11 Quantum Physics Flashcards
1
Q
What do electrons behave like?
A
Particles
2
Q
What was the first particle accelerator?
A
The electron discharge tube.
3
Q
What does an electron discharge tube accelerate?
A
electrons.
4
Q
Who used an electron discharge tube?
A
J.J. Thompson
5
Q
Why did J.J. Thompson use an electron discharge tube?
A
To show the existence of the electron.
6
Q
What is the charge of an electron?
A
1.6 × 10‐¹⁹ C
7
Q
What is the mass of an electron?
A
9.1 × 10‐³¹ kg.
8
Q
What does an electron discharge tube do?
A
Fires electrons like bullets. The electron beam is made visible by hitting a zinc sulphide screen. A magnetic can be used to deflect the electrons and hence correctly predict the deflection of the beam by treating the electron as a little chunk of mass.
9
Q
What equation connects voltage, work done and charge?
A
V = W/Q
10
Q
How do you apply the work energy principle to an electron discharge tube?
A
The work done in accelerating the electron is equal to the energy transfered to the electron.
11
Q
What is the energy transferred to an electron in the form of?
A
Kinetic energy.
12
Q
What is the speed of light?
A
3×10⁸ m s‐¹
13
Q
What does light behave like?
A
Waves.
14
Q
What does light diffract to form?
A
Interference fringes.
15
Q
How does light show wave behaviour?
A
•Double slit interference using a laser shows light diffracting to produce interference fringes meaning the light is showing wave behaviour.
•light can produce rings of constructive and destructive interference when viewed through a microscope coated with lycopodium powder. Viewing light from sodium, radium and helium lamps produces different colours of ringed interference patterns.
16
Q
What is wave-particle duality?
A
Beams of electrons were shown to behave like waves, and light could behave like particles.
17
Q
Until the 20th century, what did experimental evidence show that electrons and light behave like?
A
•electrons behave like particles.
•light behaves like wave
This is now not always the case.
18
Q
What is a gold leaf electroscope for?
A
Showing charge.
19
Q
How does a gold leaf eletroscope work?
A
Electrons are scraped on to a zinc plate from a charged polythene rod. This causes the gold leaf inside the electroscope to rise. When photons of ultra-violet light are incident on the surface of the zinc plate, the gold leaf drops meaningnelectrons are being emitted from the zinc plate.
20
Q
What colour is a sodium lamp?
A
Bright orange
21
Q
What colour is a neon lamp?
A
Red
22
Q
What happens if you shine a sodium or neon light onto a gold leaf electroscope?
A
Nothing
23
Q
What happens if you shine a sodium lamp onto a gold leaf electroscope for a long period of time?
A
Nothing
24
Q
Will an ultra-violet light being shined onto a gold leaf electroscope cause electrons to be ejected?
A
Yes
25
What is the photoelectric effect?
The emission of electrons from a metal surface when photons of light of a high enough frequency are incident upon the metal surface.
26
What is the symbol for threshold frequency?
f₀
27
When does emission of electrons only occur?
When the incident photon frequency is above a specific minimum frequency.
28
What is the minimum frequency that the incident photons frequency needs to be above for the emission of electrons to occur?
Threshhold frequency (f₀)
29
What is red light always less than?
The threshold frequency (f>f₀).
30
What is blue light always more than?
The threshold frequency (f
31
What is the threshold frequency (f₀)?
The minimum photon frequency which will overcome the work function and hence cause the emission of electrons.
32
Does the emission of electrons always occur immediately if the frequency of the light is greater than the threshold frequency?
Yes, even if the light is very dim.
33
What did Planck work out (photons)?
In some experiments light behaves like particles, described as quanta which he called photons.
34
What did Einstein say can only be explained using the idea of photons?
The photoelectric effect.
35
In the photon energy equation what is 'h'?
Planck's constant.
36
What does photon energy depend on?
Frequency (or wavelength) of the light.
37
What is a photon?
A quantum of electromagnetic energy.
38
What did Eistein visualise when an electron absorbed a photon?
The electrons energy increased by an amount equal to the energy of the photon. This extra energy might be enough for the electron to escape from the metal
39
What are Einsteins 6 explanations of the photoelectric effect?
40
Why do electrons on the metal surface need the least energy?
To escape and so are ejected with the most kinetic energy.
41
What are electrons deeper inside the metal ejected with?
A range of lower kinetic energy.
42
What is the definition for work function (Φ)?
The lowest photon energy needed to extract an electron from the surface of a particular metal.
43
What is work function (Φ) measured in?
Joules
44
What is the symbol for work function?
Φ
45
What did Einstein realise that the work function (Φ) must equal ?
The energy of the threshold frequency photon.
Φ = hf₀.
46
What equation links energy, planks constant and threshold frequency?
E = hf₀
47
If the energy of the photon that the electron absorbs is bigger than the energy needed for the electron to escape from the metal, what does the energy left over (once the electron has escaped) appear as?
Kinetic energy of the electron.
48
What equation summarised Einsteins ideas?
Energy of the photon absorbed by the electron = energy needed to remove the elctron from the metal + left over kinetic energy.
49
Which electrons will have the most kinetic energy?
Those that are easiest to extract from the metal.
50
Which electrons will have the least kinetic energy?
The electrons below the surface that will need more energy to escape from the metal.
51
What is E kmax?
Electrons are ejected from the metal witha a range of energies from zero up to a maximum value.
52
What is the equation for a photon?
Minimum energy to eject an electron + maximum energy to eject an electron.
53
What is planks constant?
6.63 × 10‐³⁴ J s
54
What is the other unit for energy?
Electron volt.
55
How many joules is one electron volt equal to?
1.6 × 10‐¹⁹ J.
56
What is the conversion from electron volts to joules the same as?
The charge of an electron.
*on the data sheet*
57
What are photoelectrons?
Electrons emitted from a metal due to light shining on the metal.
*no different to other electrons*.
58
What is Milikan's Photoelectric effect experiment?
Different frequencies of light are used to illuminate a particular metal and the max ke of the emitted electrons were measured.
59
What graph is this?
Millikan’s graph
A graph of max ke vs frequency
60
What can be seen from Millikan’s graph?
Red light is used.
61
In Millikan’s photoelectric effect experiment, what does the fact that this plot was not dependent upon the intensity of the incident light imply?
The interaction was like a particle which gave all its energy to the electron and ejected it with that energy minus that which it took to escape the surface.
62
In Milikan’s photoelectric effect experiment, light below what frequency would not eject electrons?
4.39 x 10^14 Hz.
63
In Milikan’s photoelectric effect experiment, light with a wavelength longer than what would not eject electrons?
683 nm
64
What is the equation for Ekmax?
Ekmax = hf - Φ
65
What does the gradient of Millikan’s graph represent?
Planck’s constant
66
What does the Y intercept on Millikan’s graph represent?
Work function
67
What does the X intercept on Millikan’s graph represent?
Threshold frequency
68
What is a photocell?
A vacuum tube with a concave photocathode (emitter).
69
In a photocell, when light of a frequency greater than the threshold frequency shines on the photocathode, what happens?
Electrons are emitted.
70
On a photocell, what does the concave photocathode do?
Focuses electrons onto a thin wire anode. This does not block light.
71
On a photocell, when the concave photocathode is at a positive potential, what are attracted to it?
Electrons
72
What can photocell be used for?
To allow robots to follow a line or to switch streetlights on automatically at night.
73
In the photoelectric effect, what does light behave like?
Little ‘chunks’ (or quanta) of energy that are emitted from a light source.
74
What are packets of light called?
Photons
75
Directing light from a laser onto a double slit makes light behave like a wave, producing what?
Constructive and destructive interference just like 2 wave machines in a tank of water.
76
Since light can show both wave and particle behaviour, what is light said to exhibit?
Wave-particle duality.
77
Why is light said to exhibit wave-particle duality?
Because light can show both wave and particle behaviour.
78
What is light a part of?
The electromagnetic spectrum.
79
What is the electromagnetic spectrum made up of?
Oscillating electric and magnetic fields that travel through a vacuum at a speed of 3.0 x10^8 m s^-1. They all show wave-particle duality.
80
What is einstein’s explanation of the photoelectric effect?
81
What was Louis de Brogile’s hypothesis?
‘Any moving particle or object had an associated wave’
82
What is needed to test Brogile’s hypothesis?
An electron discharge tube consisting of a small metal coil heated by a 6.3 V supply causing it to eject electrons. (Thermonic emission).
83
84
Explain how to test Broglie’s hypothesis.
-Use an electron discharge tube consisting of a small metal coil heated by a 6.3V supply.
-A large voltage (5000v) accelerates the electrons towards a lattice of carbon atoms. The electrons pass through the gaps between the carbon atoms.
-When electrons are ejected from the heated coil and then accelerated by the voltage they are behaving like particles.
-BUT when the electrons pass through the gaps between the carbon atoms they behave like waves creating a pattern of constructive and destructive interference.
-The rows of carbon atoms create gaps through which the electron beams diffract and spread out.
-The screen is coated with fluorescent material which emits light when electrons hit it causing orbiting electrons to be exited instantly to a higher energy level before de-exited with the emission of a discrete photon.
- A pattern of bright rings is produced when constructive interference occurs.
85
What is de Broglie’s wavelength also known as?
The associated wavelength.
86
What is de Broglie’s wavelength formula?
λ = h/p
*p=the partials momentum and h=planck’s constant*
87
Why can it be proven that electrons behave like particles and waves?
Because they are produced and accelerated by an electron gun, and, as waves, in forming an interference pattern after they pass through the gaps between carbon atoms in a lattice.
88
What can it be said that beams of electrons exhibit?
Wave-particle duality.
89
Due to electrons exhibiting wave-particle duality, what are beams of particles such as electrons sometimes called?
Matter waves.
90
Wave-particle summary table
91
What are orbital electrons also known as?
Atomic electrons
92
What are orbital electrons?
Electrons trapped in orbit around the atom’s nucleus.
93
When are nodes and antinodes created on a stationary wave?
They are created when a wave is trapped in a specific discrete space.
94
According to Broglie what did the atom consist of?
Orbital electrons existing as stationary waves.
95
How did De Broglie envision electrons around atoms?
As stationary waves bent around a circle as shown below.
Electrons can only orbit the nucleus in specific orbits because only those orbits produce the pattern of nodes and antinodes.
96
What did Neils Bohr suggest?
That the electrons could only travel in specific orbits around the nucleus.
These orbits are discrete are now called energy levels.
97
What is the lowest energy level?
N=1
98
What do the horizontal lines represent?
Discrete energy levels. (Shells or orbits)
99
What energy level is the energy level closest to the nucleus?
The n=1 energy level.
100
What is the n=1 energy level called?
The ground state.
101
What are energy levels assigned?
Negative energy values to represent that the electrons are trapped within the atom and that energy must be supplied to remove an orbital electron from the atom. Their value must be compared to a common reference value.
102
In atomic physics, what does 0.0eV correspond to?
A state in which all partials are at rest and infinitely distant from each other.
103
What is ionisation?
The removal of an orbital electron from an atom.
104
What is ionisation energy?
The minimum energy required to remove an electron from the ground state of an atom.
105
What does ionising an atom require? Why?
Work because an electron, which is negatively charged, must be moved completely away from the positively charged nucleus.
106
What are the 2 other methods of ionising atoms of a gas?
1. Heating a gas to a high temperature makes the gas atoms move quicker. Collisions between these fast moving atoms can knock orbital electrons off causing the gas atoms to be ionised.
2. Shining light onto a gas can cause a photon to be absorbed by one of the atoms which supplies enough energy to remove an orbital electron.
107
What does this diagram represent?
A free electron colliding inelastically with an atom. The energy transferred to the electron is only enough to make an orbital electron transition from the ground state to a higher energy level.
Their value must orbital electron remains within the atom but has made a transition from a lower energy level to a higher energy level.
The atom has been exited.
108
What is a transition?
The movement of an orbital electron from one energy level to another.
109
What is excitation?
The process by which an electron gains energy and transitions from a lower to a higher energy level.
110
When is an atom described as exited?
If one of its orbital electrons has transitioned from a lower to a higher energy level and is no longer in the ground state.
111
What happens to orbital electrons after transitioning to a higher energy level?
It is unstable and so transitions down emitting a photon of discrete energy in the process.
112
What is the energy of an emitted photon?
The difference between the energy levels that the electron transitions are between.
113
What do each transition emit a photon of?
Discrete wavelength.
114
115
How can you find the wavelengths of the photons emitted?
Using the formula
nλ = d sinθ
116
What did observing line sprectra from discharge tubes give physicists the idea of?
that orbital electrons after transitioning electrons only orbit in specific discrete energy levels but could undergo discrete transitions between those levels.
117
What does an emission spectra appear as?
Coloured lines on a black background.
118
Can only one part of a photon be absorbed?
NO. Only whole photons are absorbed, never just part of one.
119
What photons can be absorbed?
Only photons which the same energy as the difference between 2 levels will be absorbed.
120
Explain excitation caused by photons being absorbed by orbital electrons.
The electron that has been exited to a higher energy level by the absorbtion of a photon is now unstable and so transitions back to the ground state either directly (emitting 1 photon) or indirectly (by emitting several less energetic photons).
121
How does an absorption spectra appear?
As dark lines on a continuous spectrum background.
122
What is the definition of fluorescence?
The property of a material to absorb ultraviolet photons and emit visible photons.
123
What is the basis of the fluorescent tube?
some atoms can absorb ultra-violet photons then emit visible light photons.
124
explain how a fluorescent discharge tube works?
1. The tube contains low pressure mercury vapour meaning the mercury atoms are quite far apart so the electrons have time to speed up between collisions.
2. When the lamp is switched on, the electrodes are warmed by the starter unit which ejects electrons into the tube.
3. the mains-voltage accelerates the electrons to high speeds.
4. These high-speed electrons collide inelastically with the mercury atoms causing an orbital electron to be exited to a higher energy level.
5. the orbital electron de-excites to a lower energy level emitting ultra-violet photons.
6. The electrons within the fluorescent material on the inside of the glass tube, are excited by absorbing the ultra-violet photons.
7. Electrons then de-excite and emit photons within the visible spectrum
125
what does quantum physics provide?
a mathematical description of the dual particle-like and wave-like behaviour and interactions of energy and matter.
126
what are quantum effects used in?
mobile phones to sense the pressure applied to the screen, scanning tunnelling electron microscopes, LED's, microwave ovens, lasers, erasing data on a usb drive, and transistor design.
127
what do digital cameras exploit the quantum nature of light for?
to produce high resolution images and quantum computing is possibly the next big thing holding out the prospect of astonishing speed and security.
