Chapter 17

Question 1

What was Rutherford’s experiment?

Answer 1

Stream of alpha particles from a radioactive source was fired at a thin gold foil. Geiger and Marsden recorded number of alpha particles scattered at different angles, detected by a zinc sulphide screen. They were occasionally scattered at angles greater than 90*, meaning they must have collided with something bigger than themselves

Question 2

What were the conclusions from Rutherford’s experiment?

Answer 2

1 - Most of the fast charged alpha particles went straight through the gold foil, meaning the atom is mostly empty space
2 - Some of the alpha particles were deflected back through significant angles, so the centre of the atom must be tiny but contain a lot of mass
3 - The alpha particles were repelled, so the nucleus must have a positive charge
4 - Atoms are neutral overall, so the electrons must be on the outside of the atom, separating one atom from the next

Question 3

How do you calculate the distance of closest approach of an alpha particle to the nucleus?

Answer 3

At turning point, kinetic energy = electrical potential energy = Q(gold)q(alpha)/(4 pi epsilon(0) r). Now rearrange to find R

Question 4

What are hadrons?

Answer 4

Baryons and masons. Made up of quarks.

Question 5

What is the strong force of interaction?

Answer 5

Felt by hadrons, it holds the nucleons together (prevents protons repelling)

Question 6

What are baryons?

Answer 6

Neutrons and protons are examples of baryons. They are made up of 3 quarks. The proton is the only stable baryon.

Question 7

What is the baryon number?

Answer 7

The number of baryons in a reaction. Protons and neutrons have a baryon number of +1

Question 8

What are leptons?

Answer 8

Fundamental particles that don’t feel the strong interaction. They interact through the weak interaction, gravity and electromagnetic force (if they’re charged)

Question 9

What are the 3 different types of leptons?

Answer 9

The electron, muon and tau. You also get the neutrino version of these, with a charge 0, as opposed to -1. They each have a lepton number of +1, though you count them separately for each type of particle.

Question 10

How do neutrons decay?

Answer 10

n -> p + e(-) + antineutrino. This is an example of beta decay

Question 11

What are antiparticles?

Answer 11

Particles with the same mass but a different charge. Represented with the same symbol as the normal particle with a bar over the top. They also have opposite baryon and lepton numbers

Question 12

How can energy be converted to mass?

Answer 12

E=mc^2. And you have to make equal amounts of matter and antimatter

Question 13

How does creation work?

Answer 13

If a gamma ray photon has enough energy, an electron-positron pair may form (these more often than other particle pairs, as they have less mass). Often occurs near nuclei, as this helps to conserve momentum.

Question 14

How does annihilation work?

Answer 14

When a particle meets its antiparticle, they ‘disappear’ and release a large amount of energy as 2 gamma rays, equivalent to the mass of the particles.

Question 15

What is PET?

Answer 15

Positron emission tomography. Way of looking at tumours: inject patient with an isotope that releases beta plus decay, look at where it settles by detecting the gamma rays from the annihilation of positrons and electrons

Question 16

What are quarks?

Answer 16

The particles that make up hadrons. They come in up, down, strange, top, bottom and charm. You also get antiquarks. They also have a new kind of charge called ‘colour’ that comes in 3 forms: red, green and blue.

Question 17

What are the properties of up, down and strange quarks?

Answer 17

U: charge: +2/3 baryon number: +1/3 strangeness: 0
d: charge: -1/3 baryon number: +1/3 strangeness: 0
s: charge: -1/3 baryon number: +1/3 strangeness: -1
Their antiparticle have the opposite numbers

Question 18

How are protons and neutrons made up?

Answer 18

p: uud
n: ddu

Question 19

What is quark confinement?

Answer 19

The fact that you can’t make a free quark. The energy just goes into making more quarks and antiquarks

Question 20

What are gauge bosons?

Answer 20

Exchange particles that are a way of explaining attraction and repulsion. They are virtual particles that only last for a very short time

Question 21

What are the 4 fundamental forces and the gauge bosoms associated?

Answer 21

Strong: gluon (only affects hadrons)
Electromagnetic: photon (charged particles only)
Weak: W+, W-, Z0
Gravity: gravitons (?)

Question 22

What is weird about the strong force?

Answer 22

The strength of it increases with distance: the gluon field increases in energy. Eventually, the energy is creates a quark, antiquark pair

Question 23

How do linear particle accelerators work?

Answer 23

Long straight tube containing a series of electrodes; alternating current is applied across them, giving a continuously changing charge. The current is timed, so the particle is attracted to the next electrode and repelled from the previous one, increasing it in speed.

Question 24

What is a cyclotron?

Answer 24

A circular particle accelerator with 2 semicircular electrodes to accelerate charged particles across a gap. Alternating pd and magnetic field is used to keep the particle accelerating and moving in a circular pathway. Particle spirals outwards

Question 25

What is a synchrotron?

Answer 25

A particle accelerator in a circle that produces much higher energies that cyclotrons and linear accelerators. Beam is accelerated with electrodes; electromagnets keep particles in circular path. Particles can reach several TeV

Question 26

Why can’t anything with mass reach the speed of light?

Answer 26

Its mass increases as you get closer to the speed of light as its kinetic energy increases and E=mc^2

Question 27

What is the way of calculating the gamma factor based on energies?

Answer 27

Gamma = E(total)/E(rest) where E(rest)=mc^2. At low speeds, E(total) is very close to E(rest)

Question 28

How are electrons in quantum shells modelled?

Answer 28

Standing waves. Number of nodes corresponds to increasing energy required to occupy that level.

Question 29

How do electrons move between energy levels?

Answer 29

By absorbing or emitting a photon of exactly the right energy level. Leads to lines in absorption/emission spectra

Question 30

What is important to remember about the energy of electrons?

Answer 30

They are negative because its a bound system: how much energy is needed to escape?

Question 31

What is the Pauli exclusion principle?

Answer 31

The idea that no 2 fermions (electrons, protons and neutrons) can exist in the same quantum state at the same time

Question 32

What is the evidence for electron energy levels?

Answer 32

Absorption and emission spectra: lines taken out/added where photons have been absorbed/emitted at specific frequencies (corresponding to energy)

Question 33

How do you calculate the energy levels in a hydrogen atom?

Answer 33

E=-13.6eV/n^2

Question 34

What is conserved in particle interactions?

Answer 34

Total energy, momentum, charge, lepton and baryon numbers

Question 35

What are scattering experiments?

Answer 35

Experiments that reveal the structure of atoms, nuclei and the like. The smaller the scale, the greater the energy needed. At high energy, many new particles are created

Question 36

What is the de Broglie wavelength equation?

Answer 36

lambda=h/p where h is plancks’ constant and p is momentum

Question 37

What is the weak interaction?

Answer 37

The decay of protons into neutrons and vica versa by emission of one of W(0), W(+) or W(-)
n -> p by emission of W(-) (then p -> e(-) + antineutrino)
p -> n by emission of W(+) (then n -> positron + neutrino)