1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Power PLEASE Help Test THURSDAY

  1. Feb 12, 2008 #1
    Power...PLEASE Help...Test THURSDAY!!!

    I am having a hard time understanding why some power equations for finding the power dissipated by light bulb(s) don't work. I have...
    P = IV = I^2R = V^2/R
    But for some of the problems these equations are getting me different numbers for each one! I am getting confused and I now don't know when to use which one and I have a test on Thursday! One example...

    Two 97 W (114 V) lightbulbs are wired in series, then the combination is connected to a 114 V supply. How much power is dissipated by each bulb?

    I found I using P/V = .851 then when I go to find the power the only equation that gets me the right answer is P = V^2/R then divide by 2 = 24.3 W
  2. jcsd
  3. Feb 13, 2008 #2

    Shooting Star

    User Avatar
    Homework Helper

    The current reduces through each when connected in series, so that the power is not just 2*97 W. You have to find the V across each or find I.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook