# 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: • 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. Close Close