CIE A Level Physics (9702) 2019-2021

Revision Notes

21.1.1 Electric Fields & Forces on Charges

Point Charge Approximation

  • 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

Exam Tip

You might have noticed that the electric fields share many similarities to the gravitational fields. The main difference being the gravitational force is always attractive, whilst electrostatic forces can be attractive or repulsive.

You should make a list of all the similarities and differences you can find, as this could come up in an exam question.

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