A large solenoid is connected to a galvanometer (see Problem 10) as in the figur

Question

A large solenoid is connected to a galvanometer (see Problem 10) as in the figure below left. A clockwise current (as viewed from the top) will deflect the needle to the left and a counter-clockwise current will deflect the needle to the right. a) If the needle deflects to the right while a bar magnet is lowered into the solenoid as at left, determine and label the poles of the magnet (N and S). Explain how you determined your answer. b) A second ("primary coil") is connected to a D.C. power supply as shown above right, causing a current to flow in the direction indicated. As the primary coil is inserted into the secondary coil, which direction will the needle on the galvanometer deflect? Explain your reasoning. c) With the primary coil is resting inside the secondary coil, suppose the power supply is turned off. Will the galvanometer needle deflect? If so. which direction? Explain your reasoning.

Explanation / Answer

a)

since the galvanometer has deflected to right , the current in solenoid is counterclockwise. hence the induced magnetic field by the solenoid is upward using right hand rule .

Since the induced magnetic field is upward , the magnetic field by the magnet inside must be downward. hence the north pole is closer to solenoid and south pole at the other end

b)

the current in primary coil is counterclockwise , hence the magnetic field by the coil in upward direction. as the coil enters the solenoid , magnetic field is induced in down direction inducing current in clockwise direction. hence galvanometer deflects towards left.

c)

as the power is turned off , the current in coil in clockwise direction decreases, hence the magnetic field is induced in solenoid in same direction as the coil. which is counterclocwise. hence the direction of current in solenoid becomes reversed and galvanometer needle deflects towards right

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