Electrostatics and Capacitance

Electrostatics

The properties of charged particles at rest

A negative charge

Arises when an object gains electrons (e.g. polythene or ebonite) become negatively charged when rubbed with a dry cloth

A positive charge

Arises when an object loses electrons (e.g. Perspex, glass, and acetate) become positively charged when rubbed with a dry cloth.

Coulomb’s Law

States that the force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

$F=\frac{q1q2}{4\pi\varepsilon d^2}$

Electric field

Is a region of space in which electric forces can be felt

Electric field strength

At a point in an electric field is the force experienced per unit charge placed at that point in the field.

$E=\frac{F}{Q}$

Potential difference

Between two points is the work done in bringing unit positive charge from one point to another

$V=\frac{W}{Q}$

Insulator

A substance through which electric charge cannot flow

Conductor

A substance through which electric charge can flow

Coulomb

Unit of electric charge. 1 coulomb of charge passes a particular point when a current of 1 A flows for 1 Second

Volt

The potential difference between two points is 1 volt if 1 joule of work is done in bringing 1 coulomb from one point to the other

Capacitance

Is the ratio of charge to voltage in a conductor

$C=\frac{Q}{V}$

Farad

A conductor (or capacitor) has a capacitance of 1 farad, if by placing a charge of 1 coulomb on it raises its potential by 1 volt.

i.e. 1 farad = 1 coulomb per volt

Or

$$1F=1CV^{-1}$$

Potential at a point

The difference in the potential at a given point and the potential of the earth (taken to be at zero potential)