Uniform Electric Field (AQA A Level Physics): Revision Note

Uniform Electric Field Strength

• The magnitude of the electric field strength in a uniform field between two charged parallel plates is defined as:

$E=\frac{V}{d}$

• Where:

  • E = electric field strength (V m⁻¹)

  • V = potential difference between the plates (V)

  • d = separation between the plates (m)

• Note: both units for electric field strength, V m⁻¹ and N C⁻¹, are equivalent 

• The equation shows:

  • The greater the voltage between the plates, the stronger the field

  • The greater the separation between the plates, the weaker the field

• This equation cannot be used to find the electric field strength around a point charge

  • This is because the field around a point charge is radial

• The electric field between two plates is directed:

  • From the positive plate (i.e. the one connected to the positive terminal)

  • To the negative plate (i.e. the one connected to the negative terminal)

Uniform Electric Field Between two Parallel Plates

The electric field strength between two charged parallel plates is the ratio of the potential difference and separation of the plates

Derivation of Electric Field Strength Between Plates

• When two points in an electric field have a different potential, there is a potential difference between them

  • To move a charge across that potential difference, work needs to be done

  • Two parallel plates with a potential difference V across them create a uniform electric field

• The electric field strength between the plates is given by the equations:

$E=\frac{F}{Q}=\frac{V}{d}$

• Rearranging the fractions by multiplying by Q and d on both sides, gives:

$Fd=VQ$

• When a charge Q moves from one plate to the other, the work done on the charge by the field is

$W=Fd$

• Where:

  • W = work done on charge (J)

  • F = electrostatic force (N)

  • d = distance between plates (m)

The work done on the charge depends on the electric force and the distance between the plates

• Therefore, the work done in moving a charge Q through a potential difference V between parallel plates is also given by:

$W=QV$