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Physics A Level > 3.2.2 Electromagnetic Radiation and Quantum Phenomena > Flashcards

3.2.2 Electromagnetic Radiation and Quantum Phenomena Flashcards

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1
Q

What do electrons do in a metal and what are these referred to as?

A

They move around, delocalised/free electrons.

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2
Q

Why don’t free electrons escape the metal?

A

Still electrostatically attracted to the positive ions so stay in the metal. Don’t have the energy to overcome the attraction.

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3
Q

Define work function.

A

The minimum amount of energy required to remove an electron from the surface of a metal.

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4
Q

What is work function measured in and should you convert?

A

MeV or J, convert to J.

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5
Q

What symbol do we use for work function?

A

ϕ.

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6
Q

Define threshold frequency?

A

The minimum frequency of the EM radiation required to remove an electron from the surface of a metal.

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7
Q

What do we measure threshold frequency in?

A

Hz.

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8
Q

What is the symbol for threshold frequency?

A

f₀.

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9
Q

What is Einstein’s photoelectric effect equation?

A

hf = Φ + Ek(max)

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10
Q

In Einstein’s photoelectric equation what is hf?

A

Photon energy.

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11
Q

In Einstein’s photoelectric equation what is Φ?

A

Work function.

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12
Q

In Einstein’s photoelectric equation what is Ek(max)?

A

Maximum kinetic energy.

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13
Q

What do electrons escape the metal with and what is the range?

A

Ek, ranging from 0 to Ek(max).

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14
Q

Why do electrons escape with a range of Ek?

A

Because some are deeper in the surface of the metal and have to do more work to escape.

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15
Q

In a graph showing Ek max against frequency, what is the gradient?

A

h, planck constant.

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16
Q

In a graph showing Ek max against frequency, what is the x intercept?

A

f₀.

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17
Q

In a graph showing Ek max against frequency, what is the y intercept?

A

-Φ.

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18
Q

In a graph showing Ek max against frequency, if the work function is bigger how does the graph change?

A

Shifts to the right.

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19
Q

What components are in a vacuum photo cell circuit?

A

Photoemissive electrode, wire, microammeter, variable DC supply.

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20
Q

What is a micro amp in amps?

A

x10-6.

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21
Q

What is a vacuum?

A

No particles.

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22
Q

In a vacuum photocell what goes in?

A

Photons.

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23
Q

What are the two electrodes in a vacuum photocell?

A

Wire, photoemissive.

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24
Q

When the photons enter the vacuum photo cell what leaves the photoemissive electrode and what does this create?

A

Electrons, a current.

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25
Q

If we increase f in a vacuum photocell circuit what happens to A, why?

A

No change, same number of electrons crossing gap per second (they just have more Ek(max)).

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26
Q

If we increase intensity in a vacuum photocell circuit what happens to current, why?

A

Current increases, more photons arrive per second, more e-s per second cross the gap, same Ek(max) as before.

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27
Q

Define stopping potential.

A

The minimum p.d. required to stop all of the e-s crossing the gap (even ones at Ek(max)), the current becomes 0.

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28
Q

As the p.d increases the current…?

A

Decreases.

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29
Q

Work done by p.d = Ek …?

A

Lost by e-s.

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30
Q

QV =

A

1/2 m v^2.

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31
Q

eV=

A

1/2 m v^2

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32
Q

eVs = (complex)

A

1/2 m vmax^2

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33
Q

eVs = (simple)

A

Ek(max)

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34
Q

What is the e in eVs?

A

Charge of electron.

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35
Q

What can happen when an orbiting electron gains energy?

A

It can be promoted to a higher energy level.

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36
Q

When an electron is promoted to a higher energy level what do we call it?

A

Excited.

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37
Q

How does an electron gain energy to get excited?

A

Collision with a passing electron or absorbing a photon of EM radiation.

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38
Q

What do we call an electron when it’s in it’s normal energy level?

A

Ground state.

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39
Q

How do we represent in a diagram the different energy levels an e- can be promoted to?

A

Using straight lines stacked on top of eachother, get closer together as we go up.

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40
Q

What will the energies needed to promote an e- to a higher energy level be measured in?

A

eV.

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41
Q

How do we convert eV into J?

A

x by charge of an electron. 1.60 x 10^-19.

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42
Q

What happens after an e- is excited?

A

It relaxes and returns to the ground state.

43
Q

Does an e- have to return directly to the ground state?

A

No.

44
Q

What does the excited e- emit when it returns to the ground state?

A

A photon of energy.

45
Q

Change in E = ?

A

E1 - E2.

46
Q

A photon that is emitting by an e- going directly to a ground state has what?

A

Highest energy and highest frequency.

47
Q

A photon that is emitted by an e- relaxing from a higher energy level to a lower energy level but not returning directly to the ground state has what?

A

Lower energy, longer wavelength.

48
Q

How do you calculate the length of the wavelength of a photon emitted by an excited e- relaxing?

A

Convert eV into J, use E = hc/λ, rearrange to find λ.

49
Q

What is the equation that connects E, h and f?

A

E = h f.

50
Q

What is the equation that connects E, h, c and λ?

A

E = hc/λ.

51
Q

What is the equation that connects h, f, E1 and E2?

A

hf = E1 - E2.

52
Q

What do we call it when an excited e- returns to ground state?

A

De-exciting or relaxing.

53
Q

Each element has a specific set of?

A

Discrete energy levels.

54
Q

Define discrete.

A

Separate, not continuous data.

55
Q

When an electron de-excites and releases a photon the photon can only have what? What is this equal to?

A

Certain energy values, equal to the difference in energy levels.

56
Q

The frequency and wavelength are dependant on the ______?

A

Photon energy.

57
Q

What is the visible light range?

A

400nm to 700nm.

58
Q

How do you set up an absorption spectra?

A

Shoot white light through a cloud of gas and observe with a telescope.

59
Q

As white light passes through a gas why do certain photons get absorbed?

A

Because their energies match possible gaps in the atom’s energy levels.

60
Q

On absorption spectra what do the missing photons show up as?

A

Dark lines.

61
Q

What is an ion?

A

A charged atom.

62
Q

What is ionisation?

A

The process of removing/adding an electron from an atom, turning it into an ion.

63
Q

Define ionisation energy.

A

The minimum energy required to remove an electron in the ground state from an atom.

64
Q

What are the three things inside a fluorescent tube?

A

Low pressure mercury gas atoms, free electrons, powder coating.

65
Q

Why do we apply a high p.d to a fluorescent tube?

A

To remove electrons from some of the gas atoms, turning them into positive ions.

66
Q

What do the positive ions do in the fluorescent tube?

A

Accelerate towards the negative electrode and dislodge electrons from the electrode.

67
Q

What do the free electrons in a fluorescent tube do?

A

Accelerate towards the positive electrode and collide with mercury atoms along the way, exciting them.

68
Q

When a mercury atom gets excited it releases a what, of which type?

A

A high energy UV photon.

69
Q

What absorbs the UV from the mercury atom and what happens?

A

The powder coating around the tube, it excites.

70
Q

What happens when the powder coating excites?

A

The atom de-excites through smaller intermediate energy levels, releasing lower frequency photons (visible light).

71
Q

Why is there high pressure and high p.d in a fluorescent tube?

A

Ensures the e-s ca reach the required speeds (by accelerating between collisions) to excite the mercury atoms.

72
Q

Why do we need a powder coating in a fluorescent tube, why can’t we just have mercury?

A

Because mercury emits photons with too high a frequency to see and it wouldn’t make light.

73
Q

Why are compact fluorescent tubes funny shapes?

A

Increased surface area.

74
Q

EM radiation can behave as what two things?

A

Wave or particle.

75
Q

What are the qualities of EM radiation that suggest it’s a wave?

A

Reflection, refraction, diffraction, interference, polarisation.

76
Q

What are the qualities of EM radiation that suggest it’s a particle?

A

The photoelectric effect.

77
Q

What is the de Broglie hypothesis?

A

A particle, mass m, moving with speed v, has an associated wavelength, λ, the ‘de Broglie wavelength.

78
Q

What is the equation for the de Broglie wavelength?

A

λ = h/p = h/mv

79
Q

What does p stand for, and what is it measured in?

A

Momentum, kgms-1

80
Q

How has the de Broglie wavelength been proved?

A

By electron diffraction experiments.

81
Q

What is diffraction?

A

Where the edges of a wave spread out when it passes an obstacle or through a gap.

82
Q

If λ is equal to gap size what is the diffraction?

A

Strong.

83
Q

If λ > gap size what is the diffraction?

A

It reflects and doesn’t pass through.

84
Q

If λ < gap size what is the diffraction?

A

Weak.

85
Q

When getting long numbers in a calculation answer, like calculating v, what do we do?

A

Put them in standard form.

86
Q

When asked to define Ek in Einstein’s photoelectric equation, what do we always write?

A

Ek max, maximum kinetic energy an e- can have after escaping the surface of a metal.

87
Q

When asked the minimum p.d for an e- to be accelerated through for ionisation how do we answer?

A

For ionisation to occur it must supply difference from ionisation level and ground state, put this value into eV, e- is accelerated through eV value in just V.

88
Q

How do we answer questions asking about doubling light intensity? What assumptions are made?

A

x2 photons, x2 e-s, current = rate of flow of charge, x2 current. Ek max stays constant and unchanged. Assume 1 photon removes 1 e-, assume all photoelectrons are collected.

89
Q

How do we answer new metal and new Φ questions?

A

No e-s have enough energy to escape, Φ>hf and Φ>original Φ.

90
Q

What happens in an atom when line spectra are produced?

A

e-s excited from one energy level to another, e-s are emitting or absorbing definite λ/ f/ colour/ photon energy hf.

91
Q

When in eV, how to convert into J?

A

x 1.60 x10-19

92
Q

What shows e-s are particles?

A

Deflection in EM fields.

93
Q

Why does the gold leaf fall when radiation is absorbed into electroscope?

A

Energy of rad > Φ so photoelectrons are emitted and the electroscope discharges (becauses e-s and their - charge is leaving), leaf and metal stem no longer repel and the leaf falls.

94
Q

Why won’t only visible light being absorbed (for zinc) let the gold leaf fall?

A

VL f < UV f, or VL E < Φ, so leaf doesn’t fall becuase no e-s are escaping so everything is still charged and repelling.

95
Q

When plate is given + charge why doesn’t gold leaf fall?

A

Higher voltage, harder for e-s to leave, gold leaf doesn’t fall.

96
Q

What does an e- have to pass through to diffract?

A

The gap between two nuclei.

97
Q

What is observed on the fluorescent screen when the e-s diffract?

A

A diffraction pattern.

98
Q

Roughly how big is the atomic diameter?

A

10^-10m

99
Q

Why does there being no time delay for the photoelectric effect prove light is a particle?

A

Light travels as photons, transfers E in discrete packets in 1 to 1 interactions.

100
Q

Why does Ek have a max value?

A

hf is energy available/ always the same energy from the photons, energy required varies (some e-s deeper) so Ek varies.

101
Q

Why only certain values of f cause excitation

A

e-s occupy DISCRETE energy levels, need to absorb exact energy levels to move to a higher level, photons need certain E to provide f (E= hf), energy needed is same for a certain atom, all energy of photon is absorbed, 1 to 1 interaction between photon and e-.

102
Q

What does the atom receive in both ionisation and excitation?

A

Exact amount of E.

103
Q

How do we prove a cathode is negative?

A

Use a magnetic field.

104
Q

What must happen to modify/ replace a theory?

A

Theory must meet predictions tested, repeatable results and peer reviewed.

Physics A Level (15 decks)

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