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!

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:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Question circuits Date
Question about complex power in three phase circuits Mar 5, 2018
Circuits question, series vs parallel Jan 28, 2018
AC circuit Multiple choice question Dec 19, 2017