Electric capacitance of a conductor.

Capacitance is the ability of a body to store an electrical charge. Any body or structure that is capable of being charged, either with static electricity or by an electric current, exhibits capacitance. A common form of energy storage device is a parallel-platecapacitor. In a parallel plate capacitor, capacitance is directly proportional to the surface area of the conductor plates and inversely proportional to the separation distance between the plates. If the charges on the plates are +q and −q, and V gives the voltage between the plates, then the capacitance C is given by

Capacitor. Capacitance of plane capacitor.

A capacitor (originally known as condenser) is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric (insulator); for example, one common construction consists of metal foils separated by a thin layer of insulating film.

Connection of capacitors in parallel and in series.

Capacitors follow the same law using the reciprocals. The total capacitance of capacitors in series is equal to the reciprocal of the sum of the reciprocals of their individual capacitances:

.

The total capacitance of capacitors in parallel is equal to the sum of their individual capacitances:

.

The working voltage of a parallel combination of capacitors is always limited by the smallest working voltage of an individual capacitor.

Energy of electric field.

The electrostatic field stores energy. The energy density u (energy per unit volume) is given by[2]

where ε is the permittivity of the medium in which the field exists, and E is the electric field vector.

The total energy U stored in the electric field in a given volume V is therefore

Potential, potential difference.

Voltage, otherwise known as electrical potential difference or electric tension (denoted ∆V and measured in units of electric potential: volts, or joules per coulomb), is the electric potential difference between two points — or the difference in electric potential energy of a unit test charge transported between two points.[1] Voltage is equal to the work which would have to be done, per unit charge, against a static electric field to move the charge between two points. A voltage may represent either a source of energy (electromotive force), or it may represent lost, used, or stored energy (potential drop). A voltmeter can be used to measure the voltage (or potential difference) between two points in a system; usually a common reference potential such as the ground of the system is used as one of the points. Voltage can be caused by static electric fields, by electric current through a magnetic field, by time-varying magnetic fields, or a combination of all three.

Work in electrostatic field.

Electrical work is the work done on a charged particle by an electric field. The equation for 'electrical' work is, naturally, equivalent to that of 'mechanical' work:

where

Q is the charge of the particle, q, the unit charge

E is the electric field, which at a location is the force at that location divided by a unit ('test') charge

FE is the Coulomb (electric) force

r is the displacement

 is the dot product