Basic electronics: What is capacitance?

A capacitor is a device capable of storing an electrical charge. Capacitance is a measure of the amount of the electrical charge a capacitor can store. Electrical current is the movement of electrons. Everything contains electrons, but unless they are moving, it's not electric current. It doesn't matter if the same electron that leaves the voltage source actually gets to "do the work," as long ...

01/10/2004


A capacitor is a device capable of storing an electrical charge. Capacitance is a measure of the amount of the electrical charge a capacitor can store.

Electrical current is the movement of electrons. Everything contains electrons, but unless they are moving, it's not electric current. It doesn't matter if the same electron that leaves the voltage source actually gets to "do the work," as long as it makes some other electron somewhere else move.

Simply, a capacitor is two conductors that are in close proximity, but not electrically connected. A material called a dielectric insulates the conductors. If one conductor is connected to a source of electrons such as the negative pole of a battery, those electrons can collect on that conductor. If another conductor is placed very close to the first, the surface electrons on it are pushed away because like charges repel each other, giving the second conductor a positive charge. If the circuit is completed by connecting the second conductor to the positive pole of the same battery, the repelled electrons can flow back into the battery. The resulting increased positive charge on the second conductor attracts more electrons to the surface of the first conductor, pulling them out of the battery. Current flows in the circuit, although no electrons cross the dielectric of the capacitor (Fig. 1).

Upon completion of the circuit, current flows momentarily β€” until the first conductor collects all the electrons it can hold. If it's a small conductor it can't hold very many electrons; its capacity for holding a charge β€” or capacitance β€” is small. Forming both conductors into plates increases surface area within the capacitor. The larger these plates, the longer the current will flow until they are full, because more electrons can be gathered on them. Their ability to hold a charge is higher.

The closer these plates are to each other, the more effect the electrons on the first plate will have on the electrons of the second plate. The negative charge on the first plate repels more electrons from the second plate, giving the second plate a higher positive charge. Also, the capacity to hold a charge is higher when plates are moved closer to each other, just as it is with larger plates. Conversely, separating the plates decreases the capacitance.

To increase capacitance, the plates are as big and as close together as possible, remembering that they must not touch electrically. If they touch, it's like a hole in a dam. All the charge stored on the first plate drains onto the second plate, and we end up with a fancy wire. Often the plates are made of sheets of foil, and the insulator can be some type of thin plastic such as Mylar. If these materials are rolled into a tube, a large surface area can be contained in a relatively small volume. The higher the voltage used to create the charge, the more possibility that an arc might occur, causing a short. For this reason, higher voltage capacitors must have thicker insulation, and are correspondingly larger in physical size.

Quantifying capacitance

Capacitance is measured in farads (F). A 1-F capacitor would have 1 volt (V) across its plates when it is charged by 1 Coulomb of electricity. A Coulomb is the quantity of electric charge that passes any cross-section of a conductor in 1 sec when the current is maintained at 1 ampere (A). This amounts to approximately 6.25 quintillion, or 6.25 x 1018electrons. To put these abstract quantities into real-world perspective, 1 Coulomb is roughly the amount of electricity that flows through a 12-watt (W) automotive light bulb in 1 sec.

Fifty years ago, a 1-F capacitor would be physically too large to be practical, due to the materials available and the high voltages required in the circuits of the day. Speculations and exaggerations about the size of a 1-F capacitor ranged from the size of a can of tuna to a cube the size of a city block, depending on voltage rating. Today, with newer dielectric materials and decreasing voltage requirements, a useful 1-F capacitor can be smaller than a C-cell battery.

Capacitors in parallel increase the effective area of the plates and increase the total capacitance (Fig. 2). However, capacitors in series effectively move the plates further away from each other and decrease the ability to hold a charge (Fig. 3).

Capacitors in ac circuits

In dc circuits, current flows until the plates of the capacitor are fully charged. If the polarity of the battery is reversed, the electrons flow to the other side of the capacitor, and there will be current flow again for a period of time. This is what happens to a capacitor in an ac circuit. Every time the polarity of the source voltage reverses, the current flow reverses. If the polarity reverses fast enough, the current flow is essentially unaffected by the presence of the capacitor in the circuit. This gives rise to the belief that capacitors block dc, but pass ac. This is essentially true for higher values of frequency and capacitance.

At any frequency, current flows in a capacitive circuit only until the plates of the capacitor are fully charged; it is the charging current that flows. When the plates are saturated with electrons, current flow ceases. In a dc circuit, obviously the frequency is zero. As the capacitor charges, the electron flow decreases. The opposition to the flow of current, or impedance, is at its highest.

As the frequency increases, the polarity reversal is more frequent, so the charging of the plates occurs for a shorter time. If the frequency is high enough, the amount of time for charging, and therefore the number of electrons collected on the plate, is so small that nearly full current flows for the whole period, because the plates never get fully charged. Theoretically, a capacitor at that frequency has such a low value of impedance that effectively it is not even there. Similarly, if the value of capacitance is high enough, the capacity to hold electrons is so high that the plates are never fully charged. Again, nearly full current flows for the whole period, and the impedance is so low that the capacitor is effectively out of the circuit.

Summary

With this knowledge, the rules for using capacitors are: