User:Inconspicuum/Physics (A Level)/Electric Force

Electric fields are caused by charge. This charge can be either positive or negative. Like charges repel each other, and opposite charges attract each other. If we have two point charges of charge Q and q respectively, and they are a distance r apart, the force on each of them is:

$$F_{electric} = \frac{kQq}{r^2} = \frac{Qq}{4\pi \epsilon_0r^2}$$,

where k and &epsilon;0 are constants (k = 8.99 x 109 Nm2C-2,&epsilon;0 = 8.85 x 10-12C2N-1m-2). This means that, twice as far away from the point charge, the force on another charge decreases by a factor of 4. Electric force around a point charge is very similar to gravitational force around a point mass.

A uniform electric field consists of two conducting plates. These plates are oppositely charged, and infinitely wide. Obviously, infinitely wide conducting plates do not exist, so uniform electric fields do not exist. However, fields which approximate uniform electric fields do exist, provided we look towards the middle of the plates, and the plates are not too far apart - at the ends, the formulae for uniform fields no longer apply.

The force on a charge in a uniform electric field is given by:

$$F_{electric} = \frac{qV}{d}$$,

where V is the potential difference between the two plates, q is the charge of the point charge upon which the force is acting, and d is the distance between the two plates. This force remains constant as the charge travels within the electric field.