Particles Flashcards
(80 cards)
1
Q
What is a nucleon?
A
A particle found in the nucleus - either a proton or a neutron.
2
Q
What is a lepton?
A
A tiny fundamental particle e.g. electron
3
Q
What is a hadron?
A
Made up of quarks e.g. neutron, proton
4
Q
What is a positron?
A
The anti-particle to an electron.
5
Q
What is an anti-particle?
A
Same properties as the particle but with opposite charge.
6
Q
What is a quark?
A
Constituents found in hadrons.
7
Q
Nucleon Number?
A
(A) - Number of neutrons + Number of protons.
8
Q
Atomic Number?
A
(Z) - Number of protons
9
Q
What is an atom?
A
An atom consists of protons, neutrons and electrons.
10
Q
What is an isotope?
A
An atom of the same element with a different number of neutrons.
11
Q
What is specific charge?
A
The specific charge of a nucleus or ion is its charge per unit mass.
It is used in mass spectrometry to identify nuclei.
12
Q
Equation for specific charge?
A
Specific charge (Ckg^-1) = charge (C) / mass (kg)
13
Q
What particles represent charge?
A
Electrons and Protons
14
Q
What particles represent mass?
A
Protons and Neutrons
15
Q
Properties of strong force?
A
- Small range (3-4fm)
- Acts between nucleons
- Both attractive and repulsive (at<0.5fm) - the nucleus would collapse or explode otherwise e.g. like a spring that returns to equilibrium when stretched or compressed
- For light nuclei, the proton number = neutron number. The two particles must exist together.
- Heavier nuclei have more neutrons. Very large nuclei are radioactive (unstable).
16
Q
What is equilibrium separation?
A
There is a point when the resultant force is 0.
Since the electrostatic force of repulsion of two protons is generally less than 100N, the separation of the two protons is the same as the other nucleon combinations.
17
Q
What is alpha decay?
A
If an atom is unstable, it can result in an alpha particle leaving the nucleus.
This is a helium-4 nucleus with Z=2.
Alpha decay is mono-energetic - the alpha particles emitted have the same energy.
He²⁺, α²⁺
18
Q
Examples of alpha decay?
A
Am-241 → X-237 + α²⁺ (used in smoke alarms)
Po-210 → Pb-206 + α²⁺ (used in ionisers)
19
Q
Properties of weak force?
A
- There is a second nuclear force.
- This is called the weak force as it is about 1 millionth the value of the strong force.
- Its range is less too.
- It acts on both leptons and hadrons.
- It is this force that is responsible for beta decay.
20
Q
What are the two types of beta decay?
A
β− and β+
21
Q
What is beta decay?
A
- Beta decay happens when the nucleus emits an electron or a positron.
- A free neutron decays into a proton, an electron (β−) and an anti-neutrino.
- A free proton decays into a neutron, a positron (β+) and a neutrino.
- The beta particles emitted have a range of energies. The unaccounted for energy is carried away by the neutrinos (ν). Billions of neutrinos pass through our bodies every second (about 65 billion neutrinos per second pass through every cm² perpendicular to the direction of the sun).
22
Q
More on leptons?
A
They can be further split into neutrinos (ν) and anti-neutrinos (v̅).
They don’t build up to larger particles.
23
Q
More on quarks?
A
Quarks build up to larger particles.
They can be considered to be fundamental.
24
Q
Example of beta decay?
A
β− : C-14 → N-14 + β− + v̅
β+ : O-16 → N-16 + β+ + ν
25
What is gamma radiation (γ) ?
- If the nucleus is still unstable after emitting the alpha or beta radiation, it is in an excited state.
- Gamma radiation is given off. This is an electromagnetic wave and has no mass or charge.
- Gamma can be emitted at any stage of a decay series, at the same time as decay.
26
What are electromagnetic waves?
- An electric wave and a magnetic wave which travel together in phase.
- They are emitted when a charged particle loses energy. This can happen in an x-ray tube or when electrons move to a lower energy level.
- In a vacuum, electromagnetic waves travel at the speed of light where c=3x10^8ms^-1
27
Equation for wave speed?
c = fλ
28
Points of quantum theory?
- All forms of electromagnetic radiation are emitted in brief bursts or 'packets' of energy i.e. it is quantised
- The packets of energy, called photons, travel in one direction only in a straight line.
- When an atom emits a photon, its energy changes by an amount equal to the photon energy.
- The amount of energy, E, contained in each quantum is directly proportional to the frequency, f, of the radiation.
29
Planck's Equation?
```
The energy of a photon is given by:
E = hf
where:
E = energy of a photon (joule, J)
h = the planck constant (joule-second, Js)
= 6.63x10^-34 Js
f = frequency (hertz, Hz)
```
30
What is the electron-volt?
W = QV
= 1.6x10^-19 x 1
= 1.6x10^-19 J
1eV = 1.6x10^-19 J
31
What is the use of the eV?
These energies are very small, so photon energy is given in electron-volts (eV).
One electron volt is defined as the energy transferred when an electron is moved through a p.d. of 1 volt.
32
What is Dirac's theory?
For every type of particle, there is a corresponding antiparticle that:
- Annihilates the particle and itself if they meet, converting their total mass into photons.
- Has exactly the same rest mass as the particle.
- Has exactly opposite charge to the particle if the original particle has charge.
33
What is annihilation?
- When a particle and its corresponding antiparticle meet and their mass is converted into radiation energy.
- The rest mass can be calculated using the rest mass of the colliding particles and the equation E=mc²
34
What is pair production?
E=mc²
E=hf=hc/λ
-Dirac also predicted that a photon with enough energy could suddenly change into a particle antiparticle pair.
-The minimum energy required by the photon must be the rest energy of the particle pair.
-Example, if the rest energy of an electron is 0.511MeV, then the photon must have at least double that to produce the particle, antiparticle pair.
35
Pair production v β+ decay?
- In pair production, a photon changes into a positron and electron.
- In β+ decay, a proton-rich nucleus emits a proton which turns into a neutron, positron and neutrino.
36
How do particles interact?
When two objects interact, they exert equal and opposite forces on each other.
For example, if two protons approach each other, they repel and move away.
This occurs due to the exchange of a virtual photon- it is virtual because we can't detect it directly. If we intercepted it we would stop the exchange happening.
37
Four fundamental interactions?
Strong - EP: Gluon (for quarks) or pion (for nucleons)
Electromagnetic - EP: Photon
Weak - EP: W Boson (has mass + charge)
Gravitational: Graviton
38
Repulsive forces?
If two people on ice skates throw a ball between them, they move away from each other with equal momentum.
39
Attractive forces?
If two people on ice skates through a boomerang between them, the momentum causes them to move towards each other.
40
What are leptons?
These are fundamental particles that exist on their own.
They do not feel the strong force but are affected by the weak interaction.
Constituents of ordinary matter include e⁻ and νe.
Other leptons include μ⁻, νμ, τ⁻ and ντ.
Muons decay into electrons.
41
What are quarks?
Quarks are particles that exist bound together.
| Examples include u(+2/3, 1/3, 0), d(-1/3, 1/3, 0) and s(-1/3, 1/3, -1).
42
What are hadrons?
Hadrons are made up of quarks.
They are particles that feel the strong force but decay through the weak interaction.
There are two groups: Baryons and Mesons.
43
What are baryons?
- Consist of three quarks.
- Protons are UUD - they are the only stable baryon.
- Neutrons are UDD.
- The quark composition can be calculated by looking at the charges of the individual quarks.
- The antiparticles are just the anti of all the quarks.
44
What are mesons?
- Consist of a quark and an antiquark.
- Pions consist of up and down quarks. They are the exchange particles of the strong nuclear force.
- Kaons consist of a strange quark and either an up or down quark.
- Kaons decay into pions.
45
What are strange particles?
- Contain strange quarks.
- Produced through the strong interaction and decay through the weak interaction.
- Strangeness is only conserved in strong interactions.
46
What is beta decay?
Beta decay happens when the nucleus emits an electron or a positron.
A free neutron decays into a proton, an electron and an anti-neutrino - β-.
A free neutron decays into a neutron, a positron and a neutrino - β+.
Charge, spin, baryon number and lepton number must be conserved.
47
β- decay equations?
Particle: n → p⁺ + e⁻ + v̅
Quark: udd → uud + e⁻ + v̅
Simplified: d → u + e⁻ + v̅
The charge and baryon number must stay constant.
48
Weak interaction v strong interaction?
- One quark changes flavor.
- Strangeness isn't conserved (e.g. if a kaon decays into 2 pions).
- The opposite is true for a strong interaction.
49
LOOK AT INTERACTION DIAGRAMS.
---
50
What is photoelectric emission?
The emission of electrons from the surface of a metal when it is exposed to electromagnetic radiation of sufficiently high frequency.
Light of shorter wavelength is more energetic therefore the photons of the light have energy>work function.
51
Laws of photoelectric emission?
- The rate of electrons emitted is directly proportional to the intensity of the radiation.
- Photoelectrons are emitted with a range of kinetic energies. The maximum increases with frequency, and is independent of intensity.
- A minimum frequency is required to produce emission. Radiation below this threshold frequency cannot produce emission no matter how high the intensity.
52
What is the work function?
When a photon causes an electron to be ejected from the surface of a metal, the energy of the electron is always less than the energy of the incident photon.
This is because energy from the photon is used to remove the electron - the work function (Ф).
Ф=hf where f(Ф) = threshold frequency
c= f(Ф) x λ(Ф) if given threshold wavelength.
The metal with the largest Ф requires the highest frequency of light to release electrons.
53
What is Einstein's photoelectric equation?
When a photoelectron has absorbed a photon and escaped from the metal, it only has KE.
Conservation of energy show us that:
KE of electron = photon energy - energy needed to escape
Therefore, max KE:
KEmax = hf - Ф
54
What is stopping potential?
When an electron is emitted from a metal surface, it can be accelerated across a p.d.
If the p.d. is reversed, it will be decelerated.
The work done slowing the electron to 0 is calculated from the p.d. (stopping potential) across the gap.
If the metal surface is at a + potential, the photoelectrons will be attracted back to the surface If the potential is increased, the stopping potential Vs is the value at which photoemission is just prevented.
EKmax = 0
eVs = hf - Ф therefore Vs = (hf - Ф)/e
55
What happens when blue light is used?
-A dim blue light will make the metal eject a few electrons, producing a small measurable current.
The frequency of blue light is above the threshold frequency.
Therefore, the photons of blue light contain enough energy to eject electrons from the metal. A dim blue light has few photons, so few electrons are liberated. This gives a small photoelectric current.
-A brighter blue light will give a larger photoelectric current as there are more photoelectrons.
A bright blue light has more of these high-energy photons so more electrons are liberated, giving a larger photoelectric current.
56
What happens when red light is used?
-A red light, just as bright as the blue light, gives off no electrons at all and the current is 0.
The frequency of red light is too low.
Red light consists of photons that do not have enough energy to emit electrons from the metal.
Even though a bright red light has very many of these photons, not one has enough energy to eject an electron.
57
Equation for no. of electrons?
no. of electrons = total charge/charge on electron
58
Equation for no. of photons?
no. of photons = total energy/energy of photon
59
How are electrons arranged in atoms?
Electrons in an atom are trapped by the electrostatic force of attraction of the nucleus. They orbit the nucleus in discrete energy levels.
The energy of an electron in a shell is constant. An electron in a shell closer to the nucleus has less energy than an electron in a shell further from the nucleus.
60
How do energy levels work?
They have negative values as an electron is held within the atom by the electrostatic attraction of the nucleus and energy has to be supplied to remove it from the atom.
The lowest state of an atom is its ground state.
When an atom in the ground state absorbs energy, one of its electrons moves to a high energy level, so the atom is in an excited state.
61
What is excitation?
When an electron gains enough energy to move from a lower energy level to a higher energy level.
62
What is the evidence for energy levels in atoms?
Line Spectra.
- Emission spectra from hot gases obtained using a diffraction grating and spectrometer consist of discrete bright lines.
- So only certain discrete values of frequency of light are emitted.
- Photon energy, E=hf. So only discrete energies of photon are emitted.
- So only discrete energy transitions (E2 → E1) are allowed.
- This implies only certain discrete energy levels exist.
63
What are line spectra?
- Produced by atoms that have been excited either by heating or by an electrical discharge. The energy input raises the electrons to higher energy levels. When the electrons fall back to a lower level, there is an energy output. This occurs by the emission of a quantum of radiation, i.e. a spectral line.
- All elements have a line spectra.
- Photons in line spectra only have certain energy values thus electrons in those atoms can only have certain energy values.
- An electron can have an excited state or a ground state. When an electron falls from the excited state to the ground state, a photon is emitted. The energy of this photon is equal to the energy difference between the excited state and ground state.
64
What is a visible light spectrum?
Same as a line spectrum.
Formed by directing a narrow beam of white light at a triangular glass prism or at a diffraction grating.
It is continuous (i.e. there are no gaps) and contains light in wavelengths 400-700nm.
65
What is an emission line spectra?
Seen when a gas lamp is viewed through a narrow slit and diffraction grating.
It consist of separate colored lines (images of the slit) on a black background. Each colored line has its own unique wavelength.
The line spectrum for any given element is unique and can therefore be used to identify the element.
66
What is an absorption line spectra?
Obtained when white light has passed through cool gases.
Consists of black lines on a continuous white light spectrum background.
White light consists of photons having a continuous range of energies and wavelengths. Thus, when white light passes through a particular gaseous element, the only photons absorbed are those whose energy is exactly equal to one of the energy jumps between the energy levels of that element.
The spectrum is therefore continuous par black lines which have formed where the elements present in the cool gas have absorbed certain discrete wavelengths of the white light passing through the gas.
67
Examples of absorption line spectra?
A star emits white light containing all the wavelengths in the visible light spectrum. When this light passes through the cooler outer layers of the star, certain wavelengths are absorbed to give an absorption line spectrum.
The Sun's spectrum has many dark lines which are caused when light of specific wavelengths is absorbed by the cooler atmosphere around the sun.
NB:
The dark lines correspond to the emission lines of various elements in the atmosphere through which sunlight passes. This is because atoms can emit and absorb at the same wavelengths.
68
What is a transition?
Occurs in emission line spectra.
An atom emits light when one of its electrons falls from a higher to a lower energy level - this movement is a transition.
the energy of the emitted photon = the energy lost by the electron in the transition = the energy difference between the two levels involved
hf = hc/λ = E1 - E0
The greater the energy difference between the energy levels involved in a given transition, the greater the energy, higher the frequency and shorter the wavelength of the emitted photon.
If some materials are exposed to UV, they absorb the UV and then reradiate it. Some electrons fall straight to the ground state, giving off UV, but others go through several lower energy transitions, giving off visible light.
69
Why do different elements produce distinct line spectra?
Atoms of different elements have unique energy levels, therefore the wavelength of photons emitted is unique to the atom.
70
What are isolated atoms?
-Atoms in a gas are relatively far apart so have minimal interaction with each other.
Therefore, discrete line spectra can be obtained from hot gases.
-In solids and liquids, the atoms are much closer together, thus there is considerable interaction between the electrons from neighboring atoms.
Therefore, there are large numbers of closely spaced energy levels
The electromagnetic radiation emitted from solids and liquids forms spectra in which there are large numbers of lines, so close together that they appear as bands - band spectra.
71
What happens in an electron collision?
Atoms can be ionised by photons that have energies greater than the ionisation energy. The energy value doesn't have to be exact.
An incident electron with at least 8eV colliding with a particular atom can excite the electron from n=1 to n=2.
If it has 9eV, it can still excite n=1 to n=2 but continues with 1eV of KE after the collision.
72
What happens in photon absorption?
If photons strike materials, they need to have exactly the right energy to excite the electrons in an atom and raise them to higher energy levels. Otherwise, the photon will not be absorbed.
Only photons with exactly 8eV can be absorbed.
9eV photon will pass straight through.
10eV would be absorbed and excite n=1 to n=2.
73
What is ionisation?
An electron receives energy.
| An electron gains enough energy to escape the atom.
74
What is excitation?
An electron receives energy.
| An electron is promoted to a higher energy level.
75
Why can photons only of certain frequencies cause excitation of a particular atom?
- Electrons are placed in discrete energy levels and need to absorb an exact amount of energy to move to a higher level.
- Photons need to have a certain frequency to provide this energy (E=hf).
- The energy required is the same for a particular atom.
- All of the energy of the photon is absorbed in a 1 to 1 interaction.
76
How do fluorescent tubes work?
- They contain mercury vapor and inert gas.
- When current flows, ionisation occurs as electrons are accelerated by a large p.d. and collide with the mercury atoms, knocking electrons.
- Energy is transferred to the mercury electrons so they jump to higher energy levels.
- When electrons return to the ground state in the mercury atoms, they emit photons as they fall down the energy ladder.
- Some photons are in the visible range but most are in the UV range.
- A fluorescent coating absorbs the UV and electrons in the coating material are excited. They are raised to higher energy levels.
- Then they fall back to their ground state, emitting photons of visible light.
- White light is made from a mixture of red, green and blue photons, arising from different energy transitions. Fluorescent tubes give off a green color cast, indicating the presence of green photons.
77
What is De Broglie's idea?
We thought that light was a wave, and now we can see it can be a particle as well. Is it possible that electrons, considered particles, can also be waves?
78
What is electron diffraction?
When a beam of electrons strike thin layers of graphite carbon, most of them pass straight through but others pass at certain angles only, giving rings.
These rings are like the interference maxima produced when light waves pass through diffraction gratings.
This shows that electrons are diffracted by the gaps between atoms, and give maxima on the screen where they are in phase.
The wavelength of electron waves is very small, about the size of an atom. Therefore, the separation of the slits in a diffraction grating for electrons has to be small too - about the size of an atom.
79
What is De Broglie's equation?
For the electron: mv = h/λ
80
Why is the electron used to demonstrate wave-particle duality?
Comparative to other particles e.g. a proton, an electron can be easily accelerated therefore is easier to obtain.
