1. Not finding help here? Sign up for a free 30min 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!

Rms current in circuit with capacitor resistor and rms output

  1. Nov 15, 2010 #1
    1. The problem statement, all variables and given/known data

    A 39.5 uF capacitor is connected to a 47.0 ohm resistor and a generator whose rms output is 25.2 V at 60.0 Hz. Calculate the rms current in the circuit.
    (also asks for voltage drop across resistor, capacitor and the phase angle for the circuit, but I mostly want to get the first one first)

    2. Relevant equations

    I=current, V=voltage, R=resistance, f=frequency

    Irms=delta Vc, rms/Xc
    delta Vc,rms=Irms*Xc

    and then I start going in circles

    Other formulas that might be appropriate:
    L=?? not given in this problem
    Power average=Irms^2*R

    3. The attempt at a solution
    combining lots of these formulas trying to come up with the correct Vrms has proven unsuccessful. I came up with 17.819 A, .37515 A, .21739 A, none of which were right. None of them were successful and I don't remember or care to explain how I got them precisely because I wasn't confident in those anyway.
    Overall I still haven't been able to link my given information directly to the answer I need
  2. jcsd
  3. Nov 15, 2010 #2


    User Avatar
    Homework Helper

    Formulate the expression for Z, then you know that I = V/Z.
  4. Nov 15, 2010 #3
    but what do I use for L while setting it up for Z? I don't have it directly in the given information and I don't think I can derive it. Should I just use a default value like 1 or 0?

    ps thanks for a response :D
  5. Nov 15, 2010 #4


    User Avatar
    Homework Helper
    Gold Member

    Calculate the impedance Z of the RC network.

    [tex] Z = R + \frac{1}{j \omega C} [/tex]

    (true for this particular problem)

    [tex] Z = R +j \left( \frac{-1}{\omega C} \right)[/tex]

    For this problem, the resistance is R, and the reactance is -1/(ωC).

    In general,

    [tex] Z = \Re \{ Z \} + j \Im \{ Z \} [/tex]

    Use the Pythagorean Theorem to find the magnitude of Z.

    [tex] |Z| = \sqrt{\left( \Re \{ Z \} \right)^2 + \left( \Im \{ Z \} \right)^2} [/tex]

    And finally [the complex version of] Ohms law to find the current.

    [tex] i_{RMS} = \frac{v_{RMS}}{|Z|} [/tex]
  6. Nov 15, 2010 #5
    What is j for those equations?
  7. Nov 16, 2010 #6


    User Avatar
    Homework Helper
    Gold Member

    Here I used the symbol j to represent [itex] \sqrt{-1}[/itex]. This is common in electrical engineering courses, since the symbol i is already taken, representing current.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Rms current in circuit with capacitor resistor and rms output
  1. RMS Current (Replies: 4)

  2. RMS Current (Replies: 1)

  3. RMS Current (Replies: 2)

  4. Rms current (Replies: 3)