Electric Field Strength (AQA A Level Physics): Revision Note

Electric field strength

• An electric field is a region of space in which an electric charge experiences a force

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

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

• The electric 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)

• Note that the definition specifies that a positive test charge is used

• This sets a clear convention for the direction of an electric field, for example, in a 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

• Hence, electric field strength is a vector quantity and is always directed:

  • away from a positive charge

  • towards a negative charge