When a voltage is applied to a capacitive circuit there is a change in the electric flux. The ease with which this change takes place is a measure of the capacitance of the circuit.
In d.c. circuits, capacitance is only effective when the voltage is switched on and off, but in a.c. circuits where the voltage varies continuously, the effect of capacitance is continuous.
A device used specifically to introduce capacitance into a circuit is known as a capacitor (sometimes called a condenser).
CHARGING A BODY
A conductor is given a positive charge when electrons are forcibly removed from the conductor, eg, by connecting it to the positive pole of a d.c. source. Similarly, when additional electrons are pushed on to a conductor, it is given a negative charge.
The use of force means that energy has been expended by the source of d.c. and this energy is stored in an electric field. An electric field is represented by lines of flux whose direction is the direction of force which would be experienced by a free positive charge placed in the field. Lines of electric flux behave in an analogous manner to lines of magnetic flux.
As the charge on a body increases, it repels further charge with greater force until eventually the repelling force equals the charging force and the conductor is fully charged.
The charge on a fully charged body may be changed by changing the voltage supplying the charging force, but the conductor will oppose this charge due to the charge it already possesses.
Any conductor will hold a charge, the magnitude of the charge depends upon the magnitude of the voltage applied, but for a single conductor, even a large voltage produces only a relatively small charge