Particle Physics

Question 1

Work Function

Answer 1

Minimum energy for electron to leave metal surface

Question 2

Ionisation E

Answer 2

E needed to remove electron from ground state of an atom

Question 3

Threshold Frequency

Answer 3

Frequency above which work function is achieved - needed for electron emmision

Question 4

What is conserved in an interaction?

Answer 4

Charge 
Lepton #
Baryon #
Strangeness in S.F
Momentum

Question 5

What isn’t conserved in particle interaction

Answer 5

Strangeness out of Strong F

Question 6

Neutral Pion Quark structure

Answer 6

U, Anti U or D, anti D etc.

Question 7

Pion +

Answer 7

U, Anti D

Question 8

Pion -

Answer 8

Anti U, D

Question 9

K Neutral

Answer 9

S, anti D or anti S, D

Question 10

K +

Answer 10

U, anti S

Question 11

K-

Answer 11

S, anti U

Question 12

What does a Kaon decay into

Answer 12

Pions, Muons, Neutrinos - consider #s

Question 13

What does a charged pion decay into?

Answer 13

Muon and Neutrino

Question 14

What does a Muon decay into

Answer 14

Electron + neutrino

Question 15

What does a neutral pion decay into

Answer 15

High E photons

Question 16

Which E level is the ground state -16eV or -0.1eV?

Answer 16

The lowest # is the innermost shell - least energy needed to be emitted - 16eV = energy in E shell

Question 17

How does strong force vary with separation?

Answer 17

Repulsive under 1fm
Strongly attractive 1-3 fm
3-10 minor force
after 10fm nothing

Question 18

Strong Force exchange particle

Answer 18

Pions

Question 19

Weak force exchange particle

Answer 19

W Bosons

Question 20

Explain the fluorescence in a lamp

Answer 20

• High Voltage accelerates electrons across tube
• Mercury vapour fills the tube: low pressure
• Electrons collide with mercury vapour
• Mercury atoms are ionised, releasing more free electrons
• Free electrons collide with mercury atoms causing them to be excited
• Emit UV photons
• UV photons excite fluorescent coating on tube
• when it De-excited visible light is emitted

Question 21

How is the photoelectric effect evidence of the particle nature of light?

Answer 21

Wave theory suggests: Any freq of light should cause emission as energy absorbed by each electron will gradually increase with each oncoming wave. Intensity has no effect unless above the threshold frequency. If above threshold frequency more electrons are emitted per second.

Question 22

How are emission spectrum evidence of energy levels.

Answer 22

Specific wavelengths of light are emitted.
C=FxWavelength
Lines represent the differences in E levels as atoms can only absorb energy exactly equal to E1-E2.
E=hf : frequencies are representative of energy levels

Question 23

Explain in terms of energy changes how emission spectra are formed

Answer 23

Discrete E levels in an atom
photon is produced by an electron de-exciting
different wavelengths due to different frequencies from energy levels ie: E = E1 - E2

Question 24

Why must photons be above a certain frequency for emission

Answer 24

Must reach the threshold frequency for the work function (energy) to be reached.
Work is done to lift electron from energy well / change energy levels

Question 25

Why would pd vary as a radioactive source is moved to and away from it?

Answer 25

Emission of He nucleus causes ionisation and therefore a current closer means more ionisation and therefore a current. Pd decreases to a minimum, before ionisation there was no current and pd is a max. He radiation has a short range

Question 26

How are Mercury atoms in fluorescent tube excited?

Answer 26

Pd across tube stream of electrons ionise mercury atoms free electrons excite atoms by transferring energy in the collision

Question 27

Why do excited Mercury Atoms emit photons of specific frequencies?

Answer 27

Atom has discrete E levels energy emitted in form of a Photon E = E1-E2 E = hf

Question 28

How does the fluorescent coating emit visible light

Answer 28

coating absorbs UV light | It is excited by photons of lower frequencies

Question 29

Why for a transition does a photon need an exact amount of energy and an electron just a minimum KE?

Answer 29

Photon has discrete amount of E,it’s transfer it all Electron can transfer part of its energy: go to a lower KE Excitation needs the specific amount of energy

Question 30

What is not conserved in weak force?

Answer 30

Strangeness

Question 31

EM Radiation incident on photoémissive surface, why is there only current if freq above a certain value?

Answer 31

A energy of a photon needs threshold freq. to achieve work function. Photoémission occurs if E above work function

Question 32

Effect on current if intensity is increased

Answer 32

increased intensity = more photos and more energy incident per second current greater as more electrons are emitted per second

Question 33

Effect on stopping potential of increased frequency of EM

Answer 33

More energy above the work function because photons have more energy larger KE of the emitted electrons therefore more stopping potential

Question 34

What class of particles is W -

Answer 34

Bosons (exchange particle)

Question 35

What does a muon decay into

Answer 35

Electron, electron antineutrino, muon neutrino

Question 36

Identify the state of an electron in state 0

Answer 36

Just free

Question 37

Identify the state of an electron in the energy level -13.6

Answer 37

Ground state: lowest level

Question 38

Why are the energy levels negative?

Answer 38

Zero is the maximum energy: energy decreases from energy level 0 to move to lower energy levels: - energy is the amount needed to move to the lower level

Question 39

How is a discharge tube related to specific frequencies of emission? Why must there be a high pd? Discuss the E Level diagram and emmission spectrum predict each other.

Answer 39

High Pd creates a flow of electrons, electrons need enough KE to ionise energy Figure 9: discrete e levels and emitted photons relate in E photon = E1 - E2