Electric Fields & Forces on Charges (CIE A Level Physics)

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

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

• It is important to use a positive test charge in this definition, as this determines the direction of the electric field
• An electric field is always directed:
  • Away from a positive charge
  • Towards a negative charge

   Electric Field Lines for Positive and Negative Charges

Electric fields are always directed away from positive charges and towards negative charges

• Recall that opposite charges (positive and negative) charges attract each other
• Conversely, like charges (positive and positive or negative and negative) repel each other

Attraction and Repulsion of Charges

Like charges repel; opposite charges attract

• The electric field strength equation is:

F = qE

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

• The force on the charge will cause the charged particle to accelerate if it's in the same direction as the E  field, or decelerate if in the opposite direction

The Force on a Positive Charge

An electric field strength E exerts a force F on a charge +Q in a uniform electric field

• Note: the force will always be parallel to the electric field lines