WHAT IS CAPACITANCE? BASIC INFORMATION AND TUTORIALS



Capacitance (C) is the phenomenon whereby a circuit stores electrical energy. Whenever two conducting materials are separated by an insulating material, they have the ability of storing electrical energy.

Such an arrangement of materials (two conductors separated by an insulator) is called a capacitor or condenser. If a source of dc voltage is connected between the two conducting materials of a capacitor, a current will flow for a certain length of time.

The current initially will be relatively large but will rapidly diminish to zero. A certain amount of electrical energy will then be stored in the capacitor.

If the source of voltage is removed and the conductors of the capacitor are connected to the two ends of a resistor, a current will flow from the capacitor through the resistor for a certain length of time. The current initially will be relatively large but will rapidly diminish to zero.

The direction of the current will be opposite to the direction of the current when the capacitor was being charged by the dc source. When the current reaches zero, the capacitor will have dissipated the energy which was stored in it as heat energy in the resistor. The capacitor will then said to be discharged.

The two conducting materials, often called the plates of the capacitor, will be electrically charged when electrical energy is stored in the capacitor. One plate will have an excess of positive electricity and therefore will be positively charged with a certain number of coulombs of excess positive electricity.

The other plate will have an excess of negative electricity and therefore will be negatively charged with an equal number of coulombs of excess negative electricity. When in this state, the capacitor is said to be charged. When a capacitor is charged, a voltage is present between the two conductors, or plates, of the capacitor.

When a capacitor is in a discharged state, no electrical energy is stored in it, and there is no potential difference, no voltage, between its plates. Each plate contains just as much positive as negative electricity, and neither plate has any electric charge.

From the above discussion it is seen that a capacitor has a sustained current only as long as the voltage is changing. A capacitor connected to a dc supply will not have a sustained current. In an ac circuit, the voltage is continually changing from instant to instant.

Therefore, when a capacitor is connected to an ac supply, an alternating current continues to flow. The current is first in one direction, charging the capacitor, and then in the opposite direction, discharging the capacitor.

Farad (F) The unit of capacitance. It is designated by the symbol F. A circuit or capacitor will have a capacitance of 1 F if when the voltage across it is increased by 1 V, its stored electricity is increased by 1 C.

Another definition for a capacitance of 1 F, which results in the same effect, is given below. A circuit or capacitor will have a capacitance of 1 F when if the voltage impressed upon it is changed at the rate of 1 V/s, 1 A of charging current flows.

Capacitive reactance (Xc) is the name given to the opposition to the flow of alternating current due to capacitance. It is measured in ohms as resistance and inductive reactance are.

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