particles,radiation, quantum phenomena

Question 1

Characteristics of a weak interaction

Answer 1

-It must involve leptons and hadrons
- Change in quark character
- strangeness is not conserved

Question 2

Strong Nuclear Force

Answer 2

• strong nuclear force overcomes the electrostatic force of repulsion between the protons in the nucleus, keeping them together
• it’s range is no more than 3-4 femtometers (fm)
• it has the same effect between n-p, p-p and n-n
• acts as an attractive force 3-4 fm down to about 0.5 fm. At separations smaller than this it acts as a repulsive force that acts to prevent the nucleons from collapsing in on each other.

Question 3

How were scientists able to figure out the existence of neutrinos without previously detecting them?

Answer 3

neutrino was hypothesised to account for the conservation of energy in beta decay.

Question 4

When may a photon be emitted?

Answer 4

• a fast-moving electron is stopped or slows down or changes direction
• an electron in a shell of an atom moved to a different shell if lower energy
• the annihilation of a particle-anti particle pair

Question 5

What are the four fundamental interactions?

Answer 5

• Gravity
• Electromagnetic
• Weak Nuclear
• Strong Nuclear

Question 6

Annihilation & Pair Production

Answer 6

• Annihilation occurs when a particle and its antiparticle constituent interact, unlocking their rest energies and producing photons.
• Pair production occurs when a photon with sufficient energy passing near a nucleus or electron can suddenly change in to a particle-antiparticle pair

(pic)

Question 7

Conditions for a particle-antiparticle pair

Answer 7

• annihilates when it meets its antiparticle conjugate
• has exactly the same rest mass as the particle
• has exactly the opposite charge to the particle if the particle has a charge

Question 8

What causes the electromagnetic force between particles?

Answer 8

• the electromagnetic force between two charged objects is due to the exchange of virtual photons (virtual because can’t be detected directly)
• the exchange of a virtual photon between two like-charged particles transfers momentum (in the same way one recoils when catching a ball due to a transfer in momentum from the thrower to catcher)

Question 9

Why must the weak nuclear force

Answer 9

• the weak nuclear force explains the changes in quark character for bosons
• it must be weaker than the strong nuclear force because it could affect stable nuclei

Question 10

Feynman Diagram for Beta+ and Beta- decay

Answer 10

• the W- boson decays into a Beta- particle and an antineutrino
• the W+ boson decays into a Beta+ particle and neutrino

(pic)

Question 11

Feynman diagram for electron capture

Answer 11

• protons in a proton-rich nucleus turn into a neutron as a result of interacting through the weak interaction with an inner-shell electron from outside the nucleus
• exchange particle = W+ boson
• converts electron to a neutrino

(pic)

Question 12

Why do to like-charged particles separate when they repulse one another?

Answer 12

• when two charged objects interact they exert equal opposite forces on each other
• momentum is transferred between the objects
• any interaction must conserve momentum so particles will travel in equal and opposite displacements

Question 13

W Boson Characteristics

Answer 13

• weak nuclear force exchange particles
  -have a non-zero rest mass
• have a very short range of no more than about 0.001fm
• are positively charged or negatively charged

Question 14

Hadrons and Leptons

Answer 14

• Hadrons are particles and antiparticles that can interact through the string interaction
• Leptons are particles and antiparticles that do not interact through the strong interaction

interactions

Hadrons can interact through all four fundamental forces whereas leptons can only do three

• Hadrons (apart from the proton, which is stable) decay through the weak interaction.
• Hadrons are composed of Baryons and Mesons
  • Mesons are hadrons that do not include protons in their decay products and are only composed of two quarks

Question 15

Strangeness

Answer 15

• strangeness is always conserved in a strong interaction, whereas strangeness can change by 0,+1,-1 in weak interactions

Question 16

Muons, Electrons and Neutrinos

Answer 16

• muons decay in to electrons
• muons and electrons have their own respective neutrino and anti-neutrino pairs
• an electron neutrino can not be produced in a muon interaction.

Answer 17

• photoelectric emission of electrons from a metal surface does not take place if the frequency of the incident electromagnetic radiation below the threshold frequency
• the number of electrons emitted per second is proportional to the intensity of the incident radiation, provided the frequency is greater than the threshold frequency (if not at threshold, regardless of intensity no photoelectric emission will occur)

Question 18

Work Function

Answer 18

• the minimum energy needed by an electron to escape from the metal surface.

Question 19

Stopping Potential

Answer 19

• electrons that escape a metal plate can be attracted back to a plate when supplied sufficient positive charge
• the minimum potential needed to stop photoelectric emission is the stopping potential V(s)
• at this potential the max kinetic energy of the emitted electron is reduced to zero
• each electron emitted must do extra work equal to e*Vs to leave the metal surface
• ∴ KE = e*Vs

Answer 20

• ( for light above the freehold frequency)
  -the no. of photoelectrons per second that transfer from the cathode to the anode = I/e
  -the photoelectric current is proportional to the intensity of the light incident on the cathode (light intensity = Js^-1)
• the intensity of the incident light does not affect the max kinetic energy of a photoelectron

Question 21

Excitation

Answer 21

Excitation by Collision

• An electron at a lower energy level has a collision with a moving electron of sufficient energy, absorbing its energy to allow it to jump between its discrete energy levels
• the remaining energy will be the new KE of that moving electron
• if a moving electron does not have a sufficient KE then it will deflected. (elastic collision)

Excitation using photons

• a photon of energy exactly equal to the energy needed to excite the electron to the next discrete energy level is absorbed, allowing the electron to move between the energy levels.
• if the energy is not sufficient or is too large then it will not be absorbed by the electron

In both scenarios the electrons can de-excite and emit photons of energies = hf where hf is the potential energy gap between energy levels

Answer 22

• can determine the excitation energies if the atoms in the gas-filled tube by increasing the potential difference between the filament and the anode and measuring the pd when the anode current falls

Question 23

Fluorescence

Answer 23

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