Conservation Laws & Particle Interactions (AQA A Level Physics): Flashcards
Exam code: 7408
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Define exchange particle.
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Define exchange particle.
An exchange particle is a virtual particle exchanged between two interacting particles that carries a fundamental force between them.
Which fundamental interaction is the weakest, and which is the strongest?
Gravity is the weakest fundamental interaction
The strong interaction is the strongest fundamental interaction
The electromagnetic and gravitational interactions have .......... range.
The electromagnetic and gravitational interactions have infinite range.
State the ranges of the weak and strong interactions.
Weak interaction: up to 10-18 m
Strong interaction: ~10-15 m
Which property must a particle have to be affected by (a) the gravitational interaction and (b) the electromagnetic interaction?
(a) Mass — required for the gravitational interaction
(b) Charge — required for the electromagnetic interaction
True or False?
The strong interaction affects all particles.
False.
The strong interaction only affects hadrons (particles made of quarks); leptons are not affected by the strong interaction.
Name the exchange particle for the strong, weak and electromagnetic interactions.
Strong: pion
Weak: W boson or Z0 boson
Electromagnetic: photon
Define graviton.
The graviton is the theorised exchange particle for the gravitational interaction; it has not yet been discovered, since gravity is too weak to have a noticeable effect in particle interactions except between very large masses.
Define hadron.
A hadron is a particle made up of quarks, and is therefore subject to the strong interaction.
State three properties of the photon as the electromagnetic exchange particle.
No mass
No charge
Is its own antiparticle
What effect does the electromagnetic force have between two like-charged particles, for example two electrons?
Repulsion, due to the exchange of a virtual photon between them.
The electromagnetic force binds electrons to the nucleus through the .......... between the negative electrons and the positive nucleus.
The electromagnetic force binds electrons to the nucleus through the attractive force between the negative electrons and the positive nucleus.
Why can leptons not interact via the strong force?
Leptons are not made up of quarks, so they cannot interact via the strong force, which only acts on hadrons.
True or False?
The photon, as an exchange particle, has mass.
False.
The photon has no mass and no charge; it is its own antiparticle.
Name the two exchange particles of the strong interaction and state where each acts.
Pion: acts between nucleons
Gluon: acts between quarks
Define the weak interaction.
The weak interaction is the fundamental interaction responsible for the radioactive decay of atoms; its exchange particles are the W-, W+ or Z0 boson.
In β- decay, what happens to a neutron, and which particles are emitted?
A neutron turns into a proton, emitting an electron and an anti-electron neutrino.
In β+ decay, what happens to a proton, and which particles are emitted?
A proton turns into a neutron, emitting a positron and an electron neutrino.
In β- decay, the exchange particle is the .......... boson.
In β- decay, the exchange particle is the W- boson.
What is electron capture, and which particles are produced?
Electron capture is when an atomic electron is absorbed by a proton in the nucleus, producing a neutron and an electron neutrino.
What is an electron-proton collision, and which particles are produced?
An electron collides with a proton, producing a neutron and an electron neutrino — the same products as electron capture.
True or False?
Electron capture and an electron-proton collision are mediated by the same sign of W boson.
False.
Electron capture is mediated by the W+ boson, while an electron-proton collision is mediated by the W- boson — the decay equations are identical except for the sign of the W boson.
Why is it the weak interaction, rather than the electromagnetic interaction, that facilitates a collision between an electron and a proton?
Although electrons and protons are attracted to each other via the electromagnetic interaction, it is the weak interaction that facilitates the actual collision between them.
Define vertex on a Feynman diagram.
A vertex is where particles and exchange particles meet on a Feynman diagram, representing a point of interaction (electromagnetic, weak or strong).
What do the y-axis and x-axis represent on a Feynman diagram?
y-axis: time
x-axis: space
How are particles and exchange particles represented on a Feynman diagram?
Particles: straight lines with an arrow pointing forward in time
Exchange particles: wavy lines with no arrows
On a Feynman diagram, hadrons/quarks are shown on the left and .......... on the right, and they must never meet at a vertex.
On a Feynman diagram, hadrons/quarks are shown on the left and leptons on the right, and they must never meet at a vertex.
Name three quantities that must be conserved at each vertex of a Feynman diagram.
Charge
Baryon number
Lepton number
True or False?
Lines on a Feynman diagram are allowed to cross.
False.
Lines on a Feynman diagram must not cross over.
In β- decay, which quark transformation occurs, and how does the W- boson relate to it?
A down quark turns into an up quark; the W- boson carries away the negative charge of the down quark, which provides the negative charge for the electron and antineutrino.
In β+ decay, which quark transformation occurs, and how does the W+ boson relate to it?
An up quark turns into a down quark; the W+ boson carries away the positive charge of the up quark, which provides the positive charge for the positron and neutrino.
List the quantities that must be conserved in a particle interaction.
Charge, Q
Baryon number, B
Lepton number, L
Strangeness, S
Energy (mass-energy)
Momentum
Which conservation law is relaxed in weak interactions, and by how much can it change?
Strangeness does not need to be conserved in weak interactions; it can change by 0, +1 or –1.
Quantum numbers such as Q, B, L and S can only take .......... values.
Quantum numbers such as Q, B, L and S can only take discrete values (for example 0, +1, –1, 1/2).
True or False?
In the worked example of Λ0 decay, the interaction was found to be permitted.
False.
Baryon number was not conserved (+1 ≠ 0), so the interaction is not permitted.
In the example of Kaon decay, strangeness is found not to be conserved. What does this indicate about the interaction?
It shows the decay must proceed via the weak interaction, since strangeness does not need to be conserved in weak interactions.
In the Λ0 decay worked example, was charge conserved?
Yes — 0 = –1 + 1 + 0, so charge is conserved (although baryon number was not, meaning the overall interaction is not permitted).
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