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Voltages across capacitor,inductor for ac voltages

  1. Feb 8, 2012 #1

    i was wondering if we would treat inductors and capacitors as short circuits for AC voltages. i am aware that there is a 90 degrees phase shift in a RC or RL circuit with an ac voltage but the phase shift only applies to the current.

  2. jcsd
  3. Feb 8, 2012 #2


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    It depends on the frequency, the circuit, and the values of the components. You may get away with it if you are interested in something like the peak values or RMS values. For example, let's say we have a 1 kiloohm resistor in series with a 0.1 microfarad capacitor. If we excite the circuit with say 5 KHz sine wave then the effective impedance of the capacitor is so small with respect to the resistor that one can approximate the capacitor as being a short circuit in terms of the peak and RMS values across the resistor. However, there will still be a significant voltage drop across the capacitor. The main point though is due to the phase shifts this does not strongly affect the amplitude of the voltage across the resistor. So there still is a phase shift that occurs that has consequences.
  4. Feb 8, 2012 #3
    Why do you think an inductor would behave as a short circuit for AC?
  5. Feb 8, 2012 #4
    well i connected a 0.1uH inductor to a signal generator which generated 5V ac voltage and then i added a resistor connected to the inductor and ground.

    then i measured the voltage across the 10 ohms resistor on a oscilloscope and at was the same as the input so thats why i wondering if i replace the inductor with a wire what is the difference?
  6. Feb 8, 2012 #5
    The inductor has a property called reactance that is proportional to both frequency and inductance value. You could try to increase the signal generator frequency and/or replace your 0.1uH inductor with a bigger value and see what happens. It would probably also be a useful exercise to draw the circuit on paper, do the calculations, and evaluate your measurements in the light of this.
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