In the given figure, a cell is connected to a coil A and a bulb is connected to coil B.
(a) Fill in the blank.
When the key 'K' is closed, then the ............... [bulb glows continuously / bulb glows only for a moment].
(b) Support your answer to (a) above with a suitable reason.
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(a) When the key 'K' is closed, then the bulb glows only for a moment.
(b) On closing the key, the current in coil A grows from zero to its maximum value. During this short interval the magnetic flux linked with coil B changes, so an e.m.f. is induced in coil B and an induced current flows through the bulb, making it glow.
Once the current in coil A becomes steady, the magnetic flux linked with coil B no longer changes. Hence no e.m.f. is induced, no current flows in coil B and the bulb stops glowing.