3.2.2 ELECTROMAGNETIC RADIATION AND QUANTUM PHENOMENA

Question 1

What is the photoelectric effect?

Answer 1

When electrons are emitted from the surface of a metal after EM radiation of a certain frequency is directed at it

Question 2

What is intensity?

Answer 2

The rate of energy transfer to an area (Watts/m^2)

Question 3

What is the photon explanation of threshold frequency?

Answer 3

The minimum frequency of an incident photon required for photoemission

Question 4

What is the work function?

Answer 4

The minimum energy of a conduction electron to escape the surface of a metal when the metal is at zero potential.

(The minimum energy required to cause photoemission)

Question 5

What is the photoelectric equation?

Answer 5

hf = Φ + Ek (max)

Question 6

Explain why photoelectrons leave a material with varying kinetic energies?

Answer 6

Depends on closeness of electron to surface
Electrons collide with other electrons + lattice ions, transferring energy

Question 7

What is stopping potential?

Answer 7

The minimum negative voltage applied to the anode to stop the photocurrent

• Causes the Ek of electrons to fall
  to zero, as Ek is converted to potential energy

Question 8

What is the equation for stopping voltage?

Answer 8

eV = hf - Φ [eV = Ek(max)]

Question 9

What is the relationship between intensity of radiation and number of electrons emitted?

Answer 9

• Intensity is proportional to number of electrons emitted, given that the radiation is above the threshold frequency.
• Photoelectric emission occurs without delay, and each electron can only absorb one photon at a time, so increasing the intensity of radiation under the threshold frequency will not cause photoemission

Question 10

How is light “carried”?

Answer 10

In discrete packets of energy (photons)
energy of a photon = hf

Question 11

Describe a method to determine the stopping potential

Answer 11

• Connect a battery at 0V to an ammeter and 2 plates (anode
  and cathode) in an evacuated tube
• Direct radiation above the threshold frequency of the plate
  onto the cathode and record the current produced
• Connect the positive terminal of the battery to the plate
  emitting photoelectrons, and the negative terminal to the
  plate receiving them
• Adjust voltage until current recorded = 0

Question 12

Why can’t current be used solely to find the stopping potential/work function of a material?

Answer 12

The current only tells us the rate of flow of charge, and not the Ek of the electrons

Question 13

Why is an evacuated tube used to determine stopping potential?

Answer 13

To prevent collisions with air molecules, which would also do work against photoelectrons (as well as work done by battery), which could make the calculated stopping voltage lower than it should be

Question 14

What happens in an evacuated tube where photoelectrons are being emitted when a stopping potential is applied?

Answer 14

photoelectrons accelerate towards the anode, but do not come into contact with it, and return to the cathode

Question 15

How is the kinetic energy of a photoelectron converted into potential energy?

Answer 15

The electric fields produced by the charged plates opposes the motion of the electrons

Question 16

What is the rule for electrons absorbing photons?

Answer 16

An electron can only absorb one photon of energy at a time, so a single photon can eject only one electron

Question 17

How does the photoelectric effect prove that light acts as a particle?

Answer 17

• Increasing intensity didn’t affect the maximum Ek of the electrons, suggesting that one photon was absorbed by one electron - proved light existed as “quanta” - tiny bits of energy rather than one continuous wave.
• If light were simple a wave-like phenomenon then increasing the intensity and thereby increasing the total energy falling on the surface would be expected to eventually provide enough energy to release electrons no matter what the frequency.

Question 18

Describe the effect of intensity on photoemission

Answer 18

No effect on photoemission, as intensity is the energy per second per m^2 (energy is related to number of photons)
Increasing intensity increases the rate of energy absorbed on an area of the material, but not the energy received by each electron/atom

Question 19

What is the equation for the power of a beam (of photons)

Answer 19

Power = nhf

where n is the number of photons passing a fixed point each second. A beam contains photons of the same frequency, so the power of the beam is the rate at which energy is transferred from the photons

Question 20

Why does intensity of light not affect the maximum kinetic energy of a photoelectron?

Answer 20

The energy gained by an electron is only due to the absorption of ONE photon, so increasing the intensity (number of photons) won’t affect Ek

Question 21

Describe the relationship between intensity and current

Answer 21

Over the threshold frequency, intensity is proportional to the size of the photocurrent. Light intensity is a measure of energy per second per m^2. Energy is proportional to the number of photons per second. Above the threshold frequency, each electron must have absorbed one photon in order to be liberated and produce a current. By increasing the intensity, the number of electrons liberated (charge carriers) increases, so intensity is proportional to current, given that the incident light is above the cathode’s threshold frequency

Question 22

How do you calculate the number of photoelectrons transferring per second from the cathode onto the anode for a photoelectric current 𝑰

Answer 22

n = 𝑰/e

Where e is the elementary charge. The photocurrent is the rate of flow of electrons/charge (Q/t) , and each electron has a charge e, therefore the current can be rewritten as ne/t , where n is the number of electrons passing per second, and e is the elementary charge. Dividing by e gives the number of electrons flowing per second

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Question 23

What is the definition of ionisation energy?

Answer 23

The energy required to liberate an electron from the ground state of an atom
(aka principal ionisation energy)

Question 24

What is the ground state?

Answer 24

The lowest/most stable energy level of an atom

Question 25

What is excitation?

Answer 25

The process of an atom (electron) absorbing energy WITHOUT being ionised.

Question 26

Describe the process of excitation

Answer 26

Energy from an oncoming particle (photon/electron/neutron) is absorbed by an electron. The electron moves up to a discrete energy level (Ek), before dropping down to the original energy level, either directly or indirectly, releasing a photon/photons equal to the energy absorbed. When an electron has moved to a higher energy level, the atom is said to be in an excited state

Question 27

What are conditions for excitation with photons vs other particles (neutrons/electrons/ions)?

Answer 27

The energy of the photon must be EXACTLY equal to the difference in energy levels, and will be pass through the atom if it isn't With other particles, an electron can absorb all or some of the Ek of an oncoming particle. For excitation, the energy absorbed will be equal to the difference in energy lvls, and any remaining unabsorbed energy will remain in the initial oncoming particle

Question 28

Describe how the ionisation energy of a gas can be measured

Answer 28

Set up a circuit w a glass tube containing gas at low pressure, with a negative filament on on end of the tube, and an anode on the other end. Measure current in circuit w milliammeter (due to some electrons reaching anode) , and then increase pd across filament and tube to increase the Ek of electrons emitted from filament. Eventually the emitted electrons have enough Ek to ionise gas atoms in the tube, increasing number of charge carriers reaching the anode. Each electron arrives at the anode with abt the same energy required to ionise it (from oncoming electron). By measuring the voltage just as current begins to increase, ionisation energy can be calculated as eV, as the work done on each electron to ionise the atom, Gas is low pressure to reduce collisions and allow electrons to reach anode

Question 29

How can the excitation energies of a gas in a glass tube be determined?

Answer 29

Increasing voltage across tube, and measuring voltage before current drops. (As excitation will occur when energy of oncoming electron has been completely absorbed by atom, so no remaining Ek to provide a current)

Question 30

What is monochromatic light?

Answer 30

Light of a fixed frequency

Question 31

Why is the electron configuration of an excited atom unstable?

Answer 31

The excited electron has left a vacancy in the shell it moved from

Question 32

What causes each concentric ring in an electron diffraction pattern? (from a metal)

Answer 32

Electrons diffracted by the same amount from the same amount of grains of different orientations, at the same angle to the incident beam

Question 33

Why is it important to test new theories by testing its predictions experimentally?

Answer 33

If the test does not provide supporting evidence, the prediction is incorrect so the theory is incorrect and must be changed

Question 34

Explain why the Ek of photoelectrons ranges up to a maximum value

Answer 34

Incident photon has fixed energy - photon loses all its energy in a single interaction Electron can lose various amounts of energy in reaching surface of metal, e.g. through collisions with other electrons

Question 35

Why is the gas in a fluorescent tube at low pressure?

Answer 35

There must be sufficient distance between collisions for the electrons to gain enough energy for the required excitations to occur The vapour must not completely absorb the electrons

Question 36

Explain how a fluorescent tube containing mercury vapour with a phosphorous coating would emit visible light

Answer 36

Mercury vapour emits ultraviolet radiation UV radiation excites the atoms of the coating Coating then emits electromagnetic radiation of longer wavelengths Some of which is in the visible region

Question 37

Describe how electrons can behave as both particles and waves

Answer 37

- They can be diffracted, or show interference effects - Can be deflected in electric or magnetic fields, or make collisions with atoms

Question 38

Explain why an electron and muon accelerated from rest by the same pd have different wavelengths

Answer 38

Both experience the same increase in energy. As W=QδV Wavelength is inversely proportional to momentum gain in momentum is different for the muons and electrons. The smaller mass has the largest acceleration + Using λ = h/mv + Ek = p^2/2m deduce λ proportional to 1/√m So muon has a lower wavelength

Question 39

Describe wave Vs particle properties

Answer 39

Wavelength, massless, frequency, diffraction and refraction Ek, linear momentum

Question 40

What does the average Ek of a conduction electron depend on

Answer 40

The temperature of the metal

Question 41

Outline aspects of the photoelectric effect that can't be explained by the wave model

Answer 41

Wave model predicts that wave energy would accumulate and therefore photoemission would occur eventually, however no photoemission under threshold frequency (zero time delay) Wave model predicts that all frequencies of light would cause photoemission as energy depends on amplitude, isn't the case Stopping voltage independent of intensity. Wave model predicts that increasing intensity would increase energy given to each electron. However intensity is related to the size of the current, while V is related to Ek of each electron

Question 42

Is current very related to speed

Answer 42

Current is related to the speed of electrons but it is not the speed of electrons! Current is the amount of charge that passes through a cross sectional area in one second Current is given as N×A×V×E where: N is the number of free electrons per unit volume A is the area of cross section V is speed of free electrons and E is electron charge