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Intuitive reason for the capacitors/inductors in AC current

  1. May 14, 2013 #1
    Hello,

    I am wondering, why conceptually an inductor would have a large inductive reactance at high frequencies, and why they are large for capacitors are low frequencies.
     
  2. jcsd
  3. May 15, 2013 #2
    An inductor that is connected to a power source that produces a constant voltage behaves in a very simple way:

    The current through the inductor grows at constant rate, like 5 Amperes every second.

    When we first connect the inductor to the power source, voltage divided by current is infinite. (current is zero)

    Then voltage divided by current decreases, first very fast, then slower. Lets call this voltage divided by current "resistance". (with quote marks, because it's not the real resistance)

    In this circuit with an inductor "resistance" decreases as time passes.


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    A capacitor that is connected to a power source that produces a constant current behaves in a very simple way:

    The voltage over the capacitor grows at constant rate, like 5 Volts every second.

    When we first connect the capacitor to the power source, voltage divided by current is zero. (infinitely small voltage can produce the required constant current)

    Then voltage divided by current increases, first very fast, then slower. Lets also call this voltage divided by current "resistance", with quote marks.

    In this circuit with a capacitor "resistance" increases as time passes.
     
    Last edited: May 15, 2013
  4. May 16, 2013 #3

    DEvens

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    Think of it this way. An AC voltage (implied by a frequency) is positive for half the cycle, and negative for half. How long does a component spend "fighting" the change in voltage?

    A capacitor will charge up and then start "pushing back" based on that charge. So the longer it has to charge, the more it will be able to push back. So it will tend to resist slower changes more than faster ones because it has less time to charge up for fast ones. And the sine wave spends less time near the top or bottom for higher frequency, so the capacitor has less time to resist.

    An inductor resists change based on build up or decrease of magnetic field. When the voltage is rising it has to follow that rise. It takes time to build up the field and while it does it fights the change. Also, when the voltage is falling, the field starts to decrease, meaning the inductor produces its own voltage. And that tends to try to keep the voltage from falling. So the inductor fights each rise and each fall of voltage. The faster they come the more it resist them.
    Dan
     
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