Section 2: Particles and Radiation

Question 1

What is the specific charge of a particle?

Answer 1

Its charge-mass ratio, calculated by dividing the particle’s charge by its mass.

Question 2

Describe the strong nuclear force

Answer 2

A force that only acts on hadrons and counteracts the electrostatic force of repulsion between nucleons.

It is attractive up to 3fm and repulsive below 0.5fm

Question 3

When does alpha decay occur?

Answer 3

When a nucleus is too large - it has too many protons and neutrons.

Question 4

When does beta-minus decay occur?

Answer 4

When a nucleus has too many neutrons.

Question 5

Describe alpha decay

Answer 5

In alpha decay, the nucleus of an atom emits an alpha particle, which consists of 2 protons and 2 neutrons (4 nucleons total).

The results of the decay are an alpha particle, and an atom containing 2 fewer protons and 2 fewer neutrons in its nucleus.

Question 6

Describe beta-minus decay

Answer 6

A neutron inside the nucleus turns into a proton and an electron, and the electron is emitted from the nucleus alongside an electron antineutrino.

Question 7

Compare a particle with its antiparticle

Answer 7

They have the same rest energy and mass, but every other property (charge, baryon/lepton number etc) is opposite.

Question 8

Describe a photon

Answer 8

A packet that electromagnetic radiation travels in, which transfers energy and has no mass.

Question 9

Describe annihilation

Answer 9

When a particle and its antiparticle collide and as a result their masses are converted into energy.

This energy, along with the kinetic energy of both particles, is released in the form of two photons, which travel in opposite directions in order to conserve momentum.

Question 10

Explain a real world application of annihilation

Answer 10

PET scanners, which allow 3D images of the inside of the human body to be taken.

A positron-emitting radioisotope is introduced into the body, and the positrons immediately annihilate with the electrons already in the body, releasing gamma photons which can be easily detected.

Question 11

Describe pair production

Answer 11

When a photon is converted into an equal amount of matter and antimatter.

It is only possible if the energy of the photon is greater than the combined rest energy of the two particles, and any excess energy is converted into the kinetic energy of the particles.

Question 12

What is the exchange particle for the strong interaction?

Answer 12

Gluon

Question 13

What is the exchange particle for the weak interaction?

Answer 13

W boson

Question 14

What is the exchange particle for electromagnetic interactions?

Answer 14

Virtual photon

Question 15

Describe the structure of a baryon

Answer 15

3 quarks

Question 16

Describe the structure of a meson

Answer 16

A quark-antiquark pair

Question 17

What is the only stable baryon?

Answer 17

Proton

Question 18

Which two mesons do you need to know?

Answer 18

Pion and kaon.

Question 19

What are strange particles?

Answer 19

Particles that are produced by the strong interaction but decay via the weak interaction.

Question 20

What do muons decay into?

Answer 20

Electrons

Question 21

What do kaons decay into?

Answer 21

Pions

Question 22

What properties are conserved in every particle interaction?

Answer 22

Energy
Momentum
Charge
Baryon number
Lepton number

Question 23

When is strangeness conserved?

Answer 23

In the strong interaction - strange particles are produced in pairs in order for this to be true.

Question 24

When is strangeness not conserved?

Answer 24

In the weak interaction - the strangeness can change by 0, +1 or -1.

Question 25

Which interaction can change quark type?

Answer 25

Weak interaction.

Question 26

What is the photoelectric effect?

Answer 26

Where photoelectrons are emitted from the surface of a metal when light above a certain frequency - the metal's threshold frequency - is shone on it. The threshold frequency is different for different metals.

Question 27

Why can threshold frequency not be explained by the wave theory of light?

Answer 27

The wave theory suggests that any frequency of light should be able to cause the photoelectric effect as the absorbed energy would increase with every incoming wave.

Question 28

How does the particle theory of light explain the photoelectric effect?

Answer 28

EM waves travel in discrete packets called photons, which have an energy directly proportional to their frequency. Each electron can only absorb a single photon, and therefore each photon must have enough energy to allow an electron to escape. To meet the required energy, it must have a high enough frequency - the threshold frequency.

Question 29

What is the effect of increasing light intensity on the photoelectric effect?

Answer 29

As long as the frequency is above the threshold frequency, then increasing intensity increases the number of photoelectrons emitted per second, as there are more photons incident on a given area every second.

Question 30

What is the work function of a metal?

Answer 30

The minimum energy required for photoelectrons to be emitted from the surface of the metal (E = hf, where f is the threshold frequency).

Question 31

What is stopping potential?

Answer 31

The potential difference you would need to apply across the metal to stop the photoelectrons with the maximum kinetic energy. It allows you to calculate the maximum kinetic energy, as: Ek(max) = eV, where e is the charge of an electron and V is the stopping potential.

Question 32

Explain the presence of maximum kinetic energy in the photoelectric equation

Answer 32

The higher the frequency above the threshold frequency, the more energy that is transferred to each electron. Since surface electrons only need the threshold frequency to escape the metal, any additional energy on top of that is the kinetic energy of the escaped electron. The maximum kinetic energy will be the kinetic energy of the surface electrons, as they do not need to move through the metal to escape. Electrons deeper down will transfer some energy in moving out of the metal, and therefore have a lower kinetic energy when they are released.

Question 33

What is the difference between excitation and ionisation?

Answer 33

Excitation is when an electron moves up some number of energy levels, while ionisation is when an electron leaves the atom entirely.

Question 34

Describe how a fluorescent tube works

Answer 34

A high voltage is applied across a tube of mercury vapour. The voltage accelerates electrons through the tube, which collide with mercury atoms and cause electrons to become excited. When the electrons de-excite they release photons, which get absorbed by the fluorescent coating, and the electrons in the coating excite and de-excite, releasing photons of visible light.

Question 35

Define an electron volt

Answer 35

The energy gained by one electron when passing through a potential difference of 1 volt.

Question 36

Describe and explain a line spectrum

Answer 36

Passing the light from a fluorescent tube through a diffraction grating or prism yields a line spectrum, where each line represents a different wavelength of light emitted by the tube. Since there are discrete energy levels in an atom, there will be distinct lines on the spectrum corresponding to the photon energies emitted.

Question 37

Describe and explain a line absorption spectrum

Answer 37

Passing white light through a cooled gas yields a continuous spectrum of all wavelengths of light, but with black lines at certain wavelengths. The lines represents the possible differences in energy levels as the atoms in the gas can only absorb photons of an energy equal to the exact difference between two energy levels.

Question 38

How does the momentum of a particle affect the amount it diffracts by?

Answer 38

Increasing the momentum decreases the wavelength, thereby decreasing the amount of diffraction. Decreasing the momentum increases the wavelength, thereby increasing the amount of diffraction.