Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Find the difference in voltage at different points in a circuit.

  1. Feb 13, 2012 #1
    1. The problem statement, all variables and given/known data
    A section of a circuit XY shown below absorbs 50 W of power when a current I = 1.0 A passes through it as indicated by the arrow labeled i.
    (a) What is the voltage difference between X and Y?
    (b) What is the voltage difference across element C?

    2. Relevant equations
    V = IR
    P = (V2)/R
    P = (I2)R

    3. The attempt at a solution
    I have answers for both parts, but I'm not confident about them.

    (a) P = V2/R
    R = V/I
    P = V2/(V/I) = IV
    Vx= P/I = (50 W) (1.0 A) = 50 V

    P = I2R
    I =[itex]\sqrt{P/R}[/itex]
    Vy= IR = [itex]\sqrt{P/R}[/itex]*R = [itex]\sqrt{(50 W)/(2 Ω)}[/itex]*(2 Ω)
    = 20V

    Vx - Vy = 50 V - 20 V = 30 V

    (b) Wouldn't it be 0? Why would the voltage change across the capacitor?

    Edit: Maybe it's not a capacitor and I'm confused?

  2. jcsd
  3. Feb 13, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    C is not a capacitor. It's just some "unknown circuit element", which is why it just appears as a block. (For one thing, look up the circuit symbol for a capacitor -- it is different).

    To find the voltage across C:

    - You know the voltage across both.
    - You can find the voltage across the 2-ohm resistor using Ohm's law
    - The voltage across element C has to be the difference between the above two voltages , since they are in series. In other words, the voltage across the 2-ohm resistor and the voltage across element C have to add up to 50 V.
  4. Feb 14, 2012 #3
    For the second part, following your advice, I did:
    Vc1 = IR = (1.00 A) (2 Ω) = 2 V

    Vc1 + Vc2 = 50V
    2 V + Vc2 = 50 V
    Vc2 = 48 V
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook