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 force per unit positive charge

• It is important to use a positive test charge in this definition, as this determines the direction of the electric field
• Using this convention, an electric field is always directed:
  • away from a positive charge
  • towards a negative charge

Electric field strength

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

The force per unit charge experienced by a small stationary positive test charge placed at that point

• The magnitude of field strength can be calculated using the equation:

• Where:
  • E = electric field strength (N C⁻¹)
  • F = electric force on the charge (N)
  • q = magnitude of the charge (C)

• Electric field strength is a vector quantity
  • Its direction is defined in terms of the direction a small positive test charge would move if placed in the field

Electric force

• Using the equation for field strength, the force on a charge in an electric field is given by:

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

• Like electric field strength, the direction of the force on a charge is also defined in terms of a positive test charge
• For example, in an electric field of strength :
  • a positive charge  experiences a force  in the direction of the field
  • a negative charge experiences a force  in the opposite direction

Force on a positive charge

A positive charge Q in an electric field of strength E experiences a force F in the direction of the field