Chapter 24 - Particle Physics

Question 1

What were the initial ideas about the atom up until Rutherford

Answer 1

• 2000 years ago Democritus came up with the idea of tiny particles
• in 1805 John Dalton came up with the billiard ball model
• In 1897 J.J. Thompson came up with the plum pudding model

Question 2

explain what the plum pudding model was

Answer 2

it models the atom as:

• a sea of uniform positive charge the ‘dough’
• negative electrons ‘plums’ distributed throughout the dough

Question 3

explain the setup and practical method of the alpha particle scattering experiment

Answer 3

you have

• an evacuated metal container
• an alpha source (he used radium, now americium)
• a piece of gold foil
• a fluorescent screen attached to a microscope that can be moved round at an angle theta

method:

• set up apparatus
• change theta and observe the numbers of alpha particles hitting the fluorescent screen

Question 4

what were the observations of the alpha particle scattering experiment and what did they show

Answer 4

• most of the alpha particles passed straight through the foil, so most of the atom is empty space
• about 1 in 2000 was scattered and about 1 in 10,000 was deflected greater than 90 degrees, so there is a SMALL, POSITIVE nucleus at the centre of the atom

Question 5

how did Rutherford predict the size of the nucleus and what did he predict it as

Answer 5

he took the kinetic energy of the alpha particle and set it equal to the electric potential energy as it approaches the nucleus, the distance it reaches gives and UPPER BOUND on the radius of the nucleus

he found it to be 10^-14m

Question 6

what is the true radius of the nucleus and radius of the atom

Answer 6

radius of nucleus is approx 10^-15m

radius of atom is approx 10^-10m

Question 7

what does the nucleus contain, what are their masses

Answer 7

• positive protons, charge e
• uncharged neutrons
• both protons and neutrons have a mass of approximately u

Question 8

what is a nucleon

Answer 8

• any particle contained within the nucleus

protons and neutrons

Question 9

what is an isotope of an element, what are the nucleon number and atomic number

Answer 9

• nucleon number = A = number of protons and neutrons combined
• atomic number = Z = number of protons

“Isotopes are nuclei of the same element that have the same atomic number but different nucleon numbers”

Question 10

what is an atomic mass unit

Answer 10

“one atomic mass unit, 1u, is equal to 1/12th of the mass of a nucleus of carbon 12”

approximately equal to 1.66x10^-27

Question 11

what is the equation for the radius of a nucleus

Answer 11

R = ro(A^1/3)

• the radius of a nucleus only depends on the nucleon number of that atom
• it’s in the order of 10^-15
• ro = 1.2fm

Question 12

what is the density of a nucleus

Answer 12

always 10^17

around 2.3x10^17

Question 13

what is the strong nuclear force and why is it needed

Answer 13

• within a nucleus, the protons have a large repulsive electrostatic force as they have the same charge
• this is around 230N
• the gravitational force between them isn’t sufficient to overcome this so there is a strong nuclear force holding them together
• it only acts over a few femtometres and is repulsive below 0.5fm

Question 14

what is an antiparticle in comparison to a standard particle

Answer 14

• antiparticles have the exact same mass and physical properties as standard particle but the opposite charge

Question 15

what happens when particles and antiparticles collide

Answer 15

• they annihilate each other and their combined mass is converted to a pair of very high energy photons according to E = MC^2
• the energy/frequency of these photons is equal
• they travel in opposite directions

Question 16

what are the strong nuclear force’s properties

Answer 16

• it acts on nucleons
• Relative strength = 1
• range is around 10^-15m

Question 17

what are the weak nuclear force’s properties

Answer 17

• responsible for beta decay
• Relative strength = 10^-6
• acts over a range of 10^-18m

Question 18

what are the properties of EM as a force

Answer 18

• acts on static and moving charged particles
• relative strength = 10^-3
• infinite range

Question 19

what are the properties of gravity as a force

Answer 19

• acts on anything with a mass, only attractive
• relative strength of around 10^-40
• acts over an infinite range

Question 20

what are hadrons, which forces are they most affected by, give some examples of hadrons

Answer 20

• Hadrons are non-fundamental particles made from quarks
• affected by the strong nuclear force
• the weak nuclear force is responsible for their decay
• if charged, they are affected by EM
• e.g. protons, neutrons

Question 21

what are leptons, which forces are they affected by, give some examples of leptons

Answer 21

• leptons are fundamental particles which are not affected by the strong nuclear force
• they are affected by EM if they are charged
• examples are the electron and muon

Question 22

name the six quarks and antiquarks

Answer 22

up, down, strange, charm, top, bottom

and anti-up etc.

Question 23

what are the important features of the up quark

Answer 23

up quark
symbol u
charge +2e/3

Question 24

what are the important properties of the down quark

Answer 24

down quark
symbol d
charge = -e/3

Question 25

what are the important properties of the strange quark

Answer 25

strange quark
symbol s
charge -e/3

Question 26

what are the important properties of the anti-up quark

Answer 26

anti-up quark
symbol u(bar)
charge = -2e/3

Question 27

what are the important properties of the anti-down quark

Answer 27

anti-down quark
symbol d(bar)
charge = e/3

Question 28

what are the important properties of the anti-strange quark

Answer 28

anti-strange quark
symbol s(bar)
charge = e/3

Question 29

what is the quark makeup of a proton

Answer 29

2 up, 1 down

uud

Question 30

what is the quark makeup of a neutron

Answer 30

2 down, 1 up

udd

Question 31

what is a baryon

Answer 31

any hadron made up of 3 quarks

all eventually decay into protons

Question 32

what is a meson

Answer 32

any particle made of a quark and anti-quark pair

they do decay but not into protons

Question 33

what is the neutrino, why is it needed and which one do we need to know about

Answer 33

• fundamental particles, leptons
• no charge, very little mass (1/10^6 of an electron)
• needed to explain beta decay
• only interested in the electron neutrino

Question 34

what does beta decay involve and which force causes it

Answer 34

• involves protons and neutrons

- caused by the weak nuclear force

Question 35

what is the equation for beta minus decay

Answer 35

a neutron decays into a proton, electron and antineutrino
neutron —> proton + e- + antineutrino
1,0,n —> 1,1,p + 0,-1,e- + ve(bar)

Question 36

what is the equation for beta + decay

Answer 36

a proton decays into a neutron, positron and neutrino
proton —> neutron + positron + neutrino
1,1,p —> 1,0n + 0,1e+ + v

Question 37

what occurs in the quark transformation of beta minus decay

Answer 37

udd —> uud + e- + v(bar)

a down quark forms an up quark, an electron and an antineutrino

d —> u + e- + v(bar)

Question 38

what occurs in the quark transformation of beta+ decay

Answer 38

uud —> udd + e+ + v

up quark forms a down quark, a positron and a neutrino

Question 39

which physical quantities balance on both sides of the decay equations

Answer 39

• nucleon number
• charge
• proton number

Question 40

what to always include when writing electrons/positrons

Answer 40

atomic and mass number