Capacitance - (Physics 3)

Capacitor

A device that is used to store electrical potential energy (example of usage: tuning the frequency of radio receivers)

Energized (charged) capacitor

Useful because energy can be reclaimed from the capacitor when needed for a specific application

Parallel-plate capacitor

Two parallel metal plates separated by a small distance

The charge on a capacitor

The magnitude of the charge on each plate

How the capacitor is energized

Connecting the plates to the two terminals of a battery or other sources of potential difference

(Charges are removed from one of the plates, leaving the plate with a net charge (an equal and opposite amount))

Capacitance

The ability of a conductor to store energy in the form of electrically separated charges

(The ability of a system to store an electric charge)

Capacitance equation

C = Q/△V

Q symbol

Magnitude of charge on each plate

△V symbol

Potential difference

SI unit for capacitance

Farad (F), which is equivalent to C/V (coulomb per volt)

Capacitance for a parallel-plate capacitor in a vacuum equation

C = (ε₀)A/d

ε₀ (epsilon) symbol

Permittivity of a vacuum

Epsilon (ε₀) constant

8.85 × 10^-12 C²/N x m²

Capacitance of a sphere increases as

The size of the sphere increases

Dielectric

An insulating material, such as air, rubber, glass, or waxed paper

When a dielectric is placed between the plates of a capacitor,

The capacitance increases because the molecules in a dielectric can align with the applied electric field

(excess negative charge near the surface of the dielectric at the positive plate / excess positive charge near the surface of the dielectric at the negative plate)

Discharge

The charges move back from one plate to another until both plates are uncharged again (state of lowest potential energy)

Device example that uses a capacitor

The flash attachment of a camera

Why a charged capacitor stores electrical potential energy

It requires work to move charges through a circuit to the opposite plates of a capacitor

If a capacitor is initially uncharged, the plates are at the same electric potential (neutral), then…

No work is required to transfer a small amount of charge from one plate to the other

Electrical potential energy stored in a charged capacitor equation

PE = ½ (Q)(△V)