Particles test 11/23

Question 1

Specific charge definition and how it’s calculated

Answer 1

The specific charge of a particle is the charge to mass ratio and is calculated by dividing a particles charge by its mass -> charge/mass

Question 2

What is the Photoelectric affect?

Answer 2

Where photoelctrons are emitted from the surface of a metal after a light above a certain frequency is shone on it.

Question 3

What is the frequency required for different types of metals called?

Answer 3

Threshold frequency

Question 4

What is suggested in the photon model of light?

Answer 4

• EM waves travel in discrete packets called photons, that have an energy directly proportional to frequency
• each electron can absorb a single photon, thus a photo electron is only emitted if above the threshold frequency
• if the intensity of the light is increased and frequency above threshold more photoelectrons are emitted per second.

Question 5

What is the work function?

Answer 5

Minimum energy required for electrons to be emitted from the surface of a metal

Question 6

What is the stopping potential?

Answer 6

Potential difference you need to apply across the metal to stop the photo electrons with the max KE

Question 7

How photons become Photoelectrons?

Answer 7

• delocalised electrons near the surface of the metal absorb the energy (=hf) of a single incident photon
• the KE (Ek) of the delocalised electron gains the same amount of energy (=hf)
• if the KE is now bigger than the work function (phi) then the electron will escape from the surface of the metal
• while escaping the surface of the metal and becoming a photo electron, the electron loses/uses an amount of energy equal to the work function
• thus a photoelectron have a max KE calculated by:
  - KE max = energy of photon - work function
  - KE max = (planchs constant x frequency of photons) - work function

Question 8

Photoelectric graphs

Answer 8

-KE max on y axis, frequency on x axis
- gradient = Planck constant
- minimum frequency found where KE max = 0 and from equation f(min) = work function/ plan is constant
- value of -(phi) found on y-intercept
- fmin = threshold frequency

Question 9

How to find the stopping potential?

Answer 9

• create a circuit including
  - sensitive ammeter
  - photoemissive plate in an evacuated tube with an electrode opposite
  - variable voltage supply
• when monochromatic light is shone on the plate a current will flow
• increase the voltage until the current reads as 0, this is rigs stopping potential/voltage

Question 10

Converting eV to Joules

Answer 10

(eV) x 1.6x10^-19

Question 11

Converting joules to eV

Answer 11

(Joules) / 1.6x10^-19

Question 12

What does the stopping potential give us and why?

Answer 12

The maximum KE because the electrons with KE max have been stopped.
- KE max = V * 1.6*10^-19 (joules)

Question 13

What are discrete energy levels?

Answer 13

Where electrons in atoms can exist

Question 14

Define excitation

Answer 14

Electrons in atoms gain energy from collisions with free electrons
- causes them to move up in energy levels

Question 15

Define ionisation

Answer 15

When electrons in atoms gain enough energy from collisions with free electrons to be removed from the atom entirely

Question 16

When does ionisation occur?

Answer 16

If the energy of the free electron is greater than the ionisation energy

Question 17

What happens if an electron becomes exited?

Answer 17

It will return it it’s original energy level, thus release the energy gained in the form of a photon

Question 18

How to get a line spectrum with a fluorescent tube

Answer 18

Pass the light from a fluorescent tube through a diffraction grating or prism

Question 19

What does each line in a line spectrum represent

Answer 19

Different wavelength of light emitted by the tube
- contains discrete values of wavelength

Question 20

What is the relationship between photon energies and wavelengths?

Answer 20

The correspond to one another

Question 21

When does annihilation take place and what happens when it does ?

Answer 21

-When matter and anti-matter meet
- they destroy each other causing the particles to disappear from existence and a pair of photons to be produced

Question 22

Three examples of corresponding matter and antimatter

Answer 22

• electron and positron
• proton and antiproton
• neutron and antineutron

Question 23

What must be conserved in annihilation?

Answer 23

Momentum - as always
Energy - will change from particle rest energy into another form.

Question 24

What is produced in the process of annihilation?

Answer 24

An identical pair of gamma protons with the same frequency as each other

Question 25

Why is momentum conserved in annihilation?

Answer 25

Because the gamma photons will head in completely opposite directions
- thus having equal and opposite momentum

Question 26

What is the total momentum before the collision and why?

Answer 26

• the momentum of particles before the collision is equal and opposite thus total momentum before the collision is zero

Question 27

What is conserved in annihilation and pair productions?

Answer 27

Charge
Lepton number
Baryon number
Strangeness - unless weak interaction then can be 1 away
Momentum
Energy