CIE A Level Physics (9702) exams from 2022

Revision Notes

18.1.4 Motion of Charged Particles

Motion of Charged Particles

  • A charged particle in an electric field will experience a force on it that will cause it to move
  • If a charged particle remains still in a uniform electric field, it will move parallel to the electric field lines (along or against the field lines depending on its charge)
  • If a charged particle is in motion through a uniform electric field (e.g. between two charged parallel plates), it will experience a constant electric force and travel in 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
  • The force on the particle is the same at all points and is always in the same direction
  • Note: an uncharged particle, such as a neutron experiences no force in an electric field and will therefore travel straight through the plates undeflected
  • The amount of deflection depends on the following properties of the particles:
    • Mass – the greater the mass, the smaller the deflection and vice versa
    • Charge – the greater the magnitude of the charge of the particle, the greater the deflection and vice versa
    • Speed – the greater the speed of the particle, the smaller the deflection and vice versa

Worked example

Motion_of_Charged_Particles_Worked_Example_-_Proton_Trajectory_Question, downloadable AS & A Level Physics revision notes

Step 1:           

Compare the charge of the boron nucleus to the proton

    • Boron has 5 protons, meaning it has a charge 5 × greater than the proton
    • The force on boron will therefore be 5 × greater than on the proton

Step 2:           

Compare the mass of the boron nucleus to the proton

    • The boron nucleus has a mass of 11 nucleons meaning its mass is 11 × greater than the proton
    • The boron nucleus will therefore be less deflected than the proton

Step 3:           

Draw the trajectory of the boron nucleus

    • Since the mass comparison is much greater than the charge comparison, the boron nucleus will be much less deflected than the proton
    • The nucleus is positively charged since the neutrons in the nucleus have no charge
      • Therefore, the shape of the path will be the same as the proton

Motion_of_Charged_Particles_Worked_example_Boron_trajectory, downloadable AS & A Level Physics revision notes

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