Physics topic 8: Nuclear and particle physics

What is nucleon number?

The sum of the number of protons and neutrons in the nucleus.

What is atomic number?

The total number of protons in the nucleus.

Outline Rutherford’s alpha scattering experiment.

High speed alpha particles were fired at a very thin sheet of gold foil. The deflections of the particles were measured and conclusions were drawn.

What was observed in the alpha scattering experiment?

• Most of the alpha particles passed straight through the gold atoms

• Some of them were deflected

• A few of them were deflected backwards

What is thermionic emission?

Thermionic emission is the release of electrons due to heating.

Explain why electrons are released from a heated filament.

As the filament heats up, free electrons inside the metal gain kinetic energy. When the surface electrons gain sufficient energy, they are released from the surface.

What will happen to a beam of electrons if it is passed through a potential difference?

The beam of electrons will be accelerated since work is done by the potential difference.

How do you calculate the energy transferred to an electron, when it is accelerated across a potential difference?

Energy = Charge x Potential Difference

Energy = eV

What happens when a beam of electrons is directed into a magnetic field?

The electron beam will be deflected, since magnetic fields apply forces on moving charges.

What is the magnitude of the force experienced by a moving electron in a magnetic field?

Force = Magnetic Flux Density x Charge x Velocity

F=Bev

If the electrons are moving perpendicular to the field lines, which direction will the magnetic force act?

The force will act perpendicular to both the electron and field directions.

Describe the shape of the path of a beam of electrons passing through a magnetic field.

The beam will produce a circular path since the magnetic force always acts perpendicular to the electrons’ motion. This means it acts as a centripetal force and produces a circular path.

What is a cyclotron?

A cyclotron is a particle accelerator that uses magnetic fields to accelerate particles in circular paths. This allows higher speeds to be reached, without the limitation of the accelerator’s length.

Describe the basic composition of a cyclotron.

Cyclotrons consist of two D-shaped paths which are separated by a small gap. An alternating potential difference is applied across the gap.

How does a cyclotron work?

An electron beam is passed into the cyclotron, where it is deflected into a circular path by a perpendicular magnetic field. When the beam reaches the gap, it is accelerated by a potential difference. This increases the speed of the beam, causing the radius of the path to increase. This process repeats every half circle.

State the equation used to calculate the circular radius of an electron beam deflected in a magnetic field.

Radius = mv/BQ

What two equations must you combine to derive the radius equation?

1. Centripetal Force = mv²/r

2. Magnetic Force = BQv

Centripetal Force = Magnetic Force

State the mass-energy equation.

E=mc²

What is 1 MeV in joules?

(1.6×10^-19) x 10^6 = 1.6×10^-13 J

In the quark-lepton model, what are the four main categories of particles?

1. Baryons

2. Mesons

3. Leptons

4. Photons

Describe the quark composition of a baryon.

Baryons are made up of three quarks.

Describe the quark composition of a meson.

Mesons are made up of a quark and antiquark pair.

Which category of particles are classed as fundamental particles?

Leptons

Give two examples of leptons.

1. Electrons

2. Neutrinos

What category of particles do pions belong in?

Mesons.

Give two examples of baryons.

1. Protons

2. Neutrons

What did the symmetry of the quark-lepton model predict the existence of?

The top quark.

What is an antiparticle?

An antiparticle is one that has the same mass but opposite charge and conservation numbers to its corresponding particle.

What is the antiparticle of a proton?

An antiproton.

What is the antiparticle of an electron?

A positron.

Name four things that are always conserved in a particle interaction.

1. Mass/Energy

2. Baryon Number

3. Lepton Number

4. Charge

Describe the conservation of lepton number.

The lepton number for each specific type of lepton must be the same before and after an interaction.