Motion of Charged Particles (AQA A Level Physics)

Revision Note

Katie M

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Katie M

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Motion of Charged Particles in an Electric Field

  • A charged particle in an electric field will experience a force on it that will cause it to move

  • If a charged particle remains stationary 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

Parabolic trajectory, downloadable AS & A Level Physics revision notes

The parabolic path of charged particles in a uniform electric field

  • 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

A single proton travelling with a constant horizontal velocity enters a uniform electric field between two parallel charged plates.

The diagram shows the path taken by the proton.

WE Motion of Charges Particles - proton trajectory, downloadable AS & A Level Physics revision notes

Draw the path taken by a boron nucleus that enters the electric field at the same point and with the same velocity as the proton.

Atomic number of boron = 5

Mass number of boron = 11

Answer:

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

  • This is because electric force F is proportional to the charge Q

F space equals space E Q

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

Examiner Tips and Tricks

Remember less deflection (e.g. from a higher mass particle) means the particle hits the plate later and the path has a smaller curve. Think of it as the particle being heavier so it is harder to steer it towards the plate.

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.