Power Dissipated by a Lightbulb

[Sir Isaac]

1. The electric power P is dissipated by a light bulb of resistance R is P=v^2/R, where V represents the line voltage. During a brownout, the line voltage is 10.0% less than its normal value. How much power is drawn by a light bulb during the brownout if it normally draws 100.0 W (watts)? Assume that the resistance does not change.



2. P = V^2/R



3. Since resistance does not change, I disregarded it completely. I then took 100 in place for P and solved for V in the equation. Then I took 10 (my solution to the first problem) in place of V and subtracted 10.0% to find the power that is drawn by the lightbulb during the brownout. But my answer does not match up to the books.

 

[jedishrfu]

I think you use the 100 watts as P and 120v as v to get the R. Then use the v x.9 and R to get the brownout wattage.

 

[technician]

you do not need to know R because it does not change.
The power is proportional to V^2
If V is only 90% 0f the original then the power will be proportional to (0.9)^2 x original V