnet produces change in magnetic flux.
(c) Emf is induced only if the magnet is moved towards coil.
(d) Emf is induced only if the coil is moved towards magnet
Answer. B

Question. The polarity of induced emf is given by
(a) Ampere's circuital law
(b) Biot-Savart law
(c) Lenz's law
(d) Fleming's right hand rule
Answer. C

Question. Lenz's law is a consequence of the law of conservation of
(a) charge
(b) mass
(c) momentum
(d) energy
Answer. D

2. Currents can be induced not only in conducting coils, but also in conducting sheets or blocks. Current is induced in solid metallic masses when the magnetic flux threading through them changes. Such currents flow in the form of irregularly shaped loops throughout the body of the metal. These currents look like eddies or whirlpools in water so they are known as eddy currents. Eddy currents have both undesirable effects and practically useful applications. For example it causes unnecessary heating and wastage of power in electric motors, dynamos and in the cores of transformers.

Question. The working of speedometers of trains is based on
(a) wattless currents
(b) eddy currents
(c) alternating currents
(d) pulsating currents
Answer. B

Question. Identify the wrong statement
(a) Eddy currents are produced in a steady magnetic field
(b) Induction furnace uses eddy currents to produce heat.
(c) Eddy currents can be used to produce braking force in moving trains
(d) Power meters work on the principle of eddy currents.
Answer. A

Question. Which of the following is the best method to reduce eddy currents?
(a) Laminating core
(b) Using thick wires
(c) By reducing hysteresis loss
(d) None ofthese
Answer. A

Question. The direction of eddy currents is given by
(a) Fleming's left hand rule
(b) Biot-Savart law
(c) Lenz's law
(d) Ampere-circuital law
Answer. C

Question. Eddy currents can be used to heat localised tissues of the human body. This branch of medical therapy is called
(a) Hyperthermia
(b) Diathermy
(c) Inductothermy
(d) none of these
Answer. C

NUMERICALS

Question. A jet plane is travelling westward at a speed of 1800 km/h. What is the potential difference developed between the ends of a wing 25 m long? Its earth’s magnetic field at the location has a magnitude of 5.0 ×10^-4 T and the dip angle is 30°. 
Answer. 3.1 V

Question. A horizontal straight wire 10 m long extending from east to west is falling with a speed of 5.0 ms^-1 at right angles to the horizontal component of earth’s magnetic field equal to0.30 ×10^-4 Wbm^-2. (a) What is the instantaneous value of the emf induced in the wire? (b) What is the direction of emf? (c) Which emf of the wire is at the higher electrical potential?
Answer. a)1.5mV b) By Fleming’s right hand rule, the direction of induced current in wire is from west to east, therefore, direction of emf is from west to east
c) along negative x-axis, i.e., from east to west. The induced emf will oppose the flow of electrons from east to west, so eastern end will be at higher potential.

Question. A 0.4 m long straight conductor is moved in a magnetic field of induction 0.9 Wbm^-2 with velocity of 7 ms^-1. Calculate the maximum emf induced in the conductor.
Answer. 2.52 V( e =BLV)

Question. A metal disc of radius 200 cm is rotated at a constant angular speed of 60 rads^-1 In a plane at right angles to an external field of magnetic induction 0.05 Wb m^-2.Find the emf induced between the centre and a point on the rim.
Answer. 10.6V (1/2 Br²w)

One mark question

Question. 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 would reach earlier and why?
Answer. Glass would reach earlier. This is because there is no effect of electromagnetic induction in glass, due to presence of earth’s magnetic field, unlike in the case of metallic ball.

Question. When current in a coil changes with time, how is the back emf induced in the coil related to it?
Answer. The back emf induced in the coil opposes the change in current.

Question. State the law that gives the polarity of the induced emf.
Answer. Lenz’s Law: The polarity of induced emf is such that it tends to produce a current which
opposes the change in magnetic flux that produces it.

Question. A long straight current carrying wire passes normally through the centre of circular loop. If the current through the wire increases, will there be an induced emf in the loop? Justify.
Answer. No. Justification: As the magnetic field due to current carrying wire will be in the plane of the circular loop, so magnetic flux will remain zero. Also, magnetic flux does not change with the change in current.

Question. (a) How does the mutual inductance of a pair of coils change when (i) distance between the coils is increased and (ii) number of turns in the coils is increased?
Answer. 
(i) With the increase in the distance between the coils, the magnetic flux marked with the stationary coil decrease and hence the mutual inductance of the two coils will decrease. (ii) M ∝ n1n2 so with the increase in the number of turns mutual inductance increases.

Please click on below link to download CBSE Class 12 Physics Electromagnetic Induction And Alternating Current Worksheet Set B