A capacitor is a device that stores electrical potential energy by building up a difference in charge on two pieces of metal.

The ability of a capacitor to store charge is called the capacitance of the capacitor.

Capacitance is the ratio of net charge on each plate to the potential difference between the

plates. C = Q/ΔV

(or Q = VC)

The unit is the farad (F) , which is one coulomb per volt (C/V). A typical capacitor ranges from microfarads to picofarads.

F to pF1 F = 10-6 F1 pF = 10-12 F

Capacitance for a parallel plate capacitor is calculated from:C = ε0 A/dε0 is the permittivity of a vacuumA is the area of the platesd is the distance between the plates

The permittivity depends on the material between the plates. For a vacuum (or air), the value is 8.85 x 10-12 C2/N•m2. Permittivity is greater for other materials.

The material between the plates is an insulator and is called a dielectric. The presence of a dielectric increases the capacitance.

The molecules of a dielectric align themselves with the electric field, negative end toward the positive plate, and vice-versa.

The dielectric effectively reduces the charge at each plate, so the plate can store more charge for a given voltage. More charge means more capacitance for same voltage. (Q = VC)

C = ε0 A/d

As the equation indicates, the larger the plates of a capacitor, the higher the capacitance; and the further apart the plates are, the lower the capacitance.

List four ways to increase the charge on a capacitor.

Once a capacitor is charged, the source can be removed and the capacitor will retain the charge. If the two plates are connected by a conducting material, the capacitor will rapidly discharge. This discharge continues until the potential is zero.

Devices that use capacitors include: camera flash devices, computer keyboards, defibrillators, tasers and automobile blinkers.

A charged capacitor stores electrical potential energy. The amount is given by the following equation: PEelectric = ½QΔV

PEelectric = ½QΔV

Since Q = ΔVC,PEelectric = ½C(ΔV)2and:PEelectric = Q2/2C

Here are the two big capacitor equations:

Q = VC

PEelectric = ½CV2

A 3.0 F capacitor is connected to a 12 V battery. What is the magnitude of the charge on each plate of the capacitor, and how much electrical potential energy is stored in the capacitor?