[msirshad]

When we close a switch and current flows through a coil then a bulb connected in parallel does not glow. But when we open (turn off) the switch then the bulb glows for a while. Why? If it is self induction then the bulb should also glow when we close the switch in the start.It is to be noted that the voltage of the battery are very small.

[randolfo sou]

Dear Thread :

you mean that the bulb is connected parallel with a coil
? The coil with equation: Voltage = d I / dt * L ( Hz)

When you close the switch, current mostly like to flow
into the coil ( its resistance nearly to be zero )
so the bulb will not give light !
But after the open the switch, current inside the coil will
then flow into the bulb, ( Like Discharge ) so the bulb will
bright for a while...
The time that the bulb give brightness depends on the
value of L and R(ohm)

[msirshad]

Dear Thread,
Thank you for your reply but I can not understand the cause because of which bulb glows for a while when we open the switch.The problem is that the battery itself can not glow the bulb (its voltage are very low).Then what happens in the coil that glows the bulb on opening the switch.The same thing does not happen when we close the switch.If it is self inductance then the bulb should glow due to change in magnetic flux.This change occurs when we open or close the switch.I mean the bulb should glow for a while two times.

[randolfo sou]

I think the resistance of the bulb is too high when comparing
with the resistance of the Coil ( X := j X W X L where W=angular frequency, L=impedence )
You may check for that resistance of them first, at the beginning
the current flows into the coil rather than flow into the bulb , but when
you open the switch, the current no way to go , it necessary to flow
into the bulb then!

So, you may check for the resistance of that two component first!