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A Level Physics Paper 1 > Quantum > Flashcards

Quantum Flashcards

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1
Q

What is the photoelectric effect

A

When light incident on a metal surface causes electrons to be emitted from the surface

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2
Q

When was the photoelectric effect discovered and by who

A

1887

by Heinrich Hertz, but it took 18 more years for Albert Einstein to explain it

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3
Q

Which visible light colour has the highest frequency and what does this mean

A

Blue/violet

Most likely to cause a photoelectric effect

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4
Q

2 main points about increasing the frequency in terms of the photoelectric effect

A

Below a certain frequency (threshold) no electrons are emit and above it they are
Increasing the frequency increases the kinetic energy of the emitted electrons

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5
Q

What is the threshold frequency

A

The minimum frequency of light incident on a metal surface needed to cause electrons to be emitted
Different for each metal and a property of the metal

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6
Q

Effect of increasing the brightness/intensity if below the threshold frequency

A

No effect
Although more waves are transmitted per second and more waves strike the metals surface per second
The photons of light still do not have a frequency above the threshold frequency
So no electrons are emit

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7
Q

Effect of increasing the frequency above threshold

A

Photons have an increased frequency so an increased energy

Electrons emit from the metals surface with greater maximum kinetic energy

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8
Q

Effect of increasing the brightness/intensity if above the threshold frequency

A

More waves emitted per second from the light source
More photons of light striking the metal surface per second
Increased number of electrons emitted from the metals surface per second

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9
Q

What is the effect of moving a lamp twice as far away from the source

A

4x less photons of light strike the surface of the metal
Due to the inverse square law
But the kinetic energies of the photons remains the same
Since the frequency is the same
So 4x fewer electrons emitted per second
With the same kinetic energy

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10
Q

Why is it common for metals with one electron in their outer shell to exhibit the photoelectric effect in visible light

A

Less energy has to be absorbed for the electron to be emitted

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11
Q

What are photoelectrons

A

The emitted/liberated electrons

That are liberated from their metals surface due to the photoelectric effect

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12
Q

Why is it more accurate to refer to kinetic energy as maximum kinetic energy

A

The emitted electrons have a range of energies

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13
Q

What are the axis for the energy frequency graph

A

x; Frequency of radiation (x10^14Hz)

y; Maximum kinetic energy (x10-19J)

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14
Q

y intercept for the energy frequency graph

A

The work function

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15
Q

What is the x intercept for the energy frequency graph

A

The threshold frequency

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16
Q

Gradient of the energy frequency graph

A

Planck’s constant
h
In formula book (6.63x10^-34Js)

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17
Q

Comment on the speed electrons are emit during the photoelectric effect

A

Instantaneously

Negligible delay between light hitting the metal surface and electrons being emitted

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18
Q

What does the number of electrons emitted per second depend on

A

The intensity of the light

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19
Q

Why does light behave the way it does in the photoelectric effect

A

Electrons are held by electrostatic forces onto the surface of the metal
The light has to provide enough energy to rip the electron free from the metals surface and break free from these forces

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20
Q

What did the wave theory anticipate that was not true

A

If a wave hasn’t got enough energy to liberate an electron then you need a higher amplitude wave (brighter light)
But in practice if the light is below the threshold frequency then no electrons are produced even if brightness is increased
So to explain it Einstein had to move away from the wave theory

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21
Q

What was Einstein’s proposition

A

Instead of travelling in waves light can travel in discrete packets known as photons

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22
Q

What happens when a photon strikes a metals surface

A

Either absorbed if it has the sufficient energy
Or not absorbed at all
Will knock out an electron if its energy is larger than the energy holding the electron in the metal

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23
Q

Energy of photon equation

A

E=hf=hc/λ

E; Energy of each photon in Joules
h; Planck’s constant in Js
f; Frequency of each photon

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24
Q

What is the work function

A

Φ
Minimum energy required to liberate an electron from the surface of a metal
A property of the metal and different for each metal

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25
Q

What happens if monochromatic light is incident on metals surface at the threshold frequency

A

All the photons would have a frequency equal to the threshold frequency
So each photon would have just enough energy to cause the photoelectric effect to occur and electrons to be emit

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26
Q

hf

A

Photon energy is less than the work function
No photoelectrons are emitted
Photon frequency is less than the threshold frequency

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27
Q

hf=Φ

A

Photon energy is equal to the work function
Photoelectrons emitted with zero kinetic energy
Photon frequency=threshold frequency

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28
Q

hf>Φ

A

Photon energy is greater than the work function
Photoelectrons are emitted with a range of kinetic energies up to the maximum kinetic energy
Photon frequency is greater than the threshold frequency

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29
Q

Kinetic energy of electron emitted =

A

Energy of incident photon - Energy needed to remove electron

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30
Q

Comment on the electron depth and energy

A

If an electron is at the very surface of the metal the energy required is the work function and so has the maximum kinetic energy
those deeper require more energy so emitted with slightly less kinetic energy
Hence a range of kinetic energies

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31
Q

Energy of incident photon equation

A

Energy needed to remove the electron (Φ) + Kinetic energy of emitted electron

hf = Φ + Ekmax
hf = hf0 + Ekmax (in J)

Provided

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32
Q

Work function equation

A

hf0

Planck’s constant x the threshold frequency
Not provided

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33
Q

Explain the wave model prediction for the photoelectric effect

A

Electrons gradually absorb energy from multiple light waves to gain enough energy to break free from the metal
Brighter light means quicker electron accumulates required energy
Energy of a beam is determined by the number of waves emitted per second not the frequency
Changing the colour of light has no effect on the emission of electrons

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34
Q

Explain the particle model explanation for the photoelectric effect

A

Electrons either completely absorb photon energy if its energy is greater than that to break free from the metal or the photon passes through
1 to 1 interaction
Electron cannot absorb more than one photon of light
So brightness doesn’t have any affect if the photon doesn’t have enough energy
Energy of each photon is determined by the frequency/colour of light
Increasing the brightness increases the number of photons hitting the metal surface so increases the number of electrons emitted

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35
Q

Explain the first half of the gold leaf experiment

A

Rub plastic rod with cloth to transfer electrons from cloth to rod
Rod has overall negative charge
Touch rod to zinc plate to transfer electrons
Leaving plate with negative charge
Negative charge flows down the metal stem and into the gold leaf
The stem and gold leaf are both negatively charged so repel eachother
Gold leaf rises

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36
Q

Explain the set up of the electroscope in gold leaf experiment

A 
Zinc plate on a metal cap
Attached to a metal stem
That goes into an early metal cage
Insulated in a vacuum to avoid charge leaking away to the air
Stem has a gold leaf attached
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37
Q

Explain the second stage of the gold leaf experiment

A

If light with a frequency above the metals threshold frequency is incident on the plate
Electrons are liberated from the metal surface
The plate, metal stem and gold leaf lose their excess charge
The gold leaf and metal stem no longer repel one another
Leaf falls

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38
Q

Why wont light below the threshold frequency cause the golf leaf to fall

A

Photons do not have enough energy to liberate the electrons from the surface
Energy is lower than the work function
Leaf and stem remain charged as the excess charge is not removed

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39
Q

Why wont the gold leaf fall when the brightness of the light is increased when below the threshold frequency

A

Frequency still below the threshold frequency
Even though there are more photons hitting the metals surface per second, since photons and electrons interact 1-1, each individual photon still doesn’t have enough energy to liberate an electron
Leaf and stem remain charged as the excess charge is not removed

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40
Q

Energy of a single photon

A

hf-Φ

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41
Q

Explain why the photoelectric effect is not observed below the threshold frequency

A

Frequency of light is not high enough
Energy of each photon is lower than the work function
So don’t have enough energy to liberate electrons from the metal surface
Photons aren’t absorbed and instead pass through

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42
Q

What is validated evidence

A

Experimental evidence

That can be replicated

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43
Q

eV

A

Electronvolt

The kinetic energy gained by one electron passing through a potential difference of 1 volt

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44
Q

Do you use energy in joules or eV

A

Joules

eV x 10^-19

45
Q

What is the stopping potential

A

The voltage of the battery when the current is zero

46
Q

In stopping potential, the energy added by light is equal to what

A

The energy the battery removes

47
Q

How do you place the battery in the stopping potential and why

A

With the negative terminal pointed towards the wire electrode
To resist the flow of electrons and decrease current
Increasing the strength of the battery increases its voltage until the current reaches zero

48
Q

Stopping potential equation

A

Maximum kinetic energy of emitted electrons=Energy removed by stopping potential

Ekmax=eVs

Maximum kinetic energy of the emitted electrons
e; charge of an electron
Vs; stopping potential

49
Q

Why does increasing the frequency of radiation increase the stopping potential

A

Vs=Ekmax/e

Increasing the stopping potential increases the maximum kinetic energy of the photons
Since e is constant, Vs must increase
So each electron absorbs more energy

50
Q

3 types of spectra

A

Continuous
Emission line
Absorption line

51
Q

Explain continuous spectrum

A

Light is emitted across a range of wavelengths

E.g bulb or filament lamp

52
Q

Explain emission line spectrum

A

Emit specific wavelengths of light
By exciting a gas with heat or an electrical current
E.g a hot gas

53
Q

Absorption spectrum

A

If a continuous spectrum is shone though a cold gas the gas will absorb specific wavelengths of light
So when dispersed through a prism a continuous spectrum is seen but with some wavelengths missing

54
Q

Why does hydrogen/gases form an emission line spectrum

A

Can only emit light at certain discrete frequencies across the spectrum
Due to having electrons in discrete energy levels
Which only emit specific wavelengths of light/photons of specific energies

55
Q

Neils Bohr propositions

A

Electrons only travel in certain allowed orbits known as energy levels
When electron is in an allowed orbit it does not radiate but stays at a constant energy
An electron in an atom can only emit or absorb energy as it moves from one orbit to another

This explains why only certain wavelengths of light are emitted from certain gases

56
Q

Hydrogen has a ground state of 13.6eV

What does this mean

A

13.6eV energy is required to hold an electron in hydrogens ground state
Lowest energy level

57
Q

2 key points for energy level diagrams

A

Negative

Given in eV

58
Q

How is kinetic energy defined at ground state and infinity

A

Zero for highest state/infinity
So the energies for lower levels must be increasingly more negative
Since larger orbits have larger kinetic energies

59
Q

How do electrons move between energy levels in terms of speed

A

Instantly

Cannot exist between energy levels

60
Q

What is the energy absorbed equivalent to in energy level diagrams

A

The difference between the two energy levels

61
Q

What happens in de-excitation

A

Loses the exact right amount of energy to fall to a lower energy level
Lost energy of electron is emitted as a photon

62
Q

Equation for energy and frequency for photons

A

E=hf
E=hc/λ

E, the energy of a photon, must be in joules

63
Q

How do you calculate the frequency or wavelength of an emitted photon

A

Calculate the difference in energy between the two energy levels
Convert to joules
E=hf or E=hc/λ

64
Q

Why do different gases form different emission spectra

A

Each element has its own unique set of energy levels
Meaning de-excitations between the same two energy levels for two different elements will release a photon with different energies, frequencies and wavelengths

65
Q

3 parts of the hydrogen emission spectrum

A

Lyman
Balmer
Paschen

66
Q

Energy level transitions for Lyman

A

n=1 to n=6

67
Q

Wavelengths for Lyman in nm

A 
122
103
97
95
94

Smaller means a bigger transition

68
Q

Energy level transitions for Balmer

A

n=2 to n=6

69
Q

Wavelengths for Balmer in nm

A

656
486
434
410

Smaller means a bigger transition

70
Q

Energy level transitions for Paschen

A

n=3 to n=6

71
Q

Wavelengths for Paschen in nm

A

1875
1282
1094

Smaller means a bigger transition

72
Q

3 ways in which an electron can be excited to a higher state

A

Incoming photon collides with an electron in an energy level
Incoming electron from current collides with an electron in an energy level
Heat transfer to an electron in an energy level

73
Q

What happens when an electron absorbs an amount of energy

A

If its the correct amount

Excited or ionised

74
Q

What is ionisation

A

When an electron gains enough energy to leave the atom

The energy required is always equal to the energy of the ground state but positive

75
Q

How does ionisation relate to work function

A

It doesn’t
Never mention it in emission spectra
Only applies to the photoelectric effect

76
Q

Explain excitation by a photon

A

Photon incident with an energy that is the exact difference between two energy levels
Photon is completely absorbed
Electron increases its kinetic energy
Instantaneously moves to a higher energy level

77
Q

Explain excitation by collision of an electron

A

Incident electron carries energy greater than or equal to the difference between two energy levels
Loses its energy to excite the orbital electron
Orbital electron increass uts kinetic energy
Instantaneously moves to the higher energy level
Orbital maintains any left over energy to move away (its Ek)

78
Q

Explain ionisation by an electron

A

Incident electron carries greater kinetic energy than the ground state energy
Orbital electron uses this amount to free the electron from the atom
Any left over kinetic energy used by the two electrons
Due to conservation of energy

79
Q

Compare excitation by a photon and by an electron

A

Photon is absorbed but electron is not absorbed
Photon must have an energy equal to the difference between two energy levels but incident electron must have an energy greater than or equal the difference between two energy levels
Photon involves photon electron involves electrons

80
Q

Explain multiple de-excitation

A

Can de excite more than one time and emit a photon each time it de excite
The path the electron takes is usually random

81
Q

How does the existence in the spectrum of lines of a definite wavelength support the view that atoms have discrete energy levels

A

Fixed energy levels means only certain transitions are allowed
Producing photons of a few different wavelenths corresponding to the differences in energy levels
If the electron energy was continous a continous spectrum would be formed

82
Q

What is fluorescence

A

When UV light is absorbed by certain substances or materials

Which them emit visible light

83
Q

Uses of fluorescence

A

Lighting homes
Streets
Marker pens
Fluorescent inks used in bank notes for security

84
Q

Why are fluorescent tubes more efficient than traditional tungsten filament lamp

A

Fluorescent tubes convertost of the energy of the supplied into light with only a few watts lost as heat

85
Q

Why is Mercury vapour at low pressure used in fluorescent tubes

A

Lower pressure means lower collisions per second
Allowing electrons to pass through easily
So a suffienct current can flow

86
Q

Purpose of the phosphor coating atom

A

Orbitals are close together so the enegy difference between each is less than the mercury
Orbital electrons of phosphor absorb the visible light emitted by mercury
This excites its orbital electrons
And when they de-excite they emit visible light
Because the energy levels are close together

87
Q

Explain how the fluorescent tube works

A
  1. Mercury atoms collide with eachother and with electrons in the tube
  2. Electrons in Mercury absorb energy
  3. Mercury atoms become excited/ionised
  4. Electrons return to ground state in Mercury atoms
  5. Emitting ultraviolet photons as they de excite
  6. UV photons absorbed by electrons in the atoms of the tube coating
  7. Coating atoms became excited
  8. Coating atoms de excite emitting visible light photons
88
Q

Evidence for light as a wave

A

Diffraction
Refraction
Polarisation

89
Q

Evidence for light as a particle

A

Photoelectric effect

Line spectra

90
Q

What is wave particle duality

A

Light behaves like a wave under some circumstances and a particle under others and sometimes a mixture of the two

91
Q

De Broglie wavelength formula

A

4

92
Q

De Broglie wavelength formula

A

λdb=h/mv

Or

λdb=h/p

93
Q

What is the de broglie wavelength

A

The effective wavelength of a particle

As determined by its momentum

94
Q

When will a particle diffract

A

If the de broglie wavelength is similar to the gap size

Will experience diffraction effects just like a wave

95
Q

When does maximum diffraction occur

A

De broglie wavelength is close to the size of the gap

96
Q

What can be used to examine atomic structure

A

Electron diffraction

Where the spacing between atoms (aperture) is the order of 2x10-¹¹m

97
Q

Why is electronic diffraction seen as rings and not linear

A

In 2 dimensions
Still a single slit interference but in 2D there are lots of different patterns at angles to form rings of constructive and destructive (light and dark) interference

98
Q

What happens in the electron diffraction

A

Electrons fores from a hot metal filament (electron gun)
And accelerated to some kind of crystalline structure (in this case graphite)
Electrons have a λdb similar to the spacing of carbon atoms in graphite so behave like waves passing through a diffraction grating
Diffraction electrons hit fluorescent screen/phosphor coating and form an interference pattern
By measuring the separation of the rings the spacing of the carbon atoms in graphite can be calculated

CRYSTALLOGRAPGY

99
Q

What is crystallography

A

Study of atomic arrangements in materials

100
Q

State what is meant by the duality of electrons

A

Electrons behaving as both a wave and a particle

101
Q

Why is the energy of photoelectrons normally less than the maximum kinetic energy

A

Some energy is lost in collisions when leaving the metal

102
Q

How is ultraviolet light generated in a fluorescent tube

A

High voltage used to accelerate s small number of free electrons through the tube
These free electrons collide with Mercury gas atoms and ionise and excite them
When the excited atoms return to ground state they emit a ultraviolet photon of energy equal to the difference in energy levels

103
Q

How does the production if bright rings in an electron gun suggest electeons behave like waves

A

Constructive interference/superposition where waves arrive in phase and produce maximum intensity

104
Q

Why do electrons emitted have a range of kinetic energies up to a maximum

A

Photon energy depends on frequency and is constant
One to one interaction between photon and electron
Maximum kinetic energy = photon energy - work function
More energy required to remove deeper electrons

105
Q

What must happen in order for an existing scientific theory to be modified or replaced with a new theory

A

Theory makes predictions tested by other scientists and reviewed
New evidence that is repeatable is checked by other scientists

106
Q

How could you demonstrate cathode rays are negative charge particles

A

Pass them between charged particles

Use a magnetic field

107
Q

What evidence does a cathode Ray tube show about nature of moving electrons

A

Cathode rays are negatively charged
Diffraction
Electron is behaving as a wave

108
Q

Describe the process that occurs when positron collides with a free lepton in water

A

Missing energy is carried off by third particle

Law of conservation of energy appears to be violated when beta particle has less than 1.2MeV

A Level Physics Paper 1 (8 decks)

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