Electric Fields & Forces on Charges (Cambridge International A-Level Physics)

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

Worked Example A stationary electron is in an electric field of field strength 5000 N C^-1.

Calculate the magnitude of the force that acts on the electron and state the direction it acts in relation to the electric field.

Answer:

Step 1: Write out the equation for the force on a charged particle

$F=qE$

Step 2: Substitute in values

F = (1.60 × 10^-19) × 5000 = 8.0 × 10^-16 N

Step 3: State the direction of the force

• Since the charge is negative, the force is directed against the electric field lines