**Coulomb’s Law (Electric Force)** (Also seen in GCSE Physics 2)

The electrostatic force acts between all charged particles and can be attractive or repulsive. It is the charges themselves that cause the force to exist. The force that acts between two charges, *Q*_{1} and *Q*_{2}, whose centres are separated by a distance of *r* is given by:

**Electric Fields**

An electric field is the area around a charge where any other charge will experience a force. We can model a field with field lines or lines of force.

* *

*Radial Fields*

For a positive charge the field lines start at the charge and go out to infinity. For a negative charge the field lines end at the centre of a mass and tail back from infinity. We can see that they become more spread out the further from the charge we go.

*Uniform Fields*

The field lines are parallel in a uniform field. Between two conducting plates the field lines leave the positive plate and enter the negative plate.

**Electric Field Strength, E**

We can think of electric field strength as the concentration of the field lines at that point. We can see from the diagrams above that the field strength is constant in a uniform field but drops quickly as we move further out in a radial field.

The electric field strength at a point is a vector quantity and is defined as:

*The force per unit charge acting on a small charge placed at that point in the field*

We can represent this with the equation:

If we use our equation for the electric force at a distance *r* and substitute this in for *F* we get: