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AQA A Level Physics > Photoelectric effect and Energy levels > Flashcards

Photoelectric effect and Energy levels Flashcards

1
Q

What 3 particles are atoms of all elements made up of

A

Protons, neutrons and electrons

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2
Q

What is the specific charge of a proton

A

+1.60 x 10^-19 C

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3
Q

What is the specific charge of a neutron

A

0

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4
Q

What is the specific charge of an electron

A

-1.60 x 10^-19 C

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5
Q

Mass of a proton

A

1.673 x 10^-27

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6
Q

Mass of a neutron

A

1.675 x 10^-27

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7
Q

Mass of an electron

A

9.11 x 10^-31

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8
Q

Relative charge of a proton

A

+1

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9
Q

Relative mass of a proton

A

1

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10
Q

Relative charge of a neutron

A

0

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11
Q

Relative mass of a neutron

A

1

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12
Q

Relative charge of an electron

A

-1

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13
Q

Relative mass of an electron

A

1/2000

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14
Q

Why is a stable atom neutral

A

Since protons and electrons have equal and opposite charges and a stable atom has an equal number of both, the overall charge is neutral

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15
Q

Specific charge of a particle

A

Ratio of its charge to its mass

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16
Q

Specific charge =

A

charge / mass

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17
Q

What is the SI units for specific charge

A

C/kg

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18
Q

How to calculate the specific charge of a particle

A
  1. Determine the number of neutrons and protons
  2. Calculate the total mass by doing mass number x mass of 1 nucleon (1.67 x 10^-27)
  3. Calculate the total charge by doing number of electrons x ( -1.60 x 10^-19)
  4. Substitute values into the equation
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19
Q

What will the specific charge be is there is a gain in electrons

A

Negative

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20
Q

Nuclide

A

A group of atoms containing the same number of protons and neutrons

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21
Q

What does the A, Z represent in the notation

A

A - nucleon number/ mass number

Z - proton number

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22
Q

Nucleon number

A

Total number of protons and neutrons in the nucleus

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23
Q

Isotopes

A

An atom of the same element that has the same/ equal number of protons but a different number of neutrons

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24
Q

Why are isotopes unstable and what to they do as a result

A

They have an imbalance of neutrons and protons this means they constantly decay and emit radiation to achieve a more stable form

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25
Q

Isotopic data

A

Relative amounts of different isotopes of an element found within a substance

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26
Q

What is isotopic data used for

A

To identify an isotopic signature within organic and inorganic materials

Used in radioactive dating

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27
Q

What is an electronvolt used to represent

A

Very small energies because quantum energies tend to be much smaller than 1J

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28
Q

Equation for voltage in terms of energy and charge

A

V = E / Q

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29
Q

1 eV = how many joules

A

electron of charge 1.6x10^-19 x 1V = 1.6x10^-19J

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30
Q

What is the definition of an electronvolt

A

The energy gained by an electron travelling through a potential difference of one volt

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31
Q

What happens to a charge particle when it is accelerated through a PD

A

it gains KE

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32
Q

Equation for eV if an electron accelerates from rest

A

eV = 1/2mv^2 as eV=KE gained here

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33
Q

What is the photoelectric effect

A

The phenomena in which electrons are emitted from the surface of a metal upon absorption of EM radiation

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34
Q

What are electrons removed from a metal called

A

Photoelectrons

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35
Q

Why is the photoelectric effect important

A

It is evidence that light is quantised/carried in discrete packets

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36
Q

How do we know light is quantised or carried in discrete packets

A

Each electron can absorb only a single photon and this means ONLY the frequencies of light above a threshold frequency will emit a photoelectron

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37
Q

Threshold frequency

A

The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal

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38
Q

Threshold wavelength

A

The longest wavelength of incident electromagnetic radiation that would remove a photoelectron from the surface of a metal

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39
Q

High frequency =

A

Low wavelength

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40
Q

Work function

A

Minimum energy required to release a photoelectron from the surface of a metal

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41
Q

How many photons can a single electron absorb

A

1

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42
Q

When can an electron escape from the surface of the metal

A

If it absorbs a photon which has an energy equal to the work function or higher

43
Q

What colour of visible light has highest energy

A

Violet as it has highest frequency

44
Q

What colour of visible light has the lowest energy

A

Red

45
Q

Stopping potential

A

The potential difference required to stop photoelectron emission from occurring

46
Q

Emitter plate

A

The photons arriving at the metal plate causing photoelectrons to be emitted

47
Q

Collector plate

A

Electrons that cross the gap are collected at this other metal plate

48
Q

How is a photoelectric current produced

A

There is a flow of electrons across the gap which results in an emf between the plates that causes a current to flow around the rest of the circuit and it therefore becomes a photoelectric cell with a photoelectric current.

49
Q

What happens to the electrons which escape with enough KE

A

They can overcome the attraction between them and the emitter plate and can cross to the collector plates

50
Q

Energy of an incident photon =

A

Work function + Maximum kinetic energy of the photoelectron

51
Q

Energy of a photon =

A

Planck’s constant x frequency

52
Q

What happens to the energy within a photon when it strikes the surface

A

It is transferred to the electron to release it from the surface of a metal and the remaining energy is converted to KE to the emitted photoelectron

53
Q

Photoelectric equation

A

E= hxf = work function + KE max

54
Q

Why is the KE of the photoelectrons independant of the intensity of radiation

A

Each electron can only absorb one photon

55
Q

What is the KE dependant on

A

The frequency of the incident radiation

56
Q

What is intensity a measure of

A

The number of photons incident on the surface of the metal

57
Q

Where are the photoelectrons with max KE found

A

Surface of the metal as they dont require much energy to leave the material

58
Q

What is photoelectric current

A

Number of photoelectrons emitted per second

59
Q

What is photoelectric current proportional to

A

Intensity

60
Q

Ionisation of an atom

A

The removal, or addition, of an electron from, or to, an atom when given sufficient energy

61
Q

Excitation of electrons

A

When an electron is given enough energy to move up an energy level, but not enough to leave the atom

62
Q

When does fluorescence occur

A

When an electron in an atomic orbital absorbs energy from an interaction with a photon or collision with another atom

63
Q

What happens to the electrons when an electric current is passed through the vapour in a FL tube

A

They are excited and move to a higher energy level

64
Q

Why do electrons de-excite

A

Higher energy level is unstable

65
Q

What do the electrons do as they de-excite

A

Release some of the energy in the form of a UV photon

66
Q

What causes the fluorescent glow

A

The UV light released excites the electrons in the phosphor coating and so visible light photons are released which causes the glow

67
Q

What is a photon

A

A discrete packet of EM energy with no mass

68
Q

How can electrons gain energy and move up the energy levels

A

If it absorbs energy by :

Collisions with other atoms or electrons
Absorbing a photon
A physical source e.g. heat

69
Q

How can elements be identified by their line spectrum

A

No 2 elements can emit the same set of spectral lines

70
Q

Difference between 2 energy levels =

A

Energy of lower level - Energy of the higher level

71
Q

Describe how a fluoroscent tube works

A

The tube contains low presure mercury gas and thermionic emission causes electrons to be released from the cathode.

High voltage accelerates electrons from one end to the next

Electrons collide with the atoms and transfer some KE to excite the electrons in the shells to go in energy levels.

Then, they de-excite and emit a photon (mainly UV) due to high frequency

The UV is absorbed by the phosphorous paint coating, exciting an electron, and then de-excite, emitting visible light

72
Q

State and explain the effected on the emitted electrons by increasing the frequency of light (2)

A

The max KE of the released electrons increases because by increasing the frequency it also increases the energy

73
Q

State and explain the effect on the emitted electrons by increasing the intensity of light (2)

A

It will increase the number of electrons being emitted because there are now more photons striking the metal surface per second.

74
Q

Explain what is meant by validated evidence (2)

A

Been proven corrected
Experiment/observation needs to be performed

75
Q

Explain why the KE of the emitted electrons has a maximum value (2)

A

Same energy from photons
Energy required to remove electron varies

76
Q

Explain why the emitted electrons have a range of kinetic energies up to a maximum value (4)

A

Photons have energy dependant on frequency
There is a 1 to 1 interaction between photon and electron
KE max = energy of photon- work function
More energy needed to remove deeper electrons

77
Q

Describe the process by which mercury atoms become excited in a fluroscent tube (3)

A

Electrons flow through the tube, and collide with mercury atoms raising the electrons to a higher energy level

78
Q

What is the purpose of the coating on the inside surface of the glass in a fluorescent tube (3)

A

Photons emitted from mercury atoms are high energy photons.
These photons are absorbed by the powder.
The powder emits photons in the visible spectrum.

79
Q

Explain the difference between excitation and ionisation (3)

A

In either case, an electron receives energy
Excitation is when electrons absorb a photon and so gain enough energy to move up to a higher energy level.
Ionisation is when an electron receives enough energy to leave the atom

80
Q

Explain why only photons of certain frequencies cause excitation in a particular atom (4)

A

Different atoms have different energy levels.
Electrons need to absorb a photon with enough energy to move up energy levels
High frequency = high energy
Electrons occupy discrete energy levels

81
Q

How can an electron gain energy and move up energy levels

A

By absorbing energy either by:

Collisions with other atoms or electrons
Absorbing a photon
A physical source such as heat

82
Q

Line Spectra

A

A phenomenon which occurs when excited atoms emit light of certain wavelengths which correspond to different colours.

The emitted light can be observed as a series of coloured lines with dark spaces in between.

83
Q

Why can elements be identified by their line spectrum

A

Each element produces a unique set of spectral lines.

No two elements emit the same set of spectral lines

84
Q

Emission spectra (when electron goes to lower energy level and emits a photon)

A

Contains a set of discrete wavelengths, represented by coloured lines on a black background

85
Q

What is line spectra evidence of

A

Transitions between discrete energy levels in atoms

86
Q

Absorption spectra (when atom becomes excited by absorption of a photon)

A

A continuous spectrum containing all the colours with dark lines at certain wavelengths.

87
Q

What do the dark lines in an absorption spectra mean

A

They correspond exactly to the differences in energy levels in an atom

88
Q

When excited electrons return to lower levels why are some wavelengths missing

A

The photons are emitted in all directions rather than in the original direction of the white light

89
Q

difference between 2 energy levels ( E2 - E1) =

A

h x f

90
Q

What is wave-particle duality of light

A

Light can behave as a particle (i.e. photon) and a wave

91
Q

How does light act a a particle

A

Light interacts with matter such as electrons

92
Q

Evidence for light acting as a particle

A

Photoelectric effect

93
Q

How does light act as a wave

A

Light propagates through space as a wave

94
Q

Evidence for light acting as a wave

A

Diffraction
Interference of light in Young’s Double Slit experiment

95
Q

Explain the photon model of light

A

EM waves carry energy in discrete packets called photons

E = hf

Each electron can absorb only a single photon - this means only the frequencies of light above the threshold frequency will emit a photoelectron

96
Q

How do electrons behave like waves

A

They can be diffracted, and produce a pattern of concentric rings

97
Q

de broglie wavelength =

A

h / mv where mv is momentum

98
Q

Electron diffraction pattern when at low accelerating voltage

A

Low speed therefore low KE
Longer wavelength and so more diffraction/greater radius

99
Q

How does momentum affect wavelength

A

Smaller momentum = Longer wavelength because p=mv

100
Q

Larger momentum =

A

Shorter wavelength

101
Q

Process of the development of scientific theories

A

Evaluation of the theory
Peer review
Validation when there is enough evidence

102
Q

What is checked during the peer review process

A

Validity
Originality
Significance

103
Q

How to evaluate scientific claims

A

Check if :

It is repeatable and reproducible

104
Q

How are KE and stopping voltage related

A

KE = eVs
As KE increases the stopping voltage also increases

AQA A Level Physics (17 decks)

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