Electric Fields & Forces on Charges (CIE A Level Physics)

• 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:

• 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

Step 1: Write down the equation for electric field strength

Step 2: Rearrange for charge Q

Step 3: Substitute in values and calculate

• 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

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

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.

• 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

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