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: Electric Circuit

  1. May 23, 2007 #1
    1. The problem statement, all variables and given/known data
    The course that I am taking is so poorly organized. I keep having to do questions (for marks) for which I haven't even studied the principles yet. :mad:

    But anyway, I was hoping someone could help me with this, I assume basic, question about electric circuits. I studied electric circuits last year and I barely remember.

    4.24 * 10^18 electrons pass through a wire between points A and B in 0.0025 s. In the process, 24 J of heat are given to the wire. What is the difference in potential between points A and B?

    2. Relevant equations
    I = Q / t
    V = I * R
    P = V * I

    3. The attempt at a solution
    I have calculated the current going through the wire to be
    (4.24 * 10^18) * (1.60 * 10^-19) / 0.0025 = 271.36 A
    What relationship between I and V from
    V = I * R
    P = V * I
    can I use? What exactly does the 24 J represent?
    Thank you for anyone who can help.
    Last edited: May 23, 2007
  2. jcsd
  3. May 23, 2007 #2
    Power is energy(Joules) consumed in unit time. Can you proceed?
  4. May 23, 2007 #3
    Yep, yep, yep. :smile:
    Thank you so much.
    I have the formulas, but I didn't know how to plug in the numbers.
    So P = 24 J

    So P = 24 J/ 0.0025 s
    P = V * I
    Last edited: May 23, 2007
  5. May 23, 2007 #4
    Not quite.
    So W/t = V*I and so ......
  6. May 23, 2007 #5
    Yes, I realized later and I edited it.
    Ok thank you again.
  7. May 23, 2007 #6
    The voltage (potential difference) is simply the change in energy in J per Coloumb of charge.

    So as you said in the first post you have 0.6784C passing through the wire and 24J of energy is released in the form of heat. Thus

    [tex]V=\frac{\Delta E}{q}[/tex] and so on..

    Note that this is the exact same answer you get if you calculate the power and so on, but its simpler.

    Do you understand what I am saying?
  8. May 23, 2007 #7
    Oh, I overlooked that formula for a more direct solution.
    Thank you dontdisturbmycircles.

    I didn't know if the change in energy is equal to the lost heat energy. I thought there could be potential or kinetic energy being lost or gained.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook