A choke coil in series with a lamp is connected to a dc line. The lamp is seen to shine brightly. Insertion of an iron core in the choke causes no change in the lamp’s brightness. Predict the corresponding observations if the connection is to an ac line.

Explanation

In case of dc source, `f = 0 rArr X_(L) = 2pi fL = 0 rArr` Increase in the value of L will not make any difference in the glow of lamp because `X_(L)` remains zero.
In case of ac source,
`I_("rms") = ( V_(rms))/( sqrt( R^(2) + 4pi ^(2) f^(2) L^(2)))`
Here by inserting iron rod, L is increased and `I_(rms)` will decrease and hence brightness of lamp will decrease further.