Q. How does AC flow through capacitors? I thought electrons couldn’t flow through the dielectric in a capacitor.

A. You’re right, Electrons can’t flow through the dielectric in a capacitor because it is an insulator. But the electric fields from the charged plates on either side of the dielectric do pass through the dielectric.

On one half-cycle of the AC, the AC source puts a positive voltage on one plate and a negative voltage on the other plate. Electrons are drawn off the positive plate and an equal number are pushed onto the negative plate. No electrons pass through the dielectric -only the electric fields.

Electrons move through the rest of the circuit, though, as the capacitor starts to charge. But before it does, the polarity of the AC reverses. Now electrons are moved in the opposite direction. They are pushed onto the plate that had been positive and pulled off the plate that had been negative. Again, current flows through the rest of the circuit, but not between the plates.

In circuits like RC coupled amplifiers there are resistors in the current path. As a result, most of the AC voltage appears across these resistors. The voltage across the capacitor doesn’t have time to change because the AC changes polarity too fast. This is another way of saying that the reactance of the capacitor is low at the frequency ofthe AC.

DC, on the other hand, can’t get through a capacitor. This is because the dielectric is an insulator. Current will flow when the capacitor is first charged by the DC, with electrons flowing onto the negative plate and an equal number flowing off the positive plate. But once the capacitor has charged, no further current will flow.

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