A coil or N turns of insulated copper wire is closely wound over a cardboard pipe. The free ends of the coil wire are connected to a battery, a rheostat and an open key as shown. A is a point on the axis of the coil. Answer the following questions with reason:
(1) Which direction will a magnetic needle stay when placed at A.
(2) What will happen when key K is closed?
(3) Keeping K closed when moving contact C of the rheostat is moved from X to Y' what will happen.
(4) If a pivoted needle of iron were placed at A, how will the answers to (1) and (2) change.
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Solution:
(i) When current is made to pass through coil B, by plugging the key in, for that instant, the galvanometer connected to coil A shows a deflection, i.e., current passes through coil A as well.
(ii) When current is stopped through coil B by taking the plug of the key out, for that instant, the galvanometer connected to coil A again shows a deflection, this time, however, in the opposite direction. That is, a current passes through coil A in the opposite direction.
The observations are made because whenever we change the current in the coil B, the magnetic field associated with it and the coil A in its proximity changes. At the very instant of plugging the key in or taking the plug out, there is an instantaneous flow of current and a change in the magnetic field associated with the coil B. During this instantaneous moment, the current in coil B reaches from zero to the maximum or from the maximum to zero. This changing magnetic field of coil B induces a current in the coil A.
The phenomenon is known as electromagnetic induction.
(i) When current is made to pass through coil B, by plugging the key in, for that instant, the galvanometer connected to coil A shows a deflection, i.e., current passes through coil A as well.
(ii) When current is stopped through coil B by taking the plug of the key out, for that instant, the galvanometer connected to coil A again shows a deflection, this time, however, in the opposite direction. That is, a current passes through coil A in the opposite direction.
The observations are made because whenever we change the current in the coil B, the magnetic field associated with it and the coil A in its proximity changes. At the very instant of plugging the key in or taking the plug out, there is an instantaneous flow of current and a change in the magnetic field associated with the coil B. During this instantaneous moment, the current in coil B reaches from zero to the maximum or from the maximum to zero. This changing magnetic field of coil B induces a current in the coil A.
The phenomenon is known as electromagnetic induction.
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