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Homework Help: Inductors and Capacitors.

  1. Sep 10, 2007 #1
    My knowledge on how inductors and capacitors operate in AC/DC is a bit shaky. My textbook and online material don't help my understanding, so I am hoping someone would be able to explain it more clearly.

    In DC I know that capacitors store energy and charge up by time constants and then release their energy when used as independant sources. They gain voltage as they charge. I have firm knowledge here.

    In AC, I have a few questions. I do know that there is Xc and using ohms law there is voltage dropped across the capacitor.

    My question, is there energy stored in the capacitor in the same way of DC? In a FW bridge filter circuit, to smooth out the AC current the capacitor does discharge some current, hence the ripple. How does this work in AC?

    My educated guess would be to convert the AC voltage to the DC equivalent and use DC properties to see how much the capacitor charges etc. This would mean that the capacitor stores energy in DC and has a seperate AC voltage drop. Is this statement correct?

    For inductors I have a much weaker understanding.

    In DC, I don't know how inductors actually work except that they store energy in the form of an magnetic field. But I don't know how this actually works. Is it the same as capacitors where as time goes by the capacitor produces voltage opposing the DC source? Does it use time constants as well?

    In AC, I know that Inductors have XL and also drop a voltage by ohms law. But how do they work in AC? What is their purpose except to bring the current/voltage out of phase and for resonance?

    Thank you.
  2. jcsd
  3. Sep 10, 2007 #2


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    Staff Emeritus
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    In DC, a capacitor is like an open circuit. The current flows until a charge builds up and a voltage across the capacitor is established.

    In AC, the voltage and polarity are changing, and the potential on the capacitor would change correspondingly.

    In DC, an inductor is basically a short circuit. The magnetic field develops with a time changing current, which is what happens with AC.

    Both inductors and capacitors affect the relationship between voltage and current in AC.

    Is one familiar with RC, RL and RLC cirucits?

    See if these are useful -



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