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: Wired Circuits and Power

  1. Apr 21, 2006 #1

    o= blank space (ignore)
    battery= 9 V

    Determine the power dissipated in the R4 resistor in the circuit shown in the drawing.

    R1=3 ohms
    R2=2 ohms
    R3=1 ohms
    R4=6 ohms
    R5=1 ohms

    So here is what i know so far. The resistance of R4+R5=7. The resistance of R2+R3=3. So the resistance of R2345=1/(1/7+1/3)=2.1 ohms. The total resistance through the entire circuit is R1+R2345=3+2.1=5.1 ohms.
    So I use this total resistance to calculate the total current I=V/R so
    I=9V/5.1ohms=1.76A. Now I can use this total current to calculate the voltage across R4 and R5. V=IR=1.76(2.1)=3.7 V across R4 and R5 since Voltage across parallel circuits are the same. I know power=IR^2=V^2/R.
    But i cannot just plug in 3.7V^2/6ohms. This isnt giving me the right answer. I need to find the voltage across R4 only. I am not sure how to do this since the current is different passing through each resistor. please help and correct me if any of the previous steps i did were wrong. thanks.
    Last edited: Apr 21, 2006
  2. jcsd
  3. Apr 21, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I have no idea where your 6 ohms comes from!!
    You kno wthe voltage across the combined R45. Now find the current through both by using I = V/R45 = 3.7V/3 ohms. That's the current through R4. Now use Power = R_4 I^2
  4. Apr 21, 2006 #3
    sry, lil typo, it is fixed now.
    R4=6 ohms
    and I got the right answer. thanks man.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook