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: Question about RLC circuits

  1. Mar 9, 2005 #1
    A person is working near the secondary of a transformer, as shown in the figure below. The primary voltage is 120 V at 60.0 Hz. The capacitance C, which is the stray capacitance between the hand and the secondary winding, is 10.0 pF. Assuming that the person has a body resistance to ground R, determine the rms voltage across the body. (Suggestion: Redraw the circuit with the secondary of the transformer as a simple AC source.)
    FIGURE: http://east.ilrn.com/graphing/bca/user/appletImage?dbid=445132339 [Broken]

    I need some help with this stuff. This is what I know already. I know that the secondary voltage is 6000 V according to the figure. Also, I know that the rms voltage = max voltage/sqrt(2). I'm not sure how to get the max voltage. I tried using the secondary voltage as the max voltage, but the answer wasn't right. So, I think the secondary voltage is the AC source. How do I get the max voltage using the secondary voltage? Maybe I'm doing this all wrong? I don't know...any help would be great!! Thx in advance! :)
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Mar 9, 2005 #2
    How about trying this? Let the voltage (secondary) as

    [tex] v=v_0 e^{jwt}[/tex]

    Find the current through the circuit i, using ohms law.

    [tex] v_0 e^{jwt} = i (R_b -\frac{j}{wc})[/tex]

    where -j/wc is the impedance of the capacitor.

    Find the charge of the capacitor q by integrating current i since
    i = dq/dt

    use q= Cv to find the voltage accross the capacitor [tex]v_c[/tex]

    therefore, voltage accross the body [tex]v_b = v - v_c[/tex]

    absolute value of the vb is the max voltage created across the person's body. I am getting this to be about 23 volts using the human body resistance of 1 Mega ohm, causing a current of about 0.023 mA to flow through him. Hopefully it would not kill him. :rofl:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook