Topic 8: Nuclear And Particle Physics

Question 1

Define nucleon number

Answer 1

The total number of protons and neutrons in a nucleus

Question 2

Define proton number

Answer 2

The number of protons in a nucleus, which determines the element

Question 3

Explain how the results of Rutherford’s alpha scattering experiment led to the nuclear model of the atom.

Answer 3

1. Most alpha particles passed through undeflected, in a straight line -> atom is mostly empty space
2. Some alpha particles passed through with a small angle deflection -> nucleus positively charged as charged alpha particles where repelled from the centre and deflected
3. A few alpha particles deflected at large angles (> 90 degrees) -> the nucleus is small, dense, positively charged, but carry most of the mass
   Conclusion: The atom consists of a small, dense, positively charged nucleus surrounded by electrons.
   Led to the rejection of the plum pudding model, where charge and mass were spread evenly in the atom and development of the nuclear model.

Question 4

Describe the process of thermionic emission and how it is used in a linar accelerator (linac).

Answer 4

Thermionic emission: electrons are emitted when metal is heated
In a linac, alternating electric fields accelerate electrons in a straight path. Particles gain energy as they pass through successive accelerating electrodes.

Question 5

Explain how a cyclotron uses electric and magnetic fields to accelerate charged particles.

Answer 5

Uses a perpendicular electric and magnetic fields. Magnetic field causes circular motion; electric field accelerates particles between dees. As velocity increases, radius of motion increases, forming a spiral path.

Question 6

Explain why high energy collisions are required in modern particle physics experiments.

Answer 6

Small wavelengths (from 𝜆=ℎ⁄𝑝) are needed to probe small structures. ?
High energy allows creation of new particles via ∆𝐸=∆𝑚𝑐^2.
Helps study fundamental forces and quark structures of nucleons.

Question 7

Compare the quark structure of protons, neutrons and pions.

Answer 7

Proton: uud; Neutrons: udd -> barons = 3 quarks
Pions: ūd or 𝑑̅u -> mesons = quark-antiquark pairs, 2 quarks

Question 8

A moving muon has a measured lifetime longer than its rest lifetime. Explain why this happens and how it supports Einstein’s theory of relativity.

Answer 8

Time dilation in special relativity, moving objects experience time slower. Muons experience time more slowly due to high velocity. Observers on Earth measure a longer lifetime of cosmic muon than muons at rest.

Question 9

Compare the nuclear model with the earlier plum pudding model and explain why the plum pudding model was replaced. (4)

Answer 9

Plum pudding: electrons embedded in a positively charged cloud
Nuclear model: electrons orbit a dense central nucleus, most of the atom is empty space
Plum pudding model couldn’t explain why some alpha particles deflected at large angles.
Nuclear model provided a reason for deflection due to a small, dense, positively charged nucleus.

Question 10

Explain how increasing the particle’s velocity would affect its radius and how this principle is applied in a cyclotron (4).

Answer 10

Increasing v increases momentum p, so r = p/Bq increases
This means the particle follows a larger circular path
In a cyclotron, alternating electric fields increase particle energy, accelerating the particles, causing larger radius paths/spirals
Magnetic field keeps particles in circular motion