CIE A Level Physics (9702) 2019-2021

Revision Notes

10.1.2 Uniform Electric Fields

Electric Field Strength

  • The electric field strength of the uniform field between two charged parallel plates is defined by:

Electric field strength parallel plates, downloadable AS & A Level Physics revision notes

The electric field strength between two parallel plates is the ratio of the potential difference and separation of the plates

  • The direction of the electric field is from the plate connected to the positive terminal of the cell to the plate connected to the negative terminal
  • The electric fields lines are parallel and equidistant

Worked example

Worked example - parallel plates (1), downloadable AS & A Level Physics revision notes

Worked example - parallel plates (2), downloadable AS & A Level Physics revision notes

Forces on Charges

  • The force on a charge in a uniform electric field is the product of the electric field strength E and the magnitude of the charge q

 

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

Force on a charge in a uniform electric field is in the same direction as the electric field

  • A uniform electric field (e.g. between two charged parallel plates) is one where the electric field lines are parallel and equidistant
  • The force on the charge is constant
  • The force will be in parallel to the direction of the electric field
    • If the charge if positive, the force is in the same direction as the electric field
    • If the charge is negative, the force is in the opposite direction to the electric field
  • q is always the charge of the particle itself, not the charge of a particle creating an electric field

Worked example

Worked example - force on charge, downloadable AS & A Level Physics revision notes

Exam Tip

  • To remember which way the force acts on a charged particle, remember electric fields are always from positive to negative
  • A positive charge will be attracted towards a negative plate (in the same direction as the field)
  • A negative charge will be attracted towards a positive plate (the opposite direction as the field)

Motion of Charged Particles

  • A charged particle experiences an electric force F in an electric field E which will cause it to move
  • The force on the particle is constant
  • When a charged particle is kept still in a uniform electric field, it will move in the same or opposite direction to the electric field lines
  • If a charged particle is in motion through an electric field, it will experience a constant electric force and travel in a parabolic trajectory:

Parabolic trajectory, downloadable AS & A Level Physics revision notes

Charged particles in an electric field follow a parabolic trajectory

 

  • The direction of the parabola will depend on the charge of the particle
    • A positive charge will be deflected towards the negative plate
    • A negative charge will be deflected towards the positive plate

 

Worked example

Worked example - motion in an electric field, downloadable AS & A Level Physics revision notes

  • The proton has a charge of +1e so is deflected upwards towards the negative plate
  • Since a neutron is neutral (it has no charge), it experiences no electric force in the electric field and travels straight through between the plates undeflected

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