# 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

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

### Author: Katie

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.
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