CIE A Level Physics (9702) 2019-2021

Revision Notes

21.1.3 Electric Field Strength

Electric Field of a Point Charge

  • The electric field strength at a point describes how strong or weak an electric field is at that point
  • The electric field strength E at a distance r due to a point charge Q in free space is defined by:

Electric Field of a Point Charge equation

  • Where:
    • Q = the charge producing the electric field (C)
    • r = distance from the centre of the charge (m)
    • ε0 = permittivity of free space (F m-1)
  • This equation shows:
    • Electric field strength is not constant
    • As electric field strength increases, decreases by a factor of 1/r2
  • This is an inverse square law relationship with distance
  • This means the field strength decreases by a factor of four when the distance is doubled
  • Note: this equation is only for the field strength around a point charge since it produces a radial field
  • The electric field strength is a vector Its direction is the same as the electric field lines
    • If the charge is negative, the E field strength is negative and points towards the centre of the charge
    • If the charge is positive, the E field strength is positive and points away from the centre of the charge
  • This equation is analogous to the gravitational field strength around a point mass

Worked example: Surface charge of a sphere

Electric_Field_of_a_Point_Charge_-_Surface_Charge_of_a_Sphere_Question, downloadable AS & A Level Physics revision notes

Step 1:            Write down the known values

Electric field strength, E = 1.5 × 105 V m-1

Radius of sphere, r = 15 / 2 = 7.5 cm = 7.5 × 10-2 m

Step 2:            Write out the equation for electric field strength

Electric Field of a Point Charge equation

Step 3:            Rearrange for charge Q

Q = 4πε0Er2

Step 4:            Substitute in values

Q = (4π × 8.85 × 10-12) × (1.5 × 105) × (7.5 × 10-2)2 = 9.38 × 10-8 C = 94 nC (2 s.f)

Exam Tip

Remember to always square the distance!

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