AQA A Level Physics

Revision Notes

7.4.4 Uniform Electric Field

Uniform Electric Field Strength

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

Electric Field Strength Uniform Equation_2

  • Where:
    • E = electric field strength (V m-1)
    • V = potential difference between the plates (V)
    • d = separation between the plates (m)

 

  • Note: the electric field strength is now also defined by the units V m-1
  • 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

 

  • Remember this equation cannot be used to find the electric field strength around a point charge (since this would be a radial field)
  • The direction of the electric field is from the plate connected to the positive terminal of the cell to the plate connected to the negative terminal

Electric field between two plates, downloadable AS & A Level Physics revision notes

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

  • Note: if one of the parallel plates is earthed, it has a voltage of 0 V

Worked Example

Two parallel metal plates are separated by 3.5 cm and have a potential difference of 7.9 kV.

Calculate the electric force acting on a stationary charged particle between the plates that has a charge of 2.6 × 10-15 C.

Step 1: Write down the known values

    • Potential difference, V = 7.9 kV = 7.9 × 103 V
    • Distance between plates, d = 3.5 cm = 3.5 × 10-2 m
    • Charge, Q = 2.6 × 10-15 C

Step 2: Calculate the electric field strength between the parallel plates

Electric Field Strength Uniform Equation_2Electric Field Strength equation Worked Example equation 2

Step 3: Write out the equation for electric force on a charged particle

F = QE

Step 4: Substitute electric field strength and charge into electric force equation

F = QE = (2.6 × 10-15) × (2.257 × 105) = 5.87 × 10-10 N = 5.9 × 10-10 N (2 s.f.)

Exam Tip

Remember the equation for electric field strength with V and d is only used for parallel plates, and not for point charges (where you would use E = F/Q)

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 ΔV across them create a uniform electric field
  • The electric field strength between the plates is given by the equations:

Electric Field Strength Equation

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

Fd = ΔVQ

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

W = Fd

  • Where:
    • W = work done (J)
    • F = force (N)
    • d = distance (m)

Parallel Plates Work Done, downloadable AS & A Level Physics revision notes

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 ΔV between parallel plates is also given by:

W = ΔVQ

Worked Example

Calculate the force needed to move an electron between two parallel plates 2 m apart with a potential difference of 400 V between them

Work Done Parallel Plates Worked Example

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