# Homework Help: Resitor power problem

1. Feb 16, 2008

### floridianfisher

1. The problem statement, all variables and given/known data

Three 70.0 W, 120 V lightbulbs are connected across a V = 120 V power
source, as shown in Figure P18.48. (Assume that the resistance of each bulb is constant even though, in reality, the resistance increases markedly with current.)

http://i57.photobucket.com/albums/g2...er/physics.gif [Broken]
Figure P18.48
(a) Find the total power delivered to the three bulbs.
wrong check mark
Your answer differs from the correct answer by 10% to 100%. W
(b) Find the potential difference across each light bulb.
VR1 = correct check mark V
VR2 = correct check mark V
VR3 = correct check mark

2. Relevant equations
p=v^2/r

3. The attempt at a solution
I found the potential differences they were 80 and 40.12, 40.12 respectively. However I cannot find the power

I have Req as being 462.9 so then i use power = 120^2/462.9 and i get 31.1 watts?!?
I have no idea why this isn't working please give me a clue

Last edited by a moderator: May 3, 2017
2. Feb 16, 2008

### floridianfisher

3. Feb 17, 2008

### kamerling

Your Req is wrong. For a single bulb we get p = V^2/R, so R = V^2/70. Then compute Req. R_2 and R_3 are parellel, so replace them with their equivalent resitance etc.