Question

P,Q,R, and S are circuits.
P- connected with 6v DC,solenoid, and a bulb
Q- connected with 6v DC,solenoid with soft iron core inserted, and a bulb
R- connected with 6v AC,solenoid , and a bulb
S- connected with 6v AC, solenoid with soft iron core inserted, and a buld
[Question based on self induction]
a)  If the circuits are kept switched switched on,the lamp in which circuit will be seen glowing  with lowest intensity?
b) justify your answer.
c) A soft iron core in solenoid to a sensitive voltmetre is kept switched on.If so in which of the cases will the volt meter reading shows a deflection?

Answer 1

I suppose circuit S, where the magnetic field by solonoid induces emf in the opposite direction to the  AC supply. The bulb glows with power V²/R.  V will be RMS value in case of AC. V²  of rms  =  V² of DC / 2. 
circuit P and Q will glow bright.

c)  circuit Q does not show a deflection in voltmeter. As magnetic field in the core
is fixed due to fixed current in solenoid.    so circuit S shows deflection.