Particle physics DONE

Question 1

Explain why the tubes gradually increase in length in the first part of the linac accelerator.

Answer 1

The speed of the electrons increases along the accelerator
Thus the tubes have to increase in length so that the electrons spend the same time in each one
This is needed because the alternating voltage has a fixed time period so tubes switch polarity at fixed time intervals

Question 2

What is connected across the terminals of a linac?

Answer 2

A high frequency

Alternating voltage

Question 3

Why do electrons travel with constant velocity in the tubes of a linac?

Answer 3

Because there is no electric field in the tubes

So the electrons experience no force in the tubes and thus don’t accelerate

Question 4

Why are the tubes the same length at the end of a linac?

Answer 4

At the end of the accelerator the electrons approach the speed of light and their mass increases
The tubes are the same length because the speed of the electrons have become a maximum/ constant

Question 5

State the relationship between time period of particle’s circular motion and radius of circle. Include conditions on this relationship.

Answer 5

T is independent of r

Assuming m is constant/ relativistic effects don’t apply

Question 6

State the purpose of the electric field in the cyclotron.

Answer 6

The electric field between the dees accelerates the particles

Question 7

State the purpose of the magnetic field in the cyclotron

Answer 7

The magnetic field keeps the particles moving in a circular path

Question 8

Why is the p.d. across the dees in the cyclotron alternating?

Answer 8

Protons need to be accelerated between the dees

Every half rotation, polarity of dees needs to change to attract the proton from one side to another

Question 9

With reference to the magnetic field and the alternating potential difference, explain how the cyclotron produces a beam of high speed particles.

Answer 9

Electric field accelerates particles between dees
Polarity of dees switches every half cycle
Magnetic field provides a force perpendicular to the path of the particles
This is the centripetal force that causes the particles to move in a circular motion whilst in the dees

Question 10

How does the kinetic energy of the protons increase as they follow a circular path in the cyclotron?

Answer 10

Electric field between the dees causes protons to accelerate
Dee facing proton is always negative/ p.d. reversed each time proton reaches gap
KE increases each time proton crosses the gap

Question 11

What are 2 differences between how the linac and cyclotron work?

Answer 11

Cyclotron uses magnetic field, linac doesn’t
They manage increasing speed differently: linac increases length of drift tubes, cyclotron increases radius of particles’ path

Question 12

When energy is converted into mass you have to make equal amounts of matter and antimatter. What is this process called?

Answer 12

Pair production

Question 13

If the photon has energy above the minimum required for pair production, states what happens to the excess energy.

Answer 13

Converted into kinetic energy of particle and antiparticle

Question 14

State the condition for pair production to occur for a particular type of particle and antiparticle..

Answer 14

Photon must have enough energy to produce the total mass of the particle and antiparticle

E= 2mc^2

Question 15

Where does pair production tend to happen to help conserve momentum?

Answer 15

Near a nucleus

Question 16

Describe what is likely to happen to an antiparticle shortly after its creation.

Answer 16

Likely to meet their particle and they will annihilate

Releasing two photons

Question 17

When all the mass of the particle and antiparticle gets converted into energy, a pair of photons are formed. State what this process is called.

Answer 17

Annihilation

Question 18

Describe the conclusions Rutherford reached about the structure of gold atoms as a result of the alpha particle scattering experiments.

Answer 18

Atom mainly empty space
Nucleus is charged
Mass concentrated in centre of atom

Question 19

Describe what happens when a charged particle passes through a substance.

Answer 19

It causes ionisation
Electrons are knocked out of atoms
The particle leaves a trail of ions as it goes

Question 20

Cloud chambers and bubble chambers show up charged particles. Describe what can be inferred about short and heavy tracks and a particle which may cause this track.

Answer 20

Short and heavy means the particle causes lots of ionisation

Alpha particle

Question 21

Cloud chambers and bubble chambers show up charged particles. Describe what can be inferred about faint and long tracks and a particle that may cause this track.

Answer 21

Faint and long shows the particle causes less ionisation

Beta particle

Question 22

State why a photon leaves no track in a bubble chamber.

Answer 22

Because a photon causes no ionisation

Question 23

Positive and negative particles curve opposite ways. State how we distinguish which is which.

Answer 23

Fleming’s left hand rule

Question 24

Explain why particles follow a curved track instead of a circular track.

Answer 24

Interactions with the detector decreases the kinetic energy and hence momentum of the particles

Question 25

In a bubble chamber what does a straight line show?

Answer 25

Incoming beam of charged particles that pass through chamber without interacting

Question 26

A little spiral comes off one of the straight tracks in the bubble chamber. State what this shows and what can be deduced from this.

Answer 26

Little spiral shows a knock-on electron
Tells you which way the particles are going
And which way negative particles curve

Question 27

Neutral particles don’t make tracks. State the 2 events when you can see them.

Answer 27

When they decay

When they interact

Question 28

What can be deduced from a V shape starting in the middle of nowhere in a bubble chamber/ cloud chamber?

Answer 28

A neutral particle has decayed

Into 2 oppositely charged particles

Question 29

State why the tracks are sometimes almost straight in the V shape instead of curved.

Answer 29

Because the particles have such a large amount of momentum

Radius of curvature is so large

Question 30

Explain why some neutral particles seem to survive much longer than normal/ what we would expect. Name this effect.

Answer 30

Because the particles are travelling close to the speed of light
They experience relativistic time dilation

Question 31

State the structure of a baryon.

Answer 31

3 quarks

or 3 antiquarks

Question 32

State the structure of a meson.

Answer 32

A quark and an antiquark

Question 33

All baryons, except protons, are unstable. State what happens to all baryons except protons.

Answer 33

They decay to a proton

Question 34

State the main 2 groups of particles: the group of fundamental particles and the group with particles that are made up of quarks.

Answer 34

Hadrons

Leptons

Question 35

Name the 6 types of quarks.

Answer 35

Top
Bottom
Up
Down
Charm
Strange

Question 36

State what is meant by quark confinement.

Answer 36

Its not possible to isolate a quark.

Question 37

Conservation laws tell us if a reaction/ interaction is possible. State the 5 conservation laws.

Answer 37

Conservation of momentum
Conservation of mass- energy
Conservation of charge
Conservation of baryon number
Conservation of lepton number

Question 38

Explain what a charge of +2/3 means.

Answer 38

2/3 the charge of a proton

Question 39

State a similarity between a particle and its antiparticle.

Answer 39

Same mass/ magnitude of charge

Question 40

State one difference between a particle and its antiparticle.

Answer 40

Opposite charge

Question 41

State what is meant by relativistic.

Answer 41

Travelling close to the speed of light

Question 42

State why it is necessary to use very high energies in experiments where particles collide and produce other particles.

Answer 42

High energy needed to create antimatter/ overcome large repulsive forces/ break particles into their constituents

Question 43

State the strangeness value of the strange quark.

Answer 43

-1

Question 44

State what is meant by a muon or tau particle, what happens to them eventually and why and what comes with them when they are produced.

Answer 44

Heavy electrons
Eventually decay to ordinary electrons because they are unstable
Come with individual neutrinos when they are produced