Module 8: From the Universe to the Atom
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HSC Physics 2026: recall practice based on the 2017 syllabus.
92 Terms
What is the Big Bang model?
A model of the Universe's evolution from a hot, dense early state.
How can radiation produce a particle–antiparticle pair?
Sufficient energy converts into mass while conservation laws hold.
Why did more matter than antimatter remain after the early Universe?
A small matter–antimatter asymmetry survived annihilation; its origin remains unresolved.
What observation did Hubble use to infer universal expansion?
More distant galaxies generally have greater recession velocities.
What is cosmological redshift?
The stretching of light's wavelength as space expands.
What evidence indicates the early Universe was hot and dense?
Cosmic microwave background radiation and primordial light-element abundances.
What was primarily produced during Big Bang nucleosynthesis?
Hydrogen, helium and small amounts of lithium.
What produces a star's absorption lines?
Cooler outer gas absorbs particular wavelengths from the continuous spectrum.
Why are hydrogen Balmer lines visible in stellar spectra?
They involve transitions between n = 2 and higher levels.
What two main quantities are plotted on a Hertzsprung–Russell diagram?
Luminosity and surface temperature or spectral class.
How does surface temperature change from left to right on a standard H–R diagram?
It decreases.
Where are main-sequence stars found on an H–R diagram?
Along the diagonal from hot luminous to cool dim stars.
What powers a main-sequence star?
Hydrogen fusion into helium in its core.
What is the net result of the proton–proton chain?
Four hydrogen nuclei form helium-4, releasing energy and other particles.
What role does carbon play in the CNO cycle?
It acts as a catalyst in hydrogen fusion.
In which stars is the CNO cycle especially significant?
Hotter, more massive main-sequence stars.
What happens to core hydrogen fusion as a star leaves the main sequence?
Core hydrogen is depleted and fusion shifts to surrounding shells.
How are elements heavier than iron made in astrophysical environments?
Mainly through neutron capture and subsequent decay.
Why does iron fusion not normally power a star?
Fusing iron generally requires energy rather than releasing it.
What evidence showed cathode rays carried negative charge?
They deflected towards the positive electric plate.
What did the same charge-to-mass ratio in different cathode-ray tubes suggest?
Cathode rays contain a universal constituent of matter.
What quantity did Thomson determine for cathode-ray particles?
The electron's charge-to-mass ratio.
What did Millikan determine using charged oil drops?
The magnitude and quantisation of elementary charge.
How could electron mass be found from Thomson's and Millikan's results?
Divide the elementary charge magnitude by the charge-to-mass ratio.
What did most alpha particles do in the Geiger–Marsden experiment?
They passed through the thin gold foil with little deflection.
What did rare large-angle alpha scattering imply about the atom?
Positive charge and most mass are concentrated in a small nucleus.
Why could the plum-pudding model not explain large-angle alpha scattering well?
Its diffuse positive charge could not produce the observed strong deflections.
What did Chadwick's beryllium radiation experiments reveal?
A neutral particle with mass similar to a proton: the neutron.
What stability problem does Rutherford's classical atomic model face?
Orbiting accelerating electrons should radiate energy and spiral inward.
What did Bohr propose about allowed electron energies in hydrogen?
Only discrete energy levels are allowed.
How does a hydrogen atom emit a line-spectrum photon?
An electron transitions to a lower energy level.
How is emitted photon energy related to atomic energy levels?
Ephoton = Ei − Ef for a downward transition.
What transitions produce the hydrogen Balmer series?
Transitions ending at n = 2.
What is a major limitation of Bohr's atomic model?
It does not accurately describe multi-electron atoms.
What does de Broglie's hypothesis associate with a moving particle?
A matter wavelength λ = h/p.
What experiment supported the wave nature of electrons?
Davisson–Germer electron diffraction from a crystal.
How does electron diffraction support de Broglie's hypothesis?
Electron interference patterns demonstrate wave-like behaviour.
How did de Broglie explain allowed Bohr orbits qualitatively?
A whole number of electron wavelengths fits around the orbit.
What does a quantum-mechanical orbital describe?
A probability distribution for finding an electron.
What did Schrödinger contribute to the atomic model?
A wave equation used to calculate quantum states.
What is an isotope?
Atoms of the same element with different neutron numbers.
What is an alpha particle?
A helium-4 nucleus: two protons and two neutrons.
How does alpha decay change a nucleus?
Mass number decreases by four and atomic number by two.
What occurs in beta-minus decay?
A neutron becomes a proton, electron and electron antineutrino.
How does beta-minus decay change atomic and mass numbers?
Atomic number increases by one; mass number stays unchanged.
What is gamma radiation?
High-energy electromagnetic radiation from nuclear de-excitation.
How does gamma emission change atomic and mass numbers?
Neither changes.
Which radiation is generally most penetrating: alpha, beta or gamma?
Gamma, although shielding depends on energy and material.
What is radioactive half-life?
The time for the number of undecayed nuclei to halve statistically.
What does radioactive activity measure?
The number of nuclear decays per unit time.
What is the SI unit of radioactive activity?
The becquerel: one decay per second.
Which supplied equation models radioactive decay?
N = N₀e^(−λt).
Which supplied equation links decay constant and half-life?
λ = ln 2 / t½.
What happens to activity after one half-life for a pure isotope?
It halves on average.
What is nuclear fission?
The splitting of a heavy nucleus into smaller nuclei.
What is a nuclear chain reaction?
Released neutrons trigger further fission events.
How is a controlled fission chain reaction sustained?
On average one neutron from each fission causes another fission.
What do control rods do in a nuclear reactor?
Absorb neutrons to regulate the chain reaction.
What does a moderator do in a thermal nuclear reactor?
Slows neutrons to make further fission more likely.
What is nuclear fusion?
The combining of light nuclei to form a heavier nucleus.
Why can both fusion of light nuclei and fission of heavy nuclei release energy?
Their products have greater total binding energy.
What is nuclear binding energy?
Energy required to separate a nucleus into its constituent nucleons.
What is mass defect of a nucleus?
The mass of separated nucleons minus the nucleus's mass.
How is binding energy calculated from mass defect?
Ebinding = Δmc².
How is energy released in a nuclear reaction found from rest masses?
Multiply initial minus final rest mass by c².
What quantities must balance in a nuclear reaction?
Charge, nucleon number and total energy–momentum, among other conserved quantities.
What evidence shows protons contain smaller constituents?
Deep inelastic electron scattering reveals point-like constituents.
What is a hadron?
A composite particle made of quarks and/or antiquarks.
What is a baryon?
A hadron containing three quarks.
What is a meson?
A hadron containing a quark and an antiquark.
What is the proton's quark composition?
Two up quarks and one down quark: uud.
What is the neutron's quark composition?
One up quark and two down quarks: udd.
What is the electric charge of an up quark?
+2/3 of the elementary charge.
What is the electric charge of a down quark?
−1/3 of the elementary charge.
What type of fundamental particle is an electron?
A lepton.
Which fundamental interaction binds quarks within hadrons?
The strong interaction.
Which four fundamental interactions are recognised?
Strong, electromagnetic, weak and gravitational.
What does a particle accelerator use to increase charged-particle energy?
Electric fields.
What is the role of magnetic fields in many particle accelerators?
They steer and focus charged-particle beams.
Why do high-energy collisions help probe subatomic structure?
They access shorter wavelengths and can produce massive particles.
What does a particle detector record to test particle-physics models?
Collision products and their measurable properties.
How do absorption and emission wavelengths for the same atomic transition compare?
They are the same when the energy gap is the same.
What determines a star's spectral classification sequence OBAFGKM?
Primarily its surface temperature and associated spectral features.
What does a star's position above the main sequence generally indicate?
It is more luminous at that temperature, often a giant.
Why is a distant galaxy's spectral line shifted to longer wavelengths?
Cosmic expansion stretches the wavelength during propagation.
What is the energy condition for producing a particle–antiparticle pair?
Available energy must cover their rest energies and conserve momentum.
What do nuclear reaction equations use to track charge conservation?
The total atomic number on each side.
What do nuclear reaction equations use to track nucleon-number conservation?
The total mass number on each side.
What does the Rydberg equation calculate for a hydrogen transition?
The wavelength associated with a transition between energy levels.
What does a more tightly bound nucleus have per nucleon?
Greater binding energy per nucleon.
What observation confirmed that neutrinos are particles beyond protons, neutrons and electrons?
Neutrino detection following beta-decay energy and momentum studies.
How is total luminosity different from apparent brightness?
Luminosity is emitted power; brightness is power received per area.