AQA A Level Physics

Revision Notes

7.4.3 Electric Field Strength

Electric Field Strength Definition

  • An electric field is a region of space in which an electric charge “feels” a force
  • The electric field strength at a point is defined as:

The electrostatic force per unit positive charge acting on the charge at that point

  • The electric field strength can be calculated using the equation:

Electric Field Strength Equation_2

  • Where:
    • E = electric field strength (N C-1)
    • F = electrostatic force on the charge (N)
    • Q = charge (C)


  • It is important to use a positive test charge in this definition, as this determines the direction of the electric field
  • The electric field strength is a vector quantity, it is always directed:
    • Away from a positive charge
    • Towards a negative charge
  • Recall that opposite charges (positive and negative) charges attract each other
  • Conversely, like charges (positive and positive or negative and negative) repel each other

Worked Example

A charged particle is in an electric field with electric field strength 3.5 × 104 N C-1 where it experiences a force of 0.3 N.

Calculate the charge of the particle.

Electric Field Strength Worked Example


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