Electric field strength

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Electric field strength

This is defined as the force per unit charge acting at a point in the field. So an equation for it is:

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E = electric field strength

F = force acting in newtons

q = the charge in coulombs

Units of E: NC-1 or Vm-1 (these are the same thing!!)

There is one special example you also need to know:

Uniform Fields

This is a uniform field. The field strength at any point in this field is:

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V = the pd between the plates

d = the distance separating the plates.

Field strength is a vector - it has direction as well as magnitude.

This is important to remember because in electric fields you can have field strengths acting in different directions due to different signs of charge.

A particularly useful equation to find field strength around a point charge (note - the first pictures is the field diagram section were point charges) is:

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Q = the charge causing the field

r = the separation between the charge and the point you are considering

ε = permittivity (see below for notes).

You can also write this as

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This shows that:

  1. E ∝ Q - the bigger the charge, the stronger field.
  2. E ∝ Copyright S-cool - another inverse square relationship. The further you are from the charge, the weaker the field strength.
  3. The constant of proportionality is Copyright S-cool . Now this number varies depending on what the field is in. An electric field in water has different properties to a field in a vacuum, for example. So for each medium, you need a value of e, the permittivity. The one used most commonly at A-level is e0, the permittivity of a vacuum, which is almost exactly the same as permittivity for air.


What's the field strength at a point 2cm from a charge of 2 x 10-6C in air?

( = 9 x 109 mF-1)

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