# 7.2 Electric Field Strength

## 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 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

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.

#### 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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