CIE A Level Physics (9702) exams from 2022

Revision Notes

18.1.1 Electric Fields & Forces on Charges

Electric Field Definition

  • An electric field is a region of space in which an electric charge “feels” a force
  • Electric field strength at a point is defined as:

The electrostatic force per unit positive charge acting on a stationary point charge at that point

  • Electric field strength can be calculated using the equation:

Electric Field Definition equation 1

  • Where:
    • E = electric field strength (N C-1)
    • F = electrostatic force on the charge (N)
    • Q = charge (C)
  • It is important to use a positive test charge in this definition, as this determines the direction of the electric field
  • The electric field strength is a vector quantity, it is always directed:
    • Away from a positive charge
    • Towards a negative charge
  • Recall that opposite charges (positive and negative) charges attract each other
  • Conversely, like charges (positive and positive or negative and negative) repel each other

Worked example: Calculating particle charge

Electric_Field_Definition_Worked_example_-_Charge_of_the_Particle_Question, downloadable AS & A Level Physics revision notes

Step 1:            Write down the equation for electric field strength

Electric Field Definition equation 1

Step 2:            Rearrange for charge Q

Electric Field Definition equation Worked Example equation 2

Step 3:            Substitute in values and calculate

Electric Field Definition equation Worked Example equation 3

Forces on Charges

  • The electric field strength equation can be rearranged for the force F on a charge Q in an electric field E:

F = QE

  • Where:
    • F = electrostatic force on the charge (N)
    • Q = charge (C)
    • E = electric field strength (N C-1)
  • The direction of the force is determined by the charge:
    • If the charge is positive (+) the force is in the same direction as the E field
    • If the charge is negative (-) the force is in the opposite direction to the E field
  • The force on the charge will cause the charged particle to accelerate if its in the same direction as the E field, or decelerate if in the opposite
  • Note: the force will always be parallel to the electric field lines

Worked example: Electric force

Forces_on_Charges_Worked_example_-_Electric_Force_Question, downloadable AS & A Level Physics revision notes

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 × 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 and decelerates the electron.

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.

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Go to Top