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Homework Help: What is the voltage? AC circuit

  1. Nov 19, 2007 #1
    A 60 Hz power source with a peak voltage of 12.0 V is connected to a resistor. At a particular instant the voltage across the resistor is 10.0 V and decreasing. What is the voltage 1.5ms later.

    I tried using the equation

    v= V cos(2 pi ft)

    v= 12.0V cos(2pi (60Hz)(1.5 X 10^-3s))
    v= 10.13 volts.. but this is incorrect. Help please
  2. jcsd
  3. Nov 19, 2007 #2
    :uhh:Please help me out. Thank you.
  4. Nov 20, 2007 #3


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    Homework Helper

    If you are taking the instant of 12V to be your t=0, you have to first figure out at what time the 10V occurs in the waveform. Then add the 1.5 ms onto that number to find the voltage at 1.5 ms after the 10V. Does that make sense? What you have found is the voltage at 1.5 ms after your max voltage of 12V, which is not what the question is asking.
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