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Electric field intensity

The strength of the electric field is measured using a quantity called the electric field intensity. The greater the electrical field intensity the stronger the field. The electrical field intensity (EE) is defined as:

The electric field intensity is the force on a unit positive charge placed at that point in the field.

In a uniform field the electric field intensity is constant (the same at any point in the field) while in a radial field the electric field intensity decreases as the distance from the central charge increases.

Therefore for a radial field the electric field intensity distance d from a positive charge of size Q coulombs is:

Radial field:    EE = (1/4peo)Q/d2

For a uniform field between two parallel plates separated by a distance d and with a potential difference V between them the field is:

Uniform field:   EE = V/d

the units for electric field intensity are Newtons per coulomb (NC-1).

Units for electric field intensity

An alternative unit for electric field intensity may be derived as follows.

Since Joules = Volts x coulombs = Newtons x metres (J = VC = Nm) we can write NC-1 as [VC/m]C-1 = Vm-1. The electric field intensity may also be expressed in volts per metre (Vm-1).

Using these units it is probably easier to get a feel of what electric field intensity is, the change of voltage with distance (see later). The bigger the field the more rapidly does the potential change with distance. Think about this if you are near a Van de Graaff generator and your hair stands on end.

Example problem
Calculate the electric field at a distance of 2 m from a Van der Graaff generator carrying a charge of 1.0x10-6 C. [This is roughly the charge you would expect on a 0.2 m diameter sphere charged to 100 kV in air.]

Using and following equation and considering the charge as a point(!) :
EE = (1/4peo)Q/d2
EE = (1/4peo)1.0x10-6/22 = 9x109x1x10-6/4 = 2.25x1015 Vm-1.

© Keith Gibbs 2011