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Why Capacitors resist Voltage Change but not Current change?

  1. Sep 23, 2014 #1
    Hi Folks.

    I am having trouble comprehending this and sounds like a contradiction. A capacitor will resist changes in current. A capacitor will pass high frequency ac. With a higher frequency alternating current the voltage is constantly changing so how is this possible if it resists change in current????

    Thanks,

    Mark.
     
  2. jcsd
  3. Sep 23, 2014 #2

    jbriggs444

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    A capacitor will not resist changes in current. It resists total current over time (i.e. charge).

    Think of a capacitor wired in parallel with a voltage source. The capacitor resists momentary changes in voltage by sinking some charge when the voltage is momentarily high or sourcing some charge when the voltage is momentarily low. The higher the frequency of voltage changes the more current that the capacitor can source or sink before the accumulated charge becomes significant.
     
  4. Sep 23, 2014 #3

    CWatters

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    Resisting changes in current is a property normally associated with inductors - for want of a better expression they "like" to maintain the existing current and resist changes. Perhaps better to think of capacitors as resisting changes in voltage but don't get too hung up on that as a capacitor is not an ideal voltage source.

    Capacitors can be used in both high pass and low pass filters. Perhaps have a look at how they work.
     
  5. Sep 23, 2014 #4

    Drakkith

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    Capacitors don't resist changes in current. They resist changes in voltage. At high AC frequencies, the current changes direction quickly, but the voltage across the capacitor doesn't since it doesn't have time to discharge fully. If you lower the frequency the capacitor discharges more completely and the voltage across it changes more.

    In contrast, an inductor resists changes in current. At a low frequency the current in the inductor can completely alternate. As you raise the frequency you will eventually reach a point where the current no longer alternates at all.

    So at high frequencies a capacitor's voltage doesn't change but the current does, while an inductor's current doesn't change but the voltage across it does.
    At low frequencies a capacitors voltages changes significantly but the current flow is extremely little (since the capacitor will fully charge or discharge and no more current will flow), while an inductor's current is very large but the voltage across it is very little (it acts like a normal conductor at very low frequencies).
     
  6. Sep 23, 2014 #5
    Apologies for my typo above. I meant to say "A capacitor will resist changes in Voltage"

    It will pass high frequency ac but resist changes in voltage. This does not make sense to me. With a high frequency signal the voltage is changing constantly. Does it mean the peak to peak Voltage??
     
  7. Sep 23, 2014 #6

    CWatters

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    Think about what you mean by "pass". Consider the circuit shown below..

    If the signal to "passes" through the capacitor the output will follow the input. In other words the output voltage is the same as the input voltage. That means the voltage across the capacitor is constant.

    Capacitor.png
     
  8. Sep 23, 2014 #7
    A capacitor doesn't resist changes in voltage. It takes energy to increase the voltage, but the energy is released when voltage is reduced again. It acts as storage.
     
  9. Sep 23, 2014 #8

    Drakkith

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    Just look at the voltage across the capacitor, not the whole circuit. At high frequencies or a large capacitance the voltage across the capacitor barely changes since the capacitor doesn't charge/discharge much. At low frequencies or a low capacitance the capacitor charges/discharges more, so the voltage across the capacitor changes much more. In the case of a DC circuit or when the AC frequency or capacitance is low enough, the capacitor completely charges and discharges, so the voltage across it completely changes to match the applied voltage.

    This is incorrect. Consider a capacitor with a very large capacitance. When we close the circuit the capacitor acts like a short and current flows, charging it. Initially, the voltage across the capacitor is zero. As the capacitor charges the voltage increases until it matches the applied voltage. When the applied voltage falls, the voltage across the capacitor initially remains at the peak voltage and then falls off gradually as the capacitor discharges. In the case of a high frequency circuit the voltage across the capacitor barely changes even though the applied voltage alternates back and forth. If we partially charge the capacitor and then apply a high frequency AC signal, the voltage across the capacitor remains approximately the same as the signal alternates. When we say that the capacitor resists changes in voltage, we mean that it resist changes in voltage across itself.
     
  10. Sep 26, 2014 #9
    Thanks for all your replies folks. I was thinking about it the wrong way.

    Cheers,

    Mark.
     
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