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)

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)

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

$W=QV$