A charged object in an electric field experiences a force due to the field.

**The electric field strength, E, at a point in the field is defined as the force per unit charge on a positive test charge placed at that point.**

The unit of E is the newton per coulomb (NC^-1).

If a positive test charge Q at a certain point in an electric field is acted on by force F due to the electric field, the electric field strength, E, at that point is given by the equation

**E= F/Q**

The direction of the field strength is that of the force therefore field strength is a vector.

*Electric field between two parallel plates:*

Between two oppositely charged flat conductors that are parallel to each other, the field lines are at right angles to the plates and parallel to each other. The field lines always pass from the positively charged to negatively charged plates.

The field between the plates is uniform, due to the electric field having the same magnitude and direction between the plates. The electric field strength can be calculated by:

**E= V/d **

where V is the potential difference between the plates and d is the distance separating the plates. This electric field strength applies to any charged object no matter where it is inbetween the plates.

E may also have the units volt per metre (Vm^-1).

*Field Factors:*

An electric field exists near any charged body. The greater the charge, the stronger the electric field. A stronger electric field is represented by field lines that are closer together. It is important to note that field lines and imaginary lines that are used by physicists to express the nature of all fields, and cannot actually be observed under normal conditions.

The electric field between two oppositely charged parallel plates depends on the concentration of charge on the surface of the plates. The charge on each plate is spread evenly across the surface of the plate facing the other plate. Measurements show that the electric field strength between the plates is proportional to the charge per unit area on the facing surfaces.

Therefore E is proportional to Q/A where Q is charge and A is surface area of the plates. The constant of proportionality ε (Epsilon nought) gives equation Q/A = εE. ε is referred to as the permittivity of free space. It represents that charge per unit area on a surface in a vacuum that produces an electric field of strength of 1 volt per metre between the plates. Its value is 8.85 E-12 farads per metre (Fm^-1).