ForteACADEMICS
HSC Physics · Module 8
Self-guided lesson

From the Universe to the Atom

Five levels, from the Big Bang down to quarks. Read the short idea, play with the lab bench, then answer the tester questions. Your score saves in this browser.

Gold boxesThe must-know facts. Learn these first.
Dark lab benchesDrag, click and fire things. See the physics happen.
TestersAnswer, get instant feedback. Calculations accept 1.5e7 or 1.5x10^7.
1
Level 1

Origins of the elements

The Big Bang, step by step

Tap each stage. Watch energy cool into matter.

Timeline of the universe

Must know · radiation into matter
  • Matter formed from energy through E = mc²: hot photons made particle–antiparticle pairs.
  • Order: energy → quarks and leptons → protons and neutrons → H and He nuclei → atoms → stars.
  • At about 380 000 years atoms formed and the universe became transparent. That light is the CMB.

Evidence 1: galaxies are running away

Drag the slider to make a galaxy recede faster. Watch its hydrogen lines slide towards red.

Redshift labH₀ = 70 km s⁻¹ Mpc⁻¹
Hα seen at
Distance d = v / H₀
Shift z = v / c
Must know · three pieces of evidence
  • Redshift and Hubble's law v = H₀d: further galaxies recede faster, so space is expanding. Slipher measured redshifts; Hubble measured distances with Cepheid variables.
  • Cosmic microwave background (Penzias and Wilson, 1965): a 2.7 K blackbody glow from every direction.
  • Light-element abundance: about 75% hydrogen, 25% helium by mass, as the Big Bang predicts.

Reading starlight

Three kinds of spectra, then a star you can heat up.

Spectrahydrogen lines shown

Continuous: hot dense object. Emission: hot thin gas, electrons drop down. Absorption: cool gas in front of a hot source, electrons jump up. Same element, same line positions.

Star thermometer · Wien's lawλmax = b / T
Peak wavelength
Spectral class
Looks
Must know · five things a star's spectrum tells us
  • Temperature: peak wavelength (Wien) and which lines are strongest. Classes hot to cool: O B A F G K M.
  • Composition: which elements' lines appear.
  • Moving towards/away: Doppler shift (blue/red).
  • Rotation: lines broadened.
  • Density: dense dwarfs have broad lines; thin giants have narrow lines.

The HR diagram and how stars live

Tap a region, or play a star's life story.

Hertzsprung–Russell diagramEach dot is a star, coloured by temperature

Temperature decreases to the right. Luminosity goes up.

Must know · life cycles and fusion
  • Sun-like: main sequence → red giant → planetary nebula → white dwarf.
  • Massive: main sequence → red supergiant → supernova → neutron star or black hole.
  • Main-sequence stars fuse H → He. p–p chain in Sun-like stars; CNO cycle (C, N, O as catalysts) in hotter, more massive stars. Same net result: 4 H → He + energy.
  • Energy comes from lost mass: E = Δm c². Fusion stops at iron; heavier elements come from supernovae.
2
Level 2

Inside the atom

Thomson finds the electron (1897)

Switch the fields on and off. The trick is finding when the beam goes straight.

Cathode ray tubeelectron beam

Must know · Thomson's method
  • Step 1: balance the fields so the beam is straight: qE = qvB → v = E/B
  • Step 2: remove E; the beam circles in B: qvB = mv²/r → q/m = E/(B²r)
  • q/m was ~2000× bigger than hydrogen's and the same for every cathode metal, so cathode rays are tiny particles in all atoms: electrons. Model: plum pudding.

Millikan's oil drop game

Slide the voltage until the drop hovers. Then work out its charge.

Oil dropm = 2.45 × 10⁻¹⁵ kg · d = 1.0 cm
Drop is
E = V/d
Must know · Millikan
  • Stationary drop: qE = mg → q = mgd/V
  • Every charge was a whole-number multiple of 1.6 × 10⁻¹⁹ C, so charge is quantised. With Thomson's q/m this gave the electron's mass.

Geiger–Marsden: firing at gold

Compare what the plum pudding model predicted with what actually happened.

Alpha scatteringnuclei drawn far bigger than real
Straight through0
Deflected0
Bounced back0
Must know · gold foil results
  • Most went straight through → atom is mostly empty space.
  • A few deflected through big angles → concentrated positive charge.
  • About 1 in 8000 bounced back → a tiny, dense nucleus holds almost all the mass.
  • Rutherford's problem: an orbiting electron accelerates, so it should radiate and spiral in. It also can't explain line spectra.

Chadwick's neutron (1932)

The experimentfollow the arrows

Others said the mystery radiation was gamma rays. Chadwick showed gamma photons couldn't give the protons that much energy. Using conservation of momentum and energy, it had to be a neutral particle about the mass of a proton.

3
Level 3

The quantum atom

Make a photon

Pick where the electron starts and where it lands.

Hydrogen energy levelsnot to scale
Starts at
Lands on
Wavelength
Photon energy
Series · region
Must know · Bohr
  • Electrons sit in stationary states and don't radiate there.
  • Jumping between levels emits or absorbs one photon: E = hf = Eᵢ − E_f
  • Angular momentum is quantised: mvr = nh/2π
  • Rydberg: 1/λ = R(1/n_f² − 1/nᵢ²) · Lyman → n=1 (UV) · Balmer → n=2 (visible) · Paschen → n=3 (IR)
  • Limits: only works for hydrogen; can't explain line intensities or line splitting; no reason why orbits are quantised.

Why only certain orbits? de Broglie

Slide n. The wave only survives when it joins up with itself.

Electron as a standing wave2πr = nλ

Must know · matter waves
  • de Broglie: everything has a wavelength λ = h/mv
  • A whole number of wavelengths must fit the orbit: 2πr = nλ = nh/mv → mvr = nh/2π. This explains Bohr's rule.
  • Evidence: Davisson and Germer (1927) got a diffraction pattern from electrons on a nickel crystal.
  • Schrödinger: electrons described by a wave function giving the probability of where they are (orbitals). Works for all atoms.
4
Level 4

The nucleus

Decay machine

Press a decay and watch A and Z change. Challenge: turn uranium-238 into lead-206.

Nucleus
Start with:

Must know · decay types
  • α: helium nucleus ⁴₂He. A − 4, Z − 2. Stopped by paper, most ionising.
  • β⁻: neutron → proton + electron + antineutrino. Z + 1. Stopped by aluminium.
  • β⁺: proton → neutron + positron + neutrino. Z − 1.
  • γ: high-energy photon. No change to A or Z. Needs thick lead.
  • Always balance A (top) and Z (bottom). The neutrino was predicted because beta energies varied.

Half-life in action

Each atom has a 50% chance of decaying per half-life. Nobody knows which ones.

100 atoms

Gold dots = still radioactive. Dashed curve = the prediction N = N₀e^(−λt), with λ = ln2 / t½

Binding energy: why both fission and fusion work

Tap a nucleus on the curve.

Binding energy per nucleonhigher = more stable

Iron-56 sits at the top: the most stable nucleus.

Must know · mass defect and energy
  • Mass defect Δm = mass of separate nucleons − mass of nucleus.
  • Energy: E = Δm c² or Δm (u) × 931.5 MeV. For reactions, Δm = reactants − products.
  • Fusion of light nuclei and fission of heavy nuclei both move towards iron, raising binding energy per nucleon, so both release energy.
  • Fission: U-235 + slow neutron → two fragments + 2–3 neutrons → chain reaction. Reactors use control rods (absorb neutrons) and a moderator (slows neutrons).
5
Level 5

Deep inside the atom

The Standard Model

Tap any particle.

Every known fundamental particlegravity not included
quarksleptonsforce carriersHiggs

Quarks feel the strong force. Leptons don't. Bosons carry forces.

Build a hadron

Tap a slot to change the quark. Three quarks make a baryon.

Quark builderu = +⅔ · d = −⅓ · s = −⅓
Total charge
You made
Must know · the particle zoo
  • 6 quarks (up, down, charm, strange, top, bottom) and 6 leptons (electron, muon, tau + 3 neutrinos). Every particle has an antiparticle.
  • Baryons = 3 quarks: proton uud, neutron udd. Mesons = quark + antiquark.
  • Forces: strong–gluon, EM–photon, weak–W and Z (beta decay), gravity–not in the model.
  • Higgs boson gives mass; found at the LHC in 2012.

Beta decay, zoomed in

β⁻ decay at the quark levelweak force

One down quark turns into an up quark by emitting a W⁻ boson. The W⁻ instantly becomes an electron and an antineutrino. udd (neutron) → uud (proton).

Particle accelerators

Must know · why smash things together?
  • Electric fields accelerate charged particles; magnetic fields steer them (linear accelerators, cyclotrons, synchrotrons like the LHC).
  • Higher energy → shorter de Broglie wavelength → probes smaller structures.
  • Collision energy becomes new heavy particles via E = mc².
  • Evidence: quarks (SLAC, 1968: electrons bounced off three hard points inside protons) · W and Z (CERN, 1983) · Higgs (LHC, 2012).
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