Electronics Components: What Is a Capacitor?

A capacitor is an electronic component that takes advantage of the ability of electric fields to reach out across an insulator. It consists of two flat plates made from a conducting material such as silver or aluminum, separated by a thin insulating material such as Mylar or ceramic. The two conducting plates are connected to terminals so that a voltage can be applied across the plates.

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Note that because the two plates are separated by an insulator, a closed circuit isn't formed. Nevertheless, current flows — for a moment, anyway.

How can this be? When the voltage from a source such as a battery is connected, the negative side of the battery voltage immediately begins to push negative charges toward one of the plates. Simultaneously, the positive side of the battery voltage begins to pull electrons (negative charges) away from the second plate.

What permits current to flow is the electric field that quickly builds up between the two plates. As the plate on the negative side of the circuit fills with electrons, the electric field created by those electrons begins to push electrons away from the plate on the other side of the insulator, toward the positive side of the battery voltage.

As this current flows, the negative plate of the capacitor builds up an excess of electrons, whereas the positive side develops a corresponding deficiency of electrons. Thus, voltage is developed between the two plates of the capacitor.

But there's a catch: This current flows only for a brief time. As the electrons build up on the negative plate and are depleted from the positive plate, the voltage between the two plates increases because the difference in charge between the two plates increases.

The voltage continues to increase until the capacitor voltage equals the battery voltage. Once the voltages are the same, current stops flowing through the circuit, and the capacitor is said to be charged.

At this point the magic gets even better. Once a capacitor has been charged, you can disconnect the battery from the capacitor, and the voltage will remain in the capacitor. In other words, although the voltage in the capacitor is created by the battery, this voltage isn't dependent on the battery for its continued existence. Disconnect the battery, and the voltage remains across the two plates of the capacitor.

Thus, capacitors have the ability to store charge — an ability known as capacitance.

Here are a few additional things you should know about capacitors before moving on:

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The most common symbol used for capacitors in schematic diagrams is simply two parallel lines separated by a gap, as shown in the margin.
An alternative symbol uses a straight line and a curved line to represent the plates. The curved line is generally used on the negative side of the circuit.
Although some capacitors aren't sensitive to polarity, many others are. This sensitivity has to do with the choice of materials used to create the capacitors: With some materials, connecting the voltage in the wrong direction can damage the capacitor. Capacitors that have distinct positive and negative terminals are called polarized capacitors. A plus sign is used in the schematic diagram to indicate the polarity, as shown in the margin.

The insulating material between the two conducting plates is properly called dielectric, a term that refers to the ability of the insulating layer to become polarized by the electric field that exists between the two plates when they become charged.

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