Particles and Radiation

Question 1

Describe the structure of an atom

Answer 1

• A positively charged nucleus composed of protons and neutrons.
• Electrons that surround the nucleus.

Question 2

Define the term “isotope”

Answer 2

Isotopes are atoms with the same number of protons and a different number of neutrons.

Question 3

Define the term “specific charge”

Answer 3

Specific charge is defined as charge per unit mass

Question 4

What is the strong nuclear force?

Answer 4

The strong nuclear force is the force that keeps the protons and neutrons together in the nucleus. It overcomes the electrostatic force of repulsion between the protons in the nucleus.

Question 5

Describe how the strong force varies with separation between two protons or neutrons

Answer 5

• At separations less than 0.5 fm, it’s a repulsive force that prevents nucleons being pushed into one another.
• It’s an attractive force between 0.5 fm to about 3-4 fm.
• The range is no more than 3-4 fm.

Question 6

Describe what happens to an unstable nucleus during alpha emission

Answer 6

The nucleon number of the unstable nucleus decreases by 4, and the atomic number decreases by 2. The product nucleus is a different element. An alpha particle is also emitted, which is made up of 2 protons and 2 neutrons.

Question 7

Describe what happens to an unstable nucleus during beta minus emission

Answer 7

The nucleon number stays the same and the atomic number increases by one. This is because a neutron changes into a proton. A beta-minus particle (electron) is emitted alongside an anti-neutrino. The product nucleus is a different element.

Question 8

Describe what happens to an unstable nucleus during gamma radiation emission

Answer 8

The nucleus remains the same. Gamma radiation is emitted - has no charge or mass. It’s emitted from a nucleus with too much energy.

Question 9

When are electromagnetic waves emitted by charged particles?

Answer 9

Charged particles lose energy and emit an EM wave when:

• a fast moving electron is stopped, or slows down or changes direction.
• an electron in a shell of an atom moves to a different shell of lower energy.

Question 10

How would you estimate how many photons a light source emits every second.

Answer 10

Laser beams consist of photons of the same frequency, where each photon has energy E=hf. The power of the beam is the energy transferred per second by the photons, where p=nhf.

Question 11

What happens during positron emission?

Answer 11

This takes place when a proton changes into a neutron in an unstable nucleus with too many protons. The nucleon number stays the same, but the atomic number decreases by 1. A positron is emitted, alongside a neutrino.

Question 12

Describe the process of annihilation

Answer 12

Occurs when a particle and corresponding antiparticle meet, and their mass is converted into radiation energy. Two photon are produced, each with energy hf.
The minimum energy of each photon produced hf=E, where E is the rest energy of the particle.

Question 13

Describe the process of pair production

Answer 13

A photon creates a particle and corresponding antiparticle. The minimum energy of this photon is hf=2E, where E is the rest energy of the particle.

Question 14

Describe the weak nuclear force

Answer 14

A force that is responsible for beta decay - causes neutrons to change into protons, and vice versa.

Question 15

Describe the exchange particle for the weak nuclear interaction - The W boson

Answer 15

• Has a non-zero rest mass
• Has a very short range of no more than about 0.001 fm
• Can be positively or negatively charged

Question 16

What happens in beta minus decay using idea of Feynman diagrams?

Answer 16

The neutron changes into a proton. The down quark changes into an up quark. The W- boson decays into a beta minus particle and an anti-neutrino.

Question 17

What happens in beta plus decay using the idea of Feynman diagrams?

Answer 17

The proton changes into a neutron. The up quark changes into a down quark. The W+ boson decays into a beta plus particle and a neutrino.

Question 18

What is electron capture?

Answer 18

Electron capture is when a proton-rich nucleus captures an inner shell electron, causing a proton to change into a neutron via the weak interaction. An electron neutrino is emitted by the nucleus. An X-ray photon is subsequently emitted when the inner shell vacancy is filled. The W+ boson is the exchange particle.

Question 19

Describe the four fundamental forces

Answer 19

• The force of gravitational attraction between any 2 objects, due to their mass.
• The electromagnetic force acts between objects due to their electrical charge.
• The strong nuclear force holds protons and neutrons together in stable nuclei.
• The weak nuclear force causes beta decay.

Question 20

How can we find new particles?

Answer 20

By using cloud chambers and other detectors, new types of short-lived particles and antiparticles can be discovered.

Question 21

State what kaons, pions and muons decay into.

Answer 21

• A kaon decays into pions, or a muon and an anti-neutrino, or an antimuon and a neutrino.
• A charged pion can decay into a muon and an anti-neutrino, or an antimuon and a neutrino.
• Neutral pions decay into high energy photons.
• A muon decays into an electron and an anti neutrino.

Question 22

What is the difference between hadrons and leptons?

Answer 22

Leptons interact through the weak interaction, the gravitational interaction, and through the electromagnetic interaction, if charged. These include protons, neutrons, pions and kaons,

Hadrons can interact through all four fundamental interactions. They interact through the strong interaction and through the electromagnetic interaction.

Question 23

Describe the different classifications of particles

Answer 23

Matter can be split into hadrons and leptons, where leptons are fundamental particles and hadrons are composed of quarks. Hadrons can be further split into baryons and mesons.
Baryons are made up of 3 quarks, while mesons are made up of a quark and an anti quark. Baryons decay into protons directly or indirectly, while mesons don’t have protons as decay products.

Question 24

State the conservation rules for particle interactions

Answer 24

• Energy
• Momentum
• Lepton number
• Baryon number
• Charge
• Strangeness (not necessary in the weak interaction)

Question 25

What is the photoelectric effect?

Answer 25

The emission of photo electrons from a metal surface when the surface is illuminated by light of frequency equal to or greater than the threshold frequency.

Question 26

Describe an experiment that shows the photoelectric effect

Answer 26

- UV radiation from a UV lamp is directed to the surface of a zinc plate, placed on the cap of a gold leaf electroscope. - When it's charged, the gold leaf rises - repelled from the metal stem since they have the same charge. - If the electroscope is negatively charged, the leaf rises and stays in position. - If UV light is directed at the plate, the gold leaf gradually falls, due to conduction electrons leaving the zinc surface. - If the electroscope is positively charged, the leaf rises and stays in position, regardless of UV light.

Question 27

What observations of the photoelectric effect demonstrated the particle theory of light?

Answer 27

- Photo electrons aren't emitted if the frequency of the incident EM radiation is below the threshold frequency. The wavelength must be less than the maximum value, equal to speed of light / threshold frequency. - Number of photo electrons emitted per second is proportional to the intensity of the incident radiation, provided the frequency is greater than the threshold. - If the frequency is less than the threshold, no electrons are emitted from the surface. - Photoelectric emission occurs without delay as soon as incident radiation is directed at the surface, provided it is greater than the threshold frequency.

Question 28

Why can't wave theory explain the photoelectric effect?

Answer 28

- Wave theory can't explain the existence of a threshold frequency or why it occurs without delay. - Wave theory suggests that each conduction electron at the metal surface should gain some energy from the the incoming waves, regardless of how many arrive per second. This doesn't happen.

Question 29

How did Einstein explain the photoelectric effect?

Answer 29

- When light is incident on a metal surface, an electron at the surface absorbs a single photon, and gains energy equal to hf. - An electron can leave the surface if the energy gained from a single photon exceeds the work function of the metal. - Excess energy gained by the photo electron becomes KE.

Question 30

Define the term "work function"

Answer 30

This is the minimum amount of energy needed by an electron to escape from the metal surface.

Question 31

What is stopping potential? Why is it significant?

Answer 31

The stopping potential is the minimum potential needed to stop photoelectric emission. Photo electrons can be attracted back to the plate by giving it a sufficient positive charge. At this potential, the max KE of the emitted electron is reduced to 0 because each electron must do extra work equal to eVs to leave the metal surface.

Question 32

Define the term "ionisation"

Answer 32

Ionisation is where an atom gains or loses electrons, to form an ion.

Question 33

Define the term "excitation"

Answer 33

Excitation is a process in which an atom absorbs energy without becoming ionised, as a result of an electron inside an atom moving from an inner shell to an outer shell.

Question 34

Give examples of ionisation.

Answer 34

- Alpha, beta and gamma radiation create ions when they pass through substances and collide with the atoms of the substance. - Electrons passing through a fluorescent tube creates ions when they collide with the atoms of the gas/vapour in the tube.

Question 35

Define the term "electron volt"

Answer 35

The electron volt is a unit of energy equal to the work done when an electron is moved through a pd of 1 V. For a charge q moved through a pd V, the work done = qV.

Question 36

What happens if the colliding electron loses all its kinetic energy during excitation?

Answer 36

If a colliding electron loses all its kinetic energy when it causes excitation, the current due to the flow of electrons through the gas is reduced.

Question 37

What happens if the colliding electron doesn't have enough kinetic energy to cause excitation?

Answer 37

If a colliding electron doesn't have enough kinetic energy to cause excitation, it is deflected by the atom, with no overall loss of of kinetic energy.

Question 38

Why is excitation energy always less than ionisation energy?

Answer 38

The excitation energy is always less than the ionisation energy of the atom, because the atomic electron is not removed completely from the atom when excitation occurs.

Question 39

Define the term "ground state"

Answer 39

The lowest energy state of an atom.

Question 40

Describe the process of de-excitation

Answer 40

- The electron configuration in an excited atom is unstable, because an electron that moves to an outer shell leaves a vacancy in the shell it moves from. - That vacancy is later filled by an electron from an outer shell transferring to it. When this happens, the electron emits of photon. - The atom moves to a lower energy level.

Question 41

Describe the process of excitation using photons

Answer 41

- An electron in an atom can absorb a photon and move to an outer shell where a vacancy exists - ONLY IF the energy of the photon is EXACTLY EQUAL to the gain in the electron's energy. - The photon energy must be EXACTLY EQUAL to the difference between the final and initial energy levels. - If the energy is smaller or greater than that difference, it will not be absorbed by the electron.

Question 42

What is a florescent lamp?

Answer 42

The florescent tube is a glass tube with a fluorescent coating on its inner surface. The tube contains mercury vapour at low pressure.

Question 43

Explain how a fluorescent lamp works

Answer 43

When the tube is on, it emits visible light because: - Ionisation and excitation of the mercury atoms occur as they collide with each other and with electrons in the tube. - The mercury atoms emits UV photons, as well as visible photons and photons of much less energy, when they de-excite. - The UV photons are absorbed by the atoms of the fluorescent coating, causing excitation of atoms. - The coating atoms de-excite in steps - emitting visible photons.

Question 44

Explain why atoms emit characteristic line spectra

Answer 44

- The wavelengths of the lines of a line spectrum of an element are characteristic of the atoms of that element. - No other element produces the same pattern. - This is because the energy levels of each type of atom are unique to that atom. - The photons emitted are characteristic of the atom.

Question 45

What observations determine the wave-like nature of light?

Answer 45

Diffraction of light - light spreads out after passing through a gap.

Question 46

What observations determine the particle-like nature of light?

Answer 46

The photoelectric effect - electron can escape metal surface if the energy gained from the photon exceeds the work function of the metal.

Question 47

What observations determine the particle-like nature of matter?

Answer 47

Electrons in a beam can be deflected by a magnetic field.

Question 48

What observations determine the wave-like nature of matter?

Answer 48

The de Broglie wavelength of a particle, which is related to the momentum of the particle. - Electron diffraction

Question 49

Describe how electron diffraction works

Answer 49

- Narrow electron beam in a vacuum tube is directed at metal foil. - Metal composed of many tiny crystalline regions, where each region consists of positive ions arranged in fixed positions in rows in a regular pattern. - The rows cause the electrons to be diffracted. - Electrons pass through the metal foil and are diffracted in certain directions only - forms a pattern of rings on a florescent screen. - Each ring is due to electrons diffracted by the same amount from regions of different orientations, at the same angle to the beam.

Question 50

How are the beam of electrons produced?

Answer 50

The electron beam is produced by attracting electrons from a heated filament wire to a positively charged metal plate, which has a small hole at its centre. Electrons that pass through the hole form a beam.

Question 51

How can you change the speed of the electrons in the electron beam? How does this affect the observations?

Answer 51

- The speed of the electrons can be increased by increasing the pd between the filament and the metal plate. - Makes diffraction rings smaller because increased speed makes the de Broglie wavelength smaller. - Less diffraction occurs and the rings become smaller.