# Estimating Resistance

1. Nov 13, 2012

### Peter G.

"Estimating" Resistance

Hi,

So, suppose I have a bulb labelled 10 V, 12 W. This is connected to a 4 V e.m.f battery. Can I calculate the Resistance from 10 V, 12 W (Using P = VI and R = V/I) and assume that it is the resistance when it connected to a 4V battery?

Thanks!

2. Nov 13, 2012

### liblenovo

Re: "Estimating" Resistance

Generally speaking, the resistance of a bulb vary with temperature. If ignore this feature, I think your answer is right.

3. Nov 13, 2012

### SammyS

Staff Emeritus
Re: "Estimating" Resistance

If the bulb is rated 12W at 10V, that likely means that it produces a reasonable of light at 10 V. The filament of the bulb, under such conditions, will be near its maximum operating temperature.

At 4 V, the same bulb would be operating at a considerably lower temperature. The resistivity of tungsten is very temperature dependent, so the resistance of the bulb would be much less than when it operates at 10 V .

4. Nov 13, 2012

### Peter G.

Re: "Estimating" Resistance

Without a graph, however, is there any more precise method of estimation?