How Does the Resistance of a Light Bulb Change When It's On Versus Off?

[natash]

Homework Statement

we wish to measure the resistance of a lit light bulb. Unfortunately, the resistance of the light bulb when hot is not the same as when it is cold. when the light bulb is turned on it becomes hot. we cannot easily measure the resistance of the light bulb when it is on (hot) because if we use an ohmmeter to measure the resistance when the light bulb is lit, we are really measuring the resistance of the whole circuit, not that of the light bulb only. however, we can approximate its value by replacing the light bulb with a variable resistor. adjust the variable resistor to obtain the same value of the current measured with the good batteries. Now disconnect the variable resistor from the circuit and measure its resistance using an ohmmeter. also, measure the resistance of the cold light bulb with the ohmmeter using the same connection.

The question is... you measured the resistance of the flashlight when it had no current passing through it. the resistance of the flashlight is different, however, when current is passing through it. explain how your measurement of the resistance of the variable resistor is a valid approximation of the resistance of the flashlight when it had current passing through it. is the resistance higher when the flashlight is on or off?

Homework Equations

V=IR

The Attempt at a Solution

I'm confused about the variable resistor part. But I believe resistance is higher when the flashlight in off because R and V are inversely proportional

 

[gneill]

I'm confused about the variable resistor part. But I believe resistance is higher when the flashlight in off because R and V are inversely proportional

Actually, R and V are proportional, given the same current: V = I*R. I becomes the constant of proportionality.

If you did the lab then you should have actual values for the on-resistance and off-resistance that you can compare.