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

Simple Circuits Problem

  1. Jan 20, 2007 #1
    1. The problem statement, all variables and given/known data

    Determine the energy required to move 260C through 6V.

    2. Relevant equations

    q = Cv
    U = (1/2)*C*v^2

    3. The attempt at a solution

    C = q/v = 260C/6V = 130/3 F
    U = (1/2)(130/3 F)(6V)^2 = 780J

    I applied the voltage/charge relationship and energy equation for a capacitor because they seemed to be the only equations that made sense with the given values, but I'm not certain this is correct. If someone could let me know if I'm going about this correctly it would be appreciated. Thanks.
    Last edited: Jan 20, 2007
  2. jcsd
  3. Jan 20, 2007 #2
    I think you need the formula:

    [tex] V = \frac{W}{Q} [/tex]

    where V = volts, W = work(energy or joules), and Q = coulombs
  4. Jan 21, 2007 #3
    It seems the units don't work out for that formula.

    Did you mean V = W/I? If so I think it doesn't help for this problem since I don't know the current and can't find the current from the given values.
  5. Jan 21, 2007 #4
    Ok, I see where I went wrong. I was looking at the variable W as if it had the unit of watts, which in power, not energy. I now see that W is energy is joules, and the formula you wrote does work. Thanks Number2Pencil, I think I can solve the problem now.
  6. Jan 21, 2007 #5
    A Watt is a Joule/second, if you did the calculation having used Joules in place of Watts, I'd recommend going back and checking your solution over again
  7. Jan 21, 2007 #6
    I think the variable W and W as the symbol for watt caused confusion. So the correct answer would be:

    Energy = Charge*Potential = 260C*6V = 1560J

  8. Jan 22, 2007 #7
    It looks like your going to have to derrive an equasion
    Try V=E/Q=W/Q..... I=Q/t..... E=VIt... and Q=Ne

    V = voltage in volts E= energy in joules Q= charge W= work in joules I= current in amps N= number of electrons e = electron constant t=time
  9. Jan 23, 2007 #8


    User Avatar

    Staff: Mentor

    Correct, Steve. Yeah, using W for both work and Watts can cause confusion.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook