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About the reactance of capacitor

  1. Sep 25, 2011 #1
    For inductors, it is the induced e.m.f. which resists the current flow.
    However, I don't quite understand how capacitors resist the current flow.
    Why is there capacitive reactance?
    What does 'A built-up electric field resists the change of voltage on the element' mean?
     
  2. jcsd
  3. Sep 25, 2011 #2

    sophiecentaur

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    That last quote doesn't really make strict sense. Using the word "resists", in this context, implies resistance (a mechanism for losing energy as current is conducted).
    A capacitor, being basically an open circuit, will not carry on conducting forever, when connected to a battery. Initially, however, some charge can flow into it. As the charge increases, an increasing potential difference will form across the terminals and, once this PD is equal to the supply voltage, no more current will flow (there is an exponential change). The PD across the capacitor 'opposes' the supply voltage, rather than "resisting" it. (Here, I am talking in terms of the usage of those words in the context of electrical circuits and not in general English language use).
    That's why we say that a capacitor 'blocks' DC. For an alternating current , particularly at a high enough frequency, the direction of the applied AC voltage changes rapidly enough for the capacitor never to get fully charged and so the capacitor 'lets through' an alternating current. The higher the frequency, the less the opposing PD becomes so the Reactance decreases proportionally.
     
  4. Sep 26, 2011 #3
    Thank you very much.
    You have difinitely dispersed my misunderstanding.
     
  5. Sep 26, 2011 #4

    sophiecentaur

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    Good good. Keep 'em coming. :smile:

    btw, I should have included, in my hand waving model of charging a capacitor, some amount of Series Electrical Resistance in the source of the Voltage. There will always be finite resistance, in practice, so it is a reasonable thing to do. Without the added resistance, you get other awkward things happening and more headaches!
     
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