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:
- 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
- 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
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.
Examiner Tip
While the defining equation for electric field strength, E = F / 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!
