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Circuit Components: The Capacitor

We have said that an electrical current can only flow through a closed circuit. Thus, if we break or cut a wire in a circuit, that circuit is opened up, and can no longer carry a current. But we know that there will be a small electrical field between the broken ends. What if we modify the point of the break so that the area is expanded, thus providing a wide area of "not quite" contact?



A basic capacitor.

The figure to the right shows two metal plates, placed close to each other but not touching. A wire is connected to each plate as shown, so that this construction may be made part of an electrical circuit. As shown here, these plates still represent nothing more than an open circuit. A wide one to be sure, but an open circuit nevertheless.


A capacitor with an electric charge.

Now suppose we apply a fixed voltage across the plates of our construction, as shown to the left. The battery attempts to push electrons onto the negative plate (blue in the figure), and pull electrons from the positive plate (the red one). Because of the large surface area between the two plates, the battery is actually able to do this. This action in turn produces an electric field between the two plates, and actually distorts the motions of the electrons in the molecules of air in between the two plates. Our construction has been given an electric charge, such that it now holds a voltage equal to the battery voltage. If we were to disconnect the battery, we would find that this structure continues to hold its charge — until something comes along to connect the two plates directly together and allow the structure to discharge itself.

Because this structure has the capacity to hold an electrical charge, it is known as a capacitor. How much of a charge it can hold is determined by the area of the two plates and the distance between them. Large plates close together show a high capacity; smaller plates kept further apart show a lower capacity. Even the cut ends of the wire we described at the top of this page show some capacity to hold a charge, although that capacity is so small as to be negligible for practical purposes.

The electric field between capacitor plates gives this component an interesting and useful property: it resists any change in voltage applied across its terminals. It will draw or release energy in the form of an electric current, thus storing energy in its electric field, in its effort to oppose any change. As a result, the voltage across a capacitor cannot change instantaneously; it must change gradually as it overcomes this property of the capacitor.



A multi-plate capacitor.

A practical capacitor is not limited to two plates. As shown to the right, it is quite possible to place a number of plates in parallel and then connect alternate plates together. In addition, it is not necessary for the insulating material between plates to be air. Any insulating material will work, and some insulators have the effect of massively increasing the capacity of the resulting device to hold an electric charge. This ability is known generally as capacitance, and capacitors are rated according to their capacitance.

It is also unnecessary for the capacitor plates to be flat. Consider the figure below, which shows two "plates" of metal foil, interleaved with pieces of waxed paper (shown in yellow). This assembly can be rolled up to form a cylinder, with the edges of the foil extending from either end so they can be connected to the actual capacitor leads. The resulting package is small, light, rugged, and easy to use. It is also typically large enough to have its capacitance value printed on it numerically, although some small ones do still use color codes.

The schematic symbol for a capacitor, shown below and to the right of the rolled foil illustration, represents the two plates. The curved line specifically represents the outer foil when the capacitor is rolled into a cylinder as most of them are. This can become important when we start dealing with stray signals which might interfere with the desired behavior of a circuit (such as the "buzz" or "hum" you often hear in an AM radio when it is placed near fluorescent lighting). In these cases, the outer foil can sometimes act as a shield against such interference.


Constructing a paper capacitor.


Construction of a ceramic disc capacitor.

An alternate construction for capacitors is shown to the right. We start with a disc of a ceramic material. Such discs can be manufactured to very accurate thickness and diameter, for easily-controlled results.

Both sides of the disc are coated with solder, which is compounded of tin and lead. These coatings form the plates of the capacitor. Then, wire leads are bonded to the solder plates to form the structure shown here.

The completed construction is then dipped into another ceramic bath, to coat the entire structure with an insulating cover and to provide some additional mechanical protection. The capacitor ratings are then printed on one side of the ceramic coating, as shown in the example here.

Modern construction methods allow these capacitors to be made with accurate values and well-known characteristics. Also, different types of ceramic can be used in order to control such factors as how the capacitor behaves as the temperature and applied voltage change. This can be very important in critical circuits.


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