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: Units problem

  1. Sep 13, 2006 #1
    I have this problem I just can't figure out.

    Current = A (Ampere)


    where z is the initial current at t=0, R is the resistance in Ohms and C is the capacitance expressed in secondary units as F (Farad)

    - Express the units of F in terms of seconds and ohms

    So, this is what I tried to do, tell me where I went wrong

    A = A e^(s/F-Ohms)

    1 = e^(s/F-Ohms)

    ln(1) = s/F-Ohms

    0 = s/F-Ohms

    F = 0

    I obviously did something wrong, so any help would be apreciated, thank you!
  2. jcsd
  3. Sep 13, 2006 #2
    If you think of an RC circuit as a second order differential equation you will see

    dQ/dT + 1/(R*C)*Q = Junk

    Where junk = 0 for a loop of a circuit without any external components. Otherwise it can also be some forcing function driven by batteries, power sources, etc. But anyways, Q is the charge here so...

    dQ/dT = charge/time (amps)

    thus Q*1/(R*C) = charge/time since Q = charge, R*C has dimension [T]

    Thus an Ohm-Farad has dimension [T] and the quantity in the exponent in your equation is dimensionless then - as it should be
  4. Sep 14, 2006 #3
    thank you, I got it now.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook