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24.3Nuclear and Particle Physics

The Building Blocks of Matter — American System Physics Revision Notes & Topical Questions

10 min read 7 parts Lesson 5 of 5 1 quiz

📚 American System Physics Hub / Topical Revision
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Lesson summary

Name the fundamental particles of the Standard Model.Mass–energyE = mc2 (J = kg × (m/s)2).Minimum photon energy for pair production = 2mec2 = 1.02 MeV (electron rest energy 0.511 MeV).Energy gained by a charge accelerated through a p.d.: E = qV (J).Photon energy E = hf, so λ = hc…

  • Quark charges
  • Beta-minus decay at quark level
  • Beta decay is caused by the weak force
  • Pair production is the reverse
  • Show that a proton (uud) has a charge of +1e and a neutron (udd) has zero charge
  • A proton is accelerated through 5.0 MV. What kinetic energy does it gain in J and MeV?

Section 1 of 7The Building Blocks of Matter

Essential knowledge
24.3 The Building Blocks of Matter concept map: Mass–energy, Quark charges, Beta-minus decay at quark lev, Beta decay is caused by the w, Pair production is the reverse
Visual guide to the key ideas explained in this lesson.
Protons and neutrons were once thought fundamental, but high-energy scattering experiments showed they contain three point-like particles: quarks. Quarks are never found alone; they are always bound into hadrons by the strong force (carried by gluons).
Ordinary matter needs only up and down quarks plus electrons. Heavier quarks (strange, charm, bottom, top) and heavier leptons (muon, tau) are produced in high-energy collisions and cosmic rays but decay quickly.
Leptons (electron, muon, tau and three neutrinos) do not feel the strong force. Neutrinos are uncharged and almost massless; billions from the Sun pass through your body every second without interacting.
Beta decay is caused by the weak force
Beta decay is caused by the weak force inside a neutron a down quark changes to an up quark, emitting a W− boson which then decays into an electron and an electron antineutrino. The neutrino was proposed because beta electrons had a range of energies, which otherwise appeared to break conservation of energy.
Every particle has an antiparticle of equal mass and opposite charge.
  • When an electron meets a positron they annihilate, their mass turning into two gamma photons travelling in opposite directions, each of energy 0.511 MeV if the particles were nearly at rest.
    • (conserving momentum)
  • PET scanners in hospitals use this.
Pair production is the reverse
Pair production is the reverse a gamma photon with at least 1.02 MeV passing near a nucleus can create an electron–positron pair; any extra energy becomes their kinetic energy. In a magnetic field the two tracks curve in opposite directions.
The four fundamental forces are strong, electromagnetic, weak and gravitational.
  • In the Standard Model each force is transmitted by exchange of bosons: gluons (strong), photons, W and Z bosons (weak).
    • (electromagnetic)
  • The Higgs boson, discovered in 2012 at CERN, is linked to how particles acquire mass.
Linear accelerators (linacs) accelerate particles in a straight line through a series of tubes with alternating p.d.; synchrotrons use a ring of magnets and accelerating cavities, reusing the same path many times. The Large Hadron Collider is a 27 km ring.
Higher collision energies allow heavier particles to be created and smaller distances to be probed, because higher momentum means shorter de Broglie wavelength.
Conservation laws (charge, energy, momentum, baryon number, lepton number) are used to check whether a proposed particle reaction can happen.

Section 2 of 7Process & sequence

1
Charged particles (e.g. protons) are produced from a source and injected into an accelerator.
2
Alternating electric fields in accelerating cavities give the particles energy on each pass (linear accelerator or synchrotron).
3
Strong magnetic fields bend the beam into a circle and focus it (in a synchrotron the field increases as the particles speed up).
4
Two beams travelling in opposite directions are made to collide at detector sites.
5
The kinetic energy of the collision is converted into mass, creating new particles (E = mc2).
6
Detectors track the paths of products in magnetic fields; curvature direction gives charge sign and radius gives momentum, identifying the particles.

Electron-positron annihilationwrite these steps in this order; each step is a marking point.

1
An electron and a positron (its antiparticle) approach each other.
2
They meet and their total mass is converted to energy.
3
Two gamma photons are produced.
4
The photons travel in opposite directions so momentum is conserved.
5
Each photon has energy about 0.511 MeV (E = mc2).

Section 3 of 7Compare

The four fundamental forces
ForceActs onRelative strengthRangeCarrier
StrongQuarks (hadrons)1about 10−15 mGluon
ElectromagneticCharged particlesabout 10−2InfinitePhoton
WeakQuarks and leptonsabout 10−6about 10−18 mW and Z bosons
GravitationalAll particles with massabout 10−38InfiniteGraviton (not yet detected)
Quarks in nucleons
ParticleQuarksCharge calculationTotal charge
Protonu u d+⅔ + ⅔ − ⅓+1
Neutronu d d+⅔ − ⅓ − ⅓0
Antiprotonū ū d̄−⅔ − ⅔ + ⅓−1

Up quark charge +⅔e; down quark charge −⅓e. Antiquarks have opposite charges.

Section 4 of 7Worked examples

+1

Show that a proton (uud) has a charge of +1e and a neutron (udd) has zero charge.

  • Proton
    • +⅔ + ⅔ + (−⅓) = 4/3 − 1/3 = +1.
    • Neutron: +⅔ + (−⅓) + (−⅓) = 2/3 − 2/3 = 0.
An electron and a positron, both nearly at rest, annihilate producing two identical photons. Calculate the energy of each photon in J and its wavelength. (me = 9.11 × 10−31 kg)
  • Each photon energy = mec2 = 9.11 × 10−31 × (3.00 × 108)2 = 8.20 × 10−14 J (0.512 MeV). λ = hc/E = (6.63 × 10−34 × 3.00 × 108)/8.20 × 10−14 = 2.43 × 10−12 m.
5.0 MeV

A proton is accelerated through 5.0 MV. What kinetic energy does it gain in J and MeV?

  • E = qV = 1.60 × 10−19 × 5.0 × 106 = 8.0 × 10−13 J = 5.0 MeV.

Section 5 of 7Key terms

Section 6 of 7Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Section 7 of 7Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Elementary (fundamental) particle · Quark · Lepton · Hadron · Baryon · Meson · Antiparticle · Annihilation · Pair production · Neutrino · Standard Model · Particle accelerator
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary, never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

Words to avoid

1
protons and neutrons are fundamental particlesprotons and neutrons are made of quarks; quarks and leptons are fundamental.
2
an antiparticle has negative massan antiparticle has the same mass but opposite charge.
3
annihilation destroys energyin annihilation mass is converted into an equal amount of energy as gamma photons.
4
quarks have charges of +1 and −1quarks have fractional charges, +⅔e (up) and −⅓e (down).
5
the electron is a hadronthe electron is a lepton; it does not feel the strong force.

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