CIE A Level Physics复习笔记18.1.4 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 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

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