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Impedance of capacitors.

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Dwipayan Kar

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Why does a capacitor offer low impedance to a high frequency signal and high impedance to a low frequency signal?
 

A capacitor stores charge but it takes time for the current to build up charge on the plates. At low frequencies there is enough time for the charge to build up and impede the flow of current as the voltage on the capacitor approaches the charging voltage. At higher frequencies the charging voltage switches before the capacitor has enough time to build up the charge to impede flow. A good analogy is a flexible diaphragm in a water pipe.
 

Q=CV is the fundamental nature of capacitance

I=dQ/dt=CdV/dt

and for a sine wave v(t)=sin(wt) (w being omega)

d(sin(wt))/dt = w*cos(wt)

so I(t)=C*w*cos(wt)

There is your w term as a scale factor in I, impedance is
dV/dI, so w (omega) in the denominator and there you go.
 

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