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   What's a Capacitor?

BLOCK DIAGRAM

 

A basic, "parallel-plate" capacitor is simply two sheets of conductive foil, called plates, with an insulating film in between.  To save space, large plates can be rolled up like a jelly roll.  Two wire leads are attached to the capacitor, one to each plate.

When a capacitor is put into an electric current, electrons flow onto the upstream plate, charging it negatively (see figure below).  Since similar charges repel, electrons are driven off the other plate, leaving positively charged atoms called ions.

Capacitor diagram

Charge is now trapped in the capacitor.  Oppositely charged, ions and electrons pull on one another but the electrons can't cross the insulator.  They feel the force but are held in place, like an apple on a tree.

Both the apple and the capacitor harbor potential energy, one due to the separation of mass, the other to the separation of charge.  A capacitor's energy resides in an electromagnetic field surrounding the capacitor, as evidenced by a voltage across its plates.

 

CAPACITANCE

 

Capacitance, symbol C, measures the amount of charge required to produce one volt of voltage across a capacitor.  In other words, it's the charge per volt of a capacitor.

C = Q/V

A capacitor with large plates has more storage space than one with small plates but the charge is more spread out, weakening its concentration.  A large capacitor must trap more charge than a small one to achieve the same voltage across it.

The unit of capacitance is the farad, abbreviated F in honor of Michael Faraday.  The farad is an impractically large unit.  Most capacitors are measured in one of the following subunits, starting with the largest:

  • the microfarad (μF or MF) = one millionth (10-6) of a farad

  • the nanofarad (nF) = one billionth (10-9) of a farad.  Equal to 0.001 μF

  • the picofarad (pF) = one trillionth (10-12) of a farad   Equal to 0.001 nF

 

POWER SUPPLIES

 

You've seen how a capacitor is able to store charge.  This ability is used in power supply circuits, where capacitors help keep the supply's output voltage constant.

Capacitors are also used in circuits that convert alternating current (AC) into direct current (DC).  The capacitors store electrons when the source current is flowing in and put them to work when the source current is flowing out.

 

TONE CONTROL

 

As you might imagine, large capacitors can accept electrons for a longer time period than small ones because there's more space on their plates.  Small capacitors only accept electrons for a short time before additional charge is repelled.

Now consider low-frequency ac signals.  Low-frequency waves span longer time periods than high-frequency waves—the electrons go one way longer before reversing direction.  Small capacitors fill up with charge quickly and can't pass these long, low frequency signals.

On the other hand, high-frequency signals have short wavelengths.  The electrons change direction quickly, letting the alternating signal cross from plate to plate.

Because a capacitor's ability to conduct an ac signal is related to the frequency of the signal, capacitors are perfect for tone control circuits.

A capacitor wired in series with a circuit (see figure below) has a tonal effect opposite to the same capacitor wired in parallel with the circuit.


SERIES-WIRED CAPACITOR

Series wiring

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


PARALLEL-WIRED CAPACITOR

Parallel wiring

This guitar will sound bassy because high frequencies can go through the capacitor instead of going through the speaker.

 

Capacitance exists between any two conductors in close proximity, for example between the two wires in a guitar cable.  For that reason, an overly long cable will siphon off the guitar's high frequency tones, as shown in the above figure.

 

HYDRAULIC ANALOGY

 

Finally, take a look at the following hydraulic analogy to a capacitor.  It shows a flexible rubber membrane in a water pipe.  The water molecules can't cross the membrane but an alternating current, or ac signal, can.

The stretching of the membrane is like the charging and discharging of a capacitor and the amount of stretch is analogous to the voltage across the capacitor.

Potential energy is stored in the pipe's membrane as it is in the capacitor's electromagnetic field.  A very stretchy membrane corresponds to a large capacitance while a stiff membrane corresponds to a small capacitance.

 

Hydraulic analogy




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