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Physics A - Level > Particles and Radiation (3): Quantum Phenomena > Flashcards

Particles and Radiation (3): Quantum Phenomena Flashcards

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1
Q

Define the photoelectric effect

A

This is when photoelectrons are emitted from the surface of a metal when electromagnetic radiation above a threshold frequency is incident on it

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2
Q

What was the assumption before people observed the results from the photoelectric effect?

A

That light was only a wave

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3
Q

What is the definition of the threshold frequency?

A

An intrinsic numerical property of metals where incident EM radiation/photons above which will cause photoelectrons to be emitted

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4
Q

Describe 4 assumptions about how people thought that the photoelectric effect worked vs what they found after

A
  1. Electrons absorb energy continuously from the EM waves
    -> Electrons actually absorb energy in discrete ‘all or nothing’ packets called photons
  2. All frequencies of light should be able to eject electrons from the surface of a material given enough time and intensity
    -> Electrons are only ejected from a metal when the incident frequency is above the threshold frequency of that metal
  3. Photoelectric emission occurs with delay
    -> Actually occurs without delay provided the f is greater than or equal to the f(min)
  4. Wave energy is determined by E=I=A^2 therefore increasing the intensity means that the emitted photoelectrons should have more kinetic energy
    -> No relationship between intensity and Ek of photoelectrons emitted, instead as the frequency above the threshold frequency increases the Ek of the photoelectrons will increase
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5
Q

Does intensity of the EM radiation effect the Ek of the emitted photoelectrons?

A

no

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6
Q

Does intensity of the EM radiation effect the number of the emitted photoelectrons?

A

yes - but only once the frequency is above the threshold frequency of the metal

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7
Q

How many photons can each electron absorb?

A

1

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8
Q

What was the discovery that occurred due to the photoelectric effect?

A

That light can behave as a particle:
-> has both wave-like and particle-like properties

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9
Q

What did Einstein theorise light was made of?

A

Discrete ‘all or nothing’ packets called photons

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10
Q

How can you change the frequency of light?

A

Change its colour

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11
Q

Which colour of light has the highest frequency?

A

Violet

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12
Q

Which colour of light has the lowest frequency?

A

Red

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13
Q

What is the equation for the frequency of an electromagnetic wave?

A

f=c/(lambda)

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14
Q

What is the equation for the energy of a photon?

A

E = hf = hc/(lambda)

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15
Q

What is the energy gained by an electron on the surface of a metal?

A

hf - as the electron gains the energy from 1 photon

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16
Q

Work function defintion

A

The minimum energy required for an electron to escape from the surface of a metal

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17
Q

Will all emitted photoelectrons be from the surface of the metal?

A

no

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18
Q

If the value of the work function, theta, is smaller than the value of E gained by the electron from the photon, where does the extra energy go?

A

Into the kinetic energy of the emitted photoelectron

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19
Q

What is the equation for the Ek (max) of an emitted electron?

A

Ek (max) = hf - (theta)

hf = Ek (max) + theta

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20
Q

What is required for an electron emission to take place from a metal surface?

A

Ek (max) > 0 or hf > (theta)

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21
Q

What is the equation for the threshold frequency of a metal material?

A

f (min) = (theta)/h

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22
Q

What is the definition of the stopping potential?

A

The potential you would need to apply across the metal to stop the photoelectrons with the maximum kinetic energy

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23
Q

By applying a potential to your metal, what are you physically doing?

A

Giving the plate a sufficiently positive charge for the photoelectrons with the maximum kinetic energies to be attracted back

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24
Q

What is the equation for Ek (max) of a photoelectron when a stopping voltage has been applied?

A

Ek (max) = eVs

Where:
Vs - stopping voltage

  • Derived from: energy = charge x voltage
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25
Q

What is the value of Ek (max) when the stopping voltage is applied?

A

0

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26
Q

What did Plank suggest?

A

That the energy of each vibrating atom is quantised -> only certain levels of energy (energy levels) are allows corresponding to E = hf

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27
Q

What does the word quantised mean?

A

Discrete and distinct amounts

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28
Q

What happens if an electron does not gain enough energy from an incident photon in order to escape the metal surface?

A

It will collide repeatedly with other electrons and positive metal ions, quickly losing its extra kinetic energy

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29
Q

Describe the structure of a vacuum photocell

A

A glass tube containing a metal plate known as the photocathode and a smaller metal electrode opposite it known as the anode. The anode and cathode are connected to a microammeter in a series circuit.

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30
Q

Describe how a vacuum photocell works

A

When light with a frequency greater than the threshold frequency of the photocathode metal is incident, photoelectrons are emitted. These electrons are attracted to the anode and the microammeter is able to measure the photoelectric current.

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31
Q

What is the photoelectric current produced in a vacuum photocell proportional to?

A

The number of electrons per second that transfer from the cathode to the anode, hence the intensity of the incident light on the photocathode

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32
Q

What is the equation for the number of photoelectrons per second that transfer from the cathode to the anode?

A

I/e

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33
Q

What is the equipment used to measure the Ek (max) of photoelectrons emitted for different frequencies of incident light?

A

A photocell

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34
Q

Describe what graph you plot and the features of the graph when you are investigating how Ek (max) changes with different frequencies of light

A

Using equation Ek (max) = hf - (theta)
Plot Ek (max) against f
- The gradient of your line = plancks constant, h
- The y-intercept of your line = work function
- The x-intercept of your line = threshold frequency

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35
Q

What is the shape of the line of best fit on an Ek(max) - frequency graph? Why?

A
  • Straight line through the threshold frequency and work function
  • Gradient of the line of best fit always equals planks constant, h
36
Q

What is the process of creating ions called?

A

Ionisation

37
Q

What is an ion?

A

A charged atom - an atom which has lost or gained a certain number of electrons

38
Q

What are the two methods of ionisation that you need to know?

A
  • Ionising radiation, Alpha, beta or gamma, passing through substances and colliding with the atoms -> this may knock off an electron
  • Electrons passing through a fluorescent tube create ions when they collide with atoms of the gas or vapour in the tube
39
Q

What is the definition of an electron-volt?

A

A unit of energy equal to the work done when an electron is accelerated through a pd of 1V

40
Q

1 eV = …?

A

1.6 x 10^-19J

41
Q

Energy levels in atoms are…?

A

Discrete

42
Q

Do all atoms that collide with electrons get ionised?

A

no

43
Q

What is the definition of excitation?

A

When electrons in atoms gain energy from colliding with free electrons causing them to move up energy levels

44
Q

If an electron in an atom gains enough energy from a collision to leave the atom, this process is known as…?

A

Ionisation

45
Q

If you were given energy levels of an atom what value would correspond to the ionisation energy an electron at n=1?

A

The value at n=1

46
Q

What happens after an electron has been excited?

A
  • It will de-excite going back to a lower energy
  • The electron will release the energy difference in the form of a photon
47
Q

In what value are energy levels described by?

A

eV

48
Q

What is the lowest energy state of an atom also known as?

A

The ground state

49
Q

Is an excited atom stable or unstable?

A

Unstable

50
Q

What is the equation for the energy of an emitted photon during a de-excitation?

A

hf = E1 - E2

51
Q

What is the only case when a photon can be absorbed by an electron?

A

When the energy of the photon is exactly equal to the gain in the electrons energy (the difference between the final and initial energy levels of the atom)

52
Q

How do you go from eV -> J?

A

V = E/Q
eV x (1.6 x 10^-19)

53
Q

How do you go from eV -> V?

A

divide by (1.6 x 10^-19)

54
Q

Can electrons de-excite in multiple step?

A

Yes

55
Q

Why do certain substances fluoresce?

A
  • When they absorb UV radiation atoms in the substance absorb the UV photons and become excited
  • When the atoms de-excite them emit visible photons
56
Q

Describe what the set up of a fluorescent tube is?

A
  • A glass tube with fluorescent coating on its inner surface
  • Tube contains mercury gas at low pressure
  • Tube has filament electrodes connected to a mains supply at either end - they are heated to a hot enough temperature so that ionisation of the gas can occur
57
Q

Describe what happens when a fluorescent tube is on?

A
  • Ionisation and excitation of mercury atoms occur as they collide with each other and electrons in the tube (electrons produced from thermionic emission)
  • The mercury atoms emit UV photons, Visible light photons and lower energy photons when they de-excite
  • The UV photons are absorbed by some of the atoms in the fluorescent coating causing them to excite
  • The coating atoms de-excite via other energy levels emitting photons with a range of lower energies to be emitted, usually visible light photons
58
Q

Why do energy levels have more negative values as you get closer to the nucleus?

A

As the electrons in those energy levels have a greater potential energy and potential is always negative?

59
Q

What are the 3 series for the hydrogen atom energy levels that you need to know?

A
  • Lymen series
  • Balmer series
  • Paschen series
60
Q

What is the spectral line observed when there is an electron de-excitation in the Lymen series (Hydrogen atom)?

A

UV

61
Q

What energy level do electrons de-excite to in the Lymen series (Hydrogen atom)?

A

n = 1

62
Q

What is the spectral line observed when there is an electron de-excitation in the Balmer series (Hydrogen atom)?

A

Visible light

63
Q

What energy level do electrons de-excite to in the Balmer series (Hydrogen atom)?

A

n = 2

64
Q

What is the spectral line observed when there is an electron de-excitation in the Paschen series (Hydrogen atom)?

A

IR

65
Q

What energy level do electrons de-excite to in the Paschen series (Hydrogen atom)?

A

n = 3

66
Q

What is the range of wavelengths for the visible light section of EM radiation?

A

400nm -> 650nm

67
Q

What happens when you shine white light through a prism?

A

You see a continuous spectrum of colours

68
Q

What happens when you use a tube of glowing gas/fluorescent tube as a light source and shine it through a prism?

A

Discrete lines of different colours show up

69
Q

How can we identify the element from a line spectrum?

A
  • The wavelengths of the lines of a line spectrum of an element are characteristic of the atoms of that element
  • By measuring the wavelengths of a line spectrum we can therefore identify the the element producing that light as each line spectrum is unique
    -> The energy levels of each type of atom in an element are unique to that atom
70
Q

A line on a line spectrum is produced from photons all of the same…?

A

energy

71
Q

How would you get a line absorption spectrum?

A

Passing white light through a cooled gas

72
Q

What do the blacked out lines on a line absorption spectrum correspond to?

A

Possible differences in energy levels as the atoms in the gas can only absorb photons of an energy exactly equal to the energy difference between 2 energy levels

73
Q

What does a line absorption spectrum look like?

A

Continuous spectrum of all the possible colours with some blacked out lines

74
Q

What are some examples of light acting as a wave?

A

Interference and diffraction

75
Q

What are some examples of light acting as a particle

A

photoelectric effect

76
Q

Can electrons also have wave or particle properties?

A

Yes

77
Q

When does diffraction of light occur?

A

When light is passed through a narrow slit and the light emerging spreads out (in the same way as water does after passing through a gap)

78
Q

What did De Broglie hypothesise?

A

That if light could have particle properties then particles can have wave properties

79
Q

How can the wave-like nature of electrons be observed?

A

Electron diffraction

80
Q

What is the equation for the De Broglie wavelength?

A

lambda = h/mv

81
Q

What shows that electrons can behave like particles?

A

That they are deflected by a magnetic field

82
Q

What does the de Broglie wavelength of a particle depend on?

A
  • Velocity
  • Mass
83
Q

Describe how electron diffraction works

A
  • A narrow beam of electrons is passed through a thin layer of metal foil
  • As the positive ions are in rows in a regular pattern they cause the electron beam to be diffracted
  • The beam of electrons is produced by attracting electrons from a heated filament wire to a positively charged metal plate
84
Q

What shape is formed form electron diffraction?

A

Pattern of concentric rings

85
Q

What happens to the shape of the electron diffraction pattern as you increase the velocity of the electrons?

A
  • The De Broglie wavelength decreases when v increases
  • Using w = (lambda)D/s, decreasing the wavelength decreases the fringe spacing OR less diffraction occurs
  • Therefore, spacing of the concentric rings decreases
86
Q

Describe how a cathode ray tube works to produce a energy beam of fast electrons?

A
  • Thermionic emission of electrons occur on the wire
  • This is because the heating done by the electric current overcomes the work function of the material
  • The electrons are then accelerated by the electric field between the positive anode and negative cathode
87
Q

What was used as a proof of discrete energy levels being present in atoms?

A

Line spectra of atomic hydrogen

Physics A - Level (23 decks)

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