Fig. 1,280.—Hydraulic analogy illustrating capacity in an alternating current circuit. A chamber containing a rubber diaphragm is connected to a double acting cylinder and the system filled with water. In operation, as the piston moves, say to the left from the center, the diaphragm is displaced from its neutral position N, and stretched to some position M, in so doing offering increasing resistance to the flow of water. On the return stroke the flow is reversed and is assisted by the diaphragm during the first half of the stroke, and opposed during the second half. The diaphragm thus acts with the flow of water one-half of the time and in opposition to it one-half of the time. This corresponds to the electrical pressure at the terminals of a condenser connected in an alternating current circuit, and it has a maximum value when the current is zero and a zero value when the current is a maximum.
Ques. Name the unit of capacity and define it.
Ans. The unit of capacity is called the farad and its symbol is C. A condenser is said to have a capacity of one farad if one coulomb (that is, one ampere flowing one second), when stored on the plates of the condenser will cause a pressure of one volt across its terminals.
The farad being a very large unit, the capacities ordinarily encountered in practice are expressed in millionths of a farad, that is, in microfarads--a capacity equal to about three miles of an Atlantic cable.
It should be noted that the microfarad is used only for convenience, and that in working out problems, capacity should always be expressed in farads before substituting in formulæ, because the farad is chosen with respect to the volt and ampere, as above defined, and hence must be used in formulæ along with these units.
Fig. 1,281—Diagram illustrating a farad. A condenser is said to have a capacity of one farad if it will absorb one coulomb of electricity when subjected to a pressure of one volt. The farad is a very large unit, and accordingly the microfarad or one millionth of a farad is often used, though this must be reduced to farads before substituting in formulæ.
For instance, a capacity of 8 microfarads as given in a problem would be substituted in a formula as .000008 of a farad.
The charge Q forced into a condenser by a steady electric pressure E is
Q = EC