Capacitance:Different conductors have different capacities to hold electric charge. The capacity of a conductor to hold charge depends upon the shape, size and surroundings of the conductor. The capacity of a conductor to hold charge is called capacitance.
For an isolated conductor when we increase the charge on the conductor, its potential also gets increased. The charge on the conductor is directly proportional to the potential of the conductor.Q α VQ = CVHere C is proportionality constant called as capacitance. So capacitance isC = Q / V
Unit of capacitance is coulomb / volt or farad.
Principle of Capacitor and Capacitance:
A capacitor is device to store electric energy. It consists of two parallel metal plates separated by a layer of air or a dielectric.
The capacitance of a conductor can be increases by increasing the size thereby the charge stored on it or by lowering the potential of the conductor without altering the charge on it
Consider a metal plate A and give it a positive charge. If C is the conductance and C is the potential then charge on the conductor is Q = CV.Now bring an uncharged metal plate B near to the charged plate A. the positive charge Q on the conductor induces an equal amount of negative charge on B. An equal amount of +Q charge is induced on the outer side of plate B. The induced negative charge on B lowers the potential of A, whereas the induced positive charge on B increases the potential on A.
Now let the plate B is earthed (grounded). The induced positive charge on B gets neutralized due to ground. While the induced negative charge on the inner side is held in a position by attraction of positive charges on A. As this negative charge lowers the potential of A, some more charge can be added to plate A. Thereby the capacitance can be increased.Dependence of Capacitor's CapacitanceThe capacitance of a capacitor depends upon1) Geometry (shape, size) of the plates.2) Distance of separation between the plates3) Material (dielectric) between the plates