Capacitors

A capacitor is an electronic component that can do a couple of useful things. It can STORE CHARGE and it can FILTER FREQUENCIES. Before exploring these, let's see what a capacitor is.

DEFINITION

Basically, a capacitor is two sheets of conducting foil, called plates, with a film of insulation in-between. This sandwich is often rolled up like a jelly roll to save space. Take a look at this CAT SCAN of a capacitor.

When a capacitor is in the path of an electric current, electrons flow onto the upstream plate, negatively charging it.

Diagram of a Capacitor CAPACITOR

Because negative charges repel, electrons are driven off the parallel plate, inducing a positive charge there. A tide of charge is now trapped inside the capacitor.

Negative and positive attract but, unless the pressure of attraction gets high enough to break down the insulation, electrons can't jump from plate to plate. The rising pressure across the plates is an example of a voltage.

A capacitor that needs one coulomb of charge (6.24 quintillion electrons) to raise one volt of pressure across it is said to have a capacitance of one farad. This is abbreviated 1 F (capital F because Faraday was a person). A capacitor needing two coulombs to raise a volt has a capacitance of 2 F.

More everyday units of capacitance are: the microfarad (μF - one millionth of a farad); the nanofarad (nF - one billionth of a farad); and the picofarad (pF - one trillionth of a farad).

STORING CHARGE

Now you see how a capacitor can store charge like a battery does. A capacitor, however, usually stores charge for just a short period of time.

For example, most electronic circuits run on DC (direct current) but the wall supply is AC (alternating current). Capacitors are employed to store charge when the AC flow is high and discharge it when the flow is low. This helps in supplying a direct, non-alternating current.

FILTERING FREQUENCIES

Now that you're thinking about electrons sloshing in and out of capacitors, consider the following.

A large plate is able to accept electrons for a longer time than a small plate simply because it has a larger capacity. A small plate can only accept electrons for a short time before it fills up and starts repelling additional negative charge.

Low-frequency AC waves have long, slow periods compared to high-frequency waves. As a result, low-frequency electrical effects can't pass through a small capacitor. Its plates become saturated too quickly.

But high-frequency effects, with their quick turnabouts, can pass through a small capacitor unimpeded.

SERIES VS. PARALLEL CAPACITORS

A capacitor wired in series with a circuit will have an opposite effect to the same capacitor wired in parallel to the circuit:

SERIES-WIRED CAPACITOR

Simplified Schematic

This guitar will sound trebly because only higher frequencies can cross the capacitor to the speaker.

PARALLEL-WIRED CAPACITOR

Simplified Schematic

This guitar will sound 'bassy' because higher frequencies can bypass the speaker through the capacitor.

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