Electric Field Lines (AQA A Level Physics): Revision Note

Representing electric fields

• Field lines are used to represent the direction and magnitude of an electric field

• In an electric field, field lines are always directed from the positive charge to the negative charge

• In a uniform electric field, the field lines are equally spaced at all points, this means that:

  • the electric field strength is constant at all points in the field

  • the force on a test charge has the same magnitude and direction at all points in the field

• In a radial electric field, the field lines spread out with distance, this means that:

  • The field lines are equally spaced as they exit the surface of the charge

  • However, the separation between the field lines increases with distance

  • Therefore, the magnitude of electric field strength and the force on a test charge decreases with distance

Electric field around a point charge

• Around a point charge, the electric field is radial and the lines are directly radially inwards or outwards:

  • if the charge is positive (+), the field lines are radially outwards

  • if the charge is negative (-), the field lines are radially inwards

• A radial field spreads uniformly to or from the charge in all directions, but the strength of the field decreases with distance

  • The electric field is stronger where the lines are closer together

  • The electric field is weaker where the lines are further apart

• This shares many similarities to radial gravitational field lines around a point mass

  • Since gravity is only an attractive force, the field lines will look similar to the negative point charge, whilst electric field lines can be in either direction

Electric field around a conducting sphere

• When a conducting sphere becomes charged, the resulting electric field around it is the same as it would be if all the charge was concentrated at the centre

• This means that a charged sphere can be treated in the same way as a point charge in calculations

Electric field between two point charges

• For two opposite charges:

  • the field lines are directed from the positive charge to the negative charge

  • the closer the charges are brought together, the stronger the attractive electric force between them becomes

• For two charges of the same type:

  • the field lines are directed away from two positive charges or towards two negative charges

  • the closer the charges are brought together, the stronger the repulsive electric force between them becomes

  • there is a neutral point at the midpoint between the charges where the resultant electric force is zero

Electric field between two parallel plates

• When a potential difference is applied between two parallel plates, they become charged

  • The electric field between the plates is uniform

  • The electric field beyond the edges of the plates is non-uniform

Electric field between a point charge and parallel plate

• The field around a point charge travelling between two parallel plates combines:

  • The field around a point charge

  • The field between two parallel plates