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Electromagnetic Induction:C.R.Q's /Questions

15.1 Does the induced emf in a circuit depend on the resistance of the circuit?does the induced current depend on the resistance of the circuit?

Ans. The Induced emf in a coil depends upon the rate of change of flux through it (E=-Nt) .Hence its value does not depend upon the resistance of the coil.But the induced current that flows through a coil is equal to I=E/R and it’s value depends on the resistance of the coil.If , resistance increases then the current flowing through the coil decreases.Because the product of I and R must remains constant.i.e. I x R = Constant.

15.2 A square loop of wire i moving through a uniform magnetic field.The normal to the loop is oriented parallel to the magnetic field.Is emf induced in th loop?Give a reason for your answer.

Ans.No, induced emf will not be produced because there is no change of flux linking to the loop.i.e.t=0 .So according to the relation (E=-Nt) ,E=0.If the square loop is being rotated in magnetic field in such a way that the loop is cutting the magnetic field lines due to its motion,then emf will induce in the coil.

15.3 A light metallic ring is released from above into a vertical bar magnet.Viewed for above ,does the current flow clockwise or anticlockwise in the ring?

Ans.According to Faraday’s law of electromagnetic induction an induced emf and hence induced current will be produced in the metallic ring.According to Lenz law , the current in the ring should flow in such a direction as to oppose the cause producing it.So, the induced current in the coil must produce magnetic field which opposes the motion of ring towards bar magnet.The side of the ring facing magnet will be North Pole of the induced magnetic field.Right hand rule shows that the magnetic field will produce in this manner only when the current will flow in clockwise direction in ring.

15.4 What is the direction of the current through resistor R in the figure.When switch S is (a) closed (b) opened.

Ans.(a) When the switch is closed ,the current in the coil increases from zero to maximum steady value;during during this interval magnetic flux in the second coil from zero to max. and induced current will flow in it.The side of the current carrying coil facing the other coil becomes North pole.So,to oppose N pole , the current in the other coil must flow anti clockwise.Hence current in R flow from left to right according to the figure.

(b) however, when the switch is opened, the current in the coil decreases from max. to zero and flux linked to the other coil also decreases and induced current is produced in the reverse direction.So, the current will flow from Right to Left (clockwise) according to the figure.

15.5 Does the induced emf always act to decrease the magnetic flux through a circuit?

Ans.No, the induced emf does not act as to decrease magnetic flux through a circuit.According to Lenz law , the current in the ring should flow in such a direction as to oppose the cause producing it.If an induced emf appears in a circuit  due to decreasing magnetic flux linking that circuit then induced current flowing through the circuit will produce its own magnetic field that oppose the decrease of the magnetic field.In other word it is increasing the magnetic flux through a circuit.

15.6 When the switch in the circuit is closed a current is established in the coil and the metal ring jumps upward.Why?Describe what would happen to the ring is battery polarity were reversed?

Ans.From establishment of current, induced magnetic flux will be produced in the  cylinder. From Lenz’s law an opposing emf in the ring will be produced. The face of the ring opposite to coil develops similar pole of magnet and experiences repulsion, which makes it to jump upward.
The ring will jump upward in the same manner, if the battery polarity is reversed. The same process will happen as mentioned above.

15.7 The Fig. Shows a coil of wire in the xy plane with a magnetic field directed along the y- axis. Around which of the three coordinate axes should the coil be rotated in order to generate an emf and a current in the coil?

Ans.The coil must be rotated about x-axis to get change of magnetic flux and induced current through it.

15.8 How would you position a flat loop of wire in a changing magnetic field so that there
is no emf induced in the loop?

Ans.If the flat loop of wire is parallel to the field. When the coil is held parallel to the direction of B, then the angle between vector area A and B will be 90o
φB = B•A = BAcos90o= 0

15.9 In a certain region the earth’s magnetic field point vertically down. When a plane flies due north, which wingtip is positively charged?

Ans.[At the two magnetic poles, the direction of the earth’s magnetic field is vertical. At north magnetic pole it is downward into the ground, at south magnetic pole, it is upward out of the ground. Here on both places, the compass needle does not indicate any particular direction along the ground.]
Left wingtip will be positively charged. The electrons in the wing experience the magnetic force [ F = -e(vxB)] From R.H. rule, the electrons will move towards right, (the direction of conventional current is left). Due to it left wingtip (West side) will be positively charged.

15.10 Show that ε and ∆Φ / ∆t have the same units.

15.11 When an electric motor, such as an electric drill, is being used, does it also act as a
generator? If so what is the consequences of this?

Ans.Yes it also acts as a generator. When the electric motor is running, due to rotation of its coil, an emf is induced in it. It is called back emf, which produces opposing current. It increases with speed of motor. This means that it also acts as a generator.

15.12 Can a D.C. motor be turned into a D.C. generator? What changes required to be done?

Ans. Yes a d.c. motor can be turned into a d.c. generator.To change it, needs some arrangement to rotate the armature. Disconnect the brushes of the commutator from d. c. supply and connect it with some external circuit.

15.13 Is it possible to change both the area of the loop and the magnetic field passing
through the loop and still not have an induced emf in the loop?

Ans. Yes, if the flux remains constant. From the equation; ∆φ = B•A ,B and A are inversely proportional to each other. If the area of the loop and magnetic field passing through the loop are changed in such a way to make product constant, then no induced emf will be produced.
Secondly, if plane of the coil is parallel to the magnetic field, changing in area and the field will not induce any emf in the loop.

15.14 Can an electric motor be used to drive an electric generator with the output from the generator being used to operate the motor?

Ans. No. An electric motor cannot be used to drive an electric generator. Perpetual
motion machine is not possible according to law of conservation of energy.

15.15 A suspended magnet is oscillating freely in horizontal plane. Oscillations are strongly damped when a metal plate is placed under the magnet. Explain why this occurs?

Ans. The metal plate produces an induced emf, due to oscillations in the suspended magnet.This induced emf produces current, which produces its own magnetic field that will oppose the motion of the suspended magnet. So oscillations are strongly damped.

Q.16 Four unmarked wires emerge from a transformer. What steps would you take to
determine the turns ratio?

Ans. Separate primary and secondary coils by ohmmeter. Connect primary coil with a.c. supply of known voltage Vp . measure the voltage induced Vs by voltmeter. Calculate turns ratio from; Vs / Vp = Ns / Np

15.17 a) Can a step-up transformer increase the power level? b) In a transformer, there is no transfer of charge from the primary to the secondary. How is, then the power transferred?

Ans. a) No. A step up transformer cannot increase the power level. As for ideal
case : power input = power out .It can increase or decrease voltage or current but power, P = VI, will remain same.
b) Due to induced emf, power is transferred. There is no transfer of charge, but the change of flux in one coil is linked with the other coil and emf is produced.

15.18 When the primary of a transformer is connected to a.c. mains the current in it
a) is very small if the secondary circuit is open, but
b) increases when the secondary circuit is closed. Explain these facts.

Ans. a) The output power is zero, if the secondary circuit is open, very small current is drawn by the primary coil from a.c. mains.
b) Output power will increase, when the secondary circuit is closed.
Power input = Power output , Greater current is needed in primary for equalizing power in the secondary coil.


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