Electric Field Between Parallel Plates (Cambridge (CIE) A Level Physics)

Electric field between parallel plates

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

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

  • Two charged parallel plates with a potential difference of V between them create a uniform electric field

• The magnitude of the uniform electric field strength between two charged parallel plates is defined by the equation:

• Where:

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

  • ΔV = potential difference between the plates (V)

  • Δd = distance 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 potential difference 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 distance between the plates

Derivation of electric field strength between two parallel plates

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

• Where:

  • W = work done on charge (J)

  • F = force on the charge (N)

  • d = distance between plates (m)

• The work done in moving a charge q through a potential difference V between parallel plates is given by:

• Equating the two expressions:

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

The work done on a charge in an electric field depends on the force and the distance between the plates