Edexcel International A Level Physics

Revision Notes

4.13 Electric Field Strength

Test Yourself

Electric Field Strength

  • The electric field strength at a point is defined as:

The force per unit charge acting on a positive test 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 C1)
    • 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
  • Recall, the electric field strength is a vector quantity, it is always directed:
    • Away from a positive charge
    • Towards a negative charge
  • This direction is also denoted by the direction of the electric field 

Radial E field lines

Electric field lines are directed away from a positive point charge and towards a negative point charge

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

Exam Tip

While the defining equation for electric field strength, EF / Q is defined for a positive test charge, it is still useable for negative charges in an electric field. You will find that, if you substitute a negative charge in for Q, the electric field strength E is also negative. This simply means that the vector representing the field points in the opposite direction than it would for a positive charge, as you should expect. Make sure you can interpret the direction of electric field lines for your exam! 

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