## the current through a light bulb is constant: true or false?

ShylaMal
Posts: 1
Joined: Wed Sep 12, 2018 4:34 pm

### the current through a light bulb is constant: true or false?

Is the current in a light bulb greater immediately after it is turned on aor a few minutes later? explain your answer.

I thought the current was the same because V=IR and V and R are constants so I must be constant. is that correct? it was marked wrong

jeff
Posts: 39
Joined: Mon Mar 05, 2018 11:54 pm

### Re: the current through a light bulb is constant: true or false?

Hi ShylaMal,
ShylaMal wrote:
Wed Sep 12, 2018 5:22 pm
Is the current in a light bulb greater immediately after it is turned on aor a few minutes later? explain your answer.

I thought the current was the same because V=IR and V and R are constants so I must be constant. is that correct? it was marked wrong
In this case, we cannot assume that the resistance is constant. The problem is that the light bulb will heat up after it is turned on, and the resistance will increase as it gets hotter (which means the current will get smaller).

That's why bulbs normally "blow" right when they are turned on (rather than after they've been on a while). If they are at room temperature, their resistance is low, which means the current going through them at that moment is larger than normal.

(Note that this problem is assuming that the bulb is an incandescent bulb.)