Electric Force Between Two Point Charges (CIE A Level Physics)

• For a point outside a spherical conductor, the charge of the sphere may be considered to be a point charge at its centre
  • A uniform spherical conductor is one where its charge is distributed evenly

• The electric field lines around a spherical conductor are therefore identical to those around a point charge
• An example of a spherical conductor is a charged sphere
• The field lines are radial and their direction depends on the charge of the sphere
  • If the spherical conductor is positively charged, the field lines are directed away from the centre of the sphere
  • If the spherical conductor is negatively charged, the field lines are directed towards the centre of the sphere

The Electric Field Lines of Spherical Conductors

Electric field lines around a uniform spherical conductor are identical to those on a point charge

• The electrostatic force between two charges when in permittivity of free space is defined by Coulomb’s Law

• Coulomb’s Law states that:

The electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation

• The Coulomb equation is defined as:

The Electrostatic Force Between Two Charges

The electrostatic force F_E between two charges +Q and −Q with a separation of r is defined by Coulomb’s Law

• Where:
  • F_E = electrostatic force between two charges (N)
  • Q₁ and Q₂ = two point charges (C)
  • ε₀ = permittivity of free space
  • r = distance between the centre of the charges (m)

• The  relation is called the inverse square law
  • This means that when a charge is twice as far away from another, the electrostatic force between them reduces by (½)² = ¼

• If there is a positive and negative charge, then the electrostatic force is negative, this can be interpreted as an attractive force
• If the charges are the same, the electrostatic force is positive, this can be interpreted as a repulsive force
• Since uniformly charged spheres can be considered as point charges, Coulomb’s law can be applied to find the electrostatic force between them as long as the separation is taken from the centre of both spheres