8: Nuclear and Particle Physics

Question 1

How to know if a question is asking about cyclotron?

Answer 1

Key words
- electric field and magnetic field
- high kinetic energy
- particle accelerator

Question 1

Model answer - Cyclotron

Answer 1

• Two dees connected to a.c
• Elec field accelerates particle between dees
• Elec field only present across gap so KE only increases between gap
• Electric field polarity reverses every half cycle so particle always gains KE as it crosses gap
• Magnetic field perpendicular to plane of dees
• Provides force on charge at right angle to particle direction
• Acts as centripetal force; circular motion
• Radius of particle’s path increases as velocity increases

Question 2

Model answer - Linac

Answer 2

• Metal drifts tubes connected to a.c power supply; adjacent tubes opposite polarity
• Electrons accelerated across gap due to electric field produced
• a.c pd so as particle emerges from tube, polarity changes and particle is attracted to next tube due to electrostatic force
• Time spent in each tube must be same so each tube gets longer

Question 3

Model answer - Rutherford’s Atom

Answer 3

Most particle undeflected
- Atom is mainly empty space
- Particles didn’t get close enough to any matter to be affected
Few particles deflected by small angles
- *Charge is concentrated in centre
- Most particles didn’t get near enough to a sufficiently large charge to be effected
Very small proportion of particles deflected at >90°
- Mass of atom is concentrated in very small space relative to size of atom

Question 4

Experimental set-up

Answer 4

• Pump air from apparatus to create vacuum
• Ensure sufficient alpha particles reach gold foil
• Otherwise alpha particles would collide and interact with air molecules