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!

Capacitive Reactance

  1. Oct 4, 2015 #1

    A pure capacitor C is in an a.c. circuit.

    Vc = V0 * sin(w*t)
    Q = C * Vc
    = C * V0 * sin(w*t)
    I = dQ/dt
    = wCV0 sin(wt)
    then I0 = wCV0
    Xc = V0 / I0 = 1/(wC)

    So why people would say V = I * Xc?

    Is it a must to include complex number?
    Last edited: Oct 4, 2015
  2. jcsd
  3. Oct 4, 2015 #2
    The derivative of sin is cos.

    You don't need complex numbers.
  4. Oct 5, 2015 #3
    I = I0 cos(wt)
    Then V = I * Xc * tan(wt)
    not V = I * Xc
  5. Oct 5, 2015 #4
    The relationship V=I*Xc (with real numbers) is between the amplitudes or rms values and not between instantaneous values.
    Otherwise you need to consider Xc as a complex number, to take into account the phase difference between v and i.
  6. Oct 17, 2015 #5
    But why do we introduce imaginary root in it?
    Is it used to describe the phase based on the e^(i*theta)?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook