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Homework Help: 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


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    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
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