Electric Fields & Forces on Charges (Cambridge (CIE) A Level Physics): Revision Note

Electric field definition

• 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

Forces on charges

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:

$E=\frac{F}{q}$

• 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:

$F=qE$

• 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 $E$:

  • a positive charge $+q$ experiences a force $Eq$ in the direction of the field

  • a negative charge $-q$ experiences a force $Eq$ 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