CIE A Level Physics

Revision Notes

Syllabus Edition

First teaching 2020

Last exams 2024

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18.1.1 Electric Fields & Forces on Charges

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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

A charged particle is in an electric field with electric field strength 3.5 × 104 N C-1 where it experiences a force of 0.3 N.Calculate the charge of the particle.

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

Force on charge in E field, downloadable AS & A Level Physics revision notes

An electric field strength E exerts a force F on a charge +Q in a uniform electric field

  • Note: the force will always be parallel to the electric field lines

Worked example

An electron is stationary in an electric field with an electric field strength of 5000 N C-1.  Calculate the magnitude of the electric force that acts on the electron and state which direction the force will act in relation to the electric field.Electron charge e = 1.60 × 10-19 C.

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

Point charge field lines, downloadable AS & A Level Physics revision notes

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

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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