Physics A-Level (AQA) : Particles

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Simple model of the atom

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<p>Atom contains a positively charged nucleus composed of protons and neutrons and electrons that surrounds the nucleus.</p>

Atom contains a positively charged nucleus composed of protons and neutrons and electrons that surrounds the nucleus.

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Charge and mass of proton, neutron and electron in SI units

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Proton: charge +1.60x10^-19 C, mass 1.67x10^-27 kg
Neutron: charge 0 C, mass 1.67x10^-27 kg
Electron: charge -1.60x10^19 C , mass 9.11x10^-31 kg

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51 Terms

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Simple model of the atom

Atom contains a positively charged nucleus composed of protons and neutrons and electrons that surrounds the nucleus.

<p>Atom contains a positively charged nucleus composed of protons and neutrons and electrons that surrounds the nucleus.</p>
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Charge and mass of proton, neutron and electron in SI units

Proton: charge +1.60x10^-19 C, mass 1.67x10^-27 kg
Neutron: charge 0 C, mass 1.67x10^-27 kg
Electron: charge -1.60x10^19 C , mass 9.11x10^-31 kg

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Charge and mass of proton, neutron and electron in relative units

Proton: charge +1, mass 1
Neutron: charge 0, mass 1
Electron: charge -1, mass 1/1840

<p>Proton: charge +1, mass 1<br />
Neutron: charge 0, mass 1<br />
Electron: charge -1, mass 1/1840</p>
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What is specific charge?

Charge divided by mass, unit C kg-1

<p>Charge divided by mass, unit C kg-1</p>
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What is proton number (atomic number)?

Number of protons in the nucleus, symbol Z

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What is nucleon number (mass number)?

Number of protons and neutrons in the nucleus, symbol A

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Example of nuclide notation

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What are isotopes?

Atoms with the same number of protons and different number of neutrons

<p>Atoms with the same number of protons and different number of neutrons</p>
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What is the role of the strong nuclear force?

Overcomes electrostatic repulsion between protons and keeps the nucleus stable

<p>Overcomes electrostatic repulsion between protons and keeps the nucleus stable</p>
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How does the strong nuclear force vary with separation?

Closer than 0.5fm - repulsive
Between 0.5-3.0fm - attractive
Further than 3.0fm - no effect / zero

<p>Closer than 0.5fm - repulsive<br />
Between 0.5-3.0fm - attractive<br />
Further than 3.0fm - no effect / zero</p>
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What is alpha decay?

Unstable nucleus emits alpha particle (helium nucleus)
Equation: X(A,Z) -> Y(A-4,Z-2) + α(4,2)

<p>Unstable nucleus emits alpha particle (helium nucleus)<br />
Equation: X(A,Z) -&gt; Y(A-4,Z-2) + α(4,2)</p>
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What is beta (minus) decay?

A neutron in the nucleus changes into a proton and emits fast-moving electron and electron antineutrino
Equation: X(A,Z) -> Y(A,Z+1) + e-(0,-1) + _νe(0,0)

<p>A neutron in the nucleus changes into a proton and emits fast-moving electron and electron antineutrino<br />
Equation: X(A,Z) -&gt; Y(A,Z+1) + e-(0,-1) + _νe(0,0)</p>
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Why was the existence of the neutrino hypothesised?

To account for conservation of energy in beta decay. Observation showed energy of particles after beta decay was less than it was before. Some of the energy must had been carried away by undetected particles (neutrino).

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What is antiparticle?

For every type of particle, there is a corresponding antiparticle

<p>For every type of particle, there is a corresponding antiparticle</p>
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Comparison of particle and antiparticle masses, charge and rest energy

Particle and its corresponding particle have equal masses and rest energy, but opposite charge.

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Antiparticles of the electron, proton, neutron and neutrino

Positron, antiproton, antineutron, antineutrino

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Photon model of electromagnetic radiation

Electromagnetic waves are emitted as discrete wavepackets and each wavepacket is referred to as a photon.
E = hf, E = hc/λ
E = photon energy, J
h = planck constant, 6.63x10^-34 J s
f = frequency, Hz
c = speed of light, 3.00x10^8 m s-1
λ = wavelength, m

<p>Electromagnetic waves are emitted as discrete wavepackets and each wavepacket is referred to as a photon.<br />
E = hf, E = hc/λ<br />
E = photon energy, J<br />
h = planck constant, 6.63x10^-34 J s<br />
f = frequency, Hz<br />
c = speed of light, 3.00x10^8 m s-1<br />
λ = wavelength, m</p>
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What is annihilation?

A particle and a corresponding antiparticle meet and their mass is converted into radiation energy as two photons. Two photons are produced in this process to conserve momentum.

<p>A particle and a corresponding antiparticle meet and their mass is converted into radiation energy as two photons. Two photons are produced in this process to conserve momentum.</p>
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What is pair production?

A photon interacts with a nucleus or an electron and creates a particle-antiparticle pair, its radiation energy is converted into mass.

<p>A photon interacts with a nucleus or an electron and creates a particle-antiparticle pair, its radiation energy is converted into mass.</p>
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Energies involved in annihilation and pair production

Rest energy and kinetic energy of the particle-antiparticle pair is equal to the energy of the photon / two photons

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What are the four fundamental interactions?

Gravity, electromagnetic, weak nuclear, strong nuclear/interaction

<p>Gravity, electromagnetic, weak nuclear, strong nuclear/interaction</p>
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What is the concept of exchange particles?

Exchange particles are transferred between particles when a force acts between them. Exchange particles transfer energy and momentum.

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What is the electromagnetic force?

The force that acts between charged particles

<p>The force that acts between charged particles</p>
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What is the exchange particle of the electromagnetic force?

Virtual photons - they have zero mass, infinite range and no charge.

<p>Virtual photons - they have zero mass, infinite range and no charge.</p>
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What is the weak interaction?

The force that is responsible for β- decay, β+ decay, electron capture and electron-proton collisions.

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What is exchange particle of the weak interaction?

W bosons - they have a non-zero rest mass, a short range of no more than 0.001fm, and are positively charged (W+ boson) or negatively charged (W- boson).

<p>W bosons - they have a non-zero rest mass, a short range of no more than 0.001fm, and are positively charged (W+ boson) or negatively charged (W- boson).</p>
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Feynman diagram: β- decay

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Feynman diagram: β+ decay

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Feynman diagram: electron capture

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Feynman diagram: electron-proton collisions

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What are hadrons?

Particles that are subject to the strong interaction

<p>Particles that are subject to the strong interaction</p>
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The two classes of hadrons

Baryons / antibaryons and mesons

<p>Baryons / antibaryons and mesons</p>
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What are (anti)baryons?

Particles that consist of three (anti)quarks

<p>Particles that consist of three (anti)quarks</p>
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Examples of baryons and antibaryons

Baryons: proton, neutron
Antibaryons: antiproton, antineutron

<p>Baryons: proton, neutron<br />
Antibaryons: antiproton, antineutron</p>
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What are mesons?

Particles that consist of one quark and one antiquark

<p>Particles that consist of one quark and one antiquark</p>
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What is baryon number?

A quantum number that must be conserved in all interactions.
Baryon: 1
Antibaryon: -1
Non-baryon: 0

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What is the only stable baryon?

The proton, into which other baryons eventually decay

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What is the pion?

The exchange particle of the strong nuclear force

<p>The exchange particle of the strong nuclear force</p>
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What is the kaon?

A strange particle that can decay into pions

<p>A strange particle that can decay into pions</p>
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What are leptons?

Leptons are fundamental particles and are not subject to the strong interaction

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Example of leptons and anti-leptons

Leptons: Electron (e-), muon (μ-), electron neutrino (νe), muon neutrino (νμ)
Their antiparticles: Positron (e+), anti-muon (μ+), electron antineutrino (νe), muon antineutrino (νμ)

<p>Leptons: Electron (e-), muon (μ-), electron neutrino (νe), muon neutrino (νμ)<br />
Their antiparticles: Positron (e+), anti-muon (μ+), electron antineutrino (<em>νe), muon antineutrino (</em>νμ)</p>
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What is lepton number?

A quantum number that must be conserved in all interactions; lepton number for electron leptons and muon leptons must be conserved in all interactions.
Lepton: 1
Anti-lepton: -1
Non-lepton: 0

<p>A quantum number that must be conserved in all interactions; lepton number for electron leptons and muon leptons must be conserved in all interactions.<br />
Lepton: 1<br />
Anti-lepton: -1<br />
Non-lepton: 0</p>
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What is the muon?

A particle that decays into an electron

<p>A particle that decays into an electron</p>
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What are strange particles?

Strange particles contain strange quark. They are produced through the strong interaction and decay through the weak interaction.

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What is strangeness?

A quantum number to reflect the fact that strange particles are always created in pairs. It is conserved in strong interaction but can change by 0, +1 or -1 in weak interaction.
Strange quark: -1
Anti-strange quark: 1

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Properties of quarks and antiquarks

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Quark combinations of hadrons

Proton: u u d
Neutron: u d d
Antiproton: _u _u _d
Antineutron: _u _d _d
π+: u _d
π-: _u d
π0: u _u, d _d, s _s
K+: u _s
K-: _u s
K0: d _s
_K0: _d s

<p>Proton: u u d<br />
Neutron: u d d<br />
Antiproton: _u _u _d<br />
Antineutron: _u _d _d<br />
π+: u _d<br />
π-: _u d<br />
π0: u _u, d _d, s _s<br />
K+: u _s<br />
K-: _u s<br />
K0: d _s<br />
_K0: _d s</p>
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Decay of the neutron

n -> p + e- + _ve

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Change of quark character in β- decay

d -> u + e- + _ve

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Change of quark character in β+ decay

u -> d + e+ + ve

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What are conserved in interactions?

Energy, charge, momentum, baryon number, lepton number are conserved in all interactions. Strangeness is not conserved in weak interaction.