Particles and Radiation
Particles and radiation is the A-level Physics topic covering the structure of the atom, radioactive decay, particle classification (hadrons, leptons and quarks) and the photon model of electromagnetic radiation. It builds on GCSE atomic structure and introduces the conservation laws that govern which particle interactions can occur.
Method
- Learn the standard particles (proton, neutron, electron, neutrino) and their charges, masses and specific charges (charge divided by mass).
- Use nuclide notation (mass number A, atomic/proton number Z) to write balanced equations for alpha, beta-minus and beta-plus decay, checking that A and Z balance on both sides.
- Classify particles as hadrons (baryons made of three quarks, or mesons made of a quark-antiquark pair) or leptons (fundamental particles that do not feel the strong force).
- Apply conservation laws (charge, baryon number, lepton number, strangeness) to decide whether a proposed particle interaction is allowed, naming the conservation law violated if it is not.
- Use E = m x c^2 and E = h x f together to link the energy released in annihilation or pair production to photon frequency.
- For the photoelectric effect, apply Ek(max) = h x f - phi to explain why a threshold frequency exists, using the photon (particle) model of light.
Worked example
A proton and an antiproton, each with negligible kinetic energy, annihilate to produce two identical gamma-ray photons travelling in opposite directions. Mass of proton = mass of antiproton = 1.673 x 10^-27 kg. Calculate the frequency of each photon produced. (c = 3.00 x 10^8 m/s, h = 6.63 x 10^-34 Js)
- Calculate the total mass involved: 2 x 1.673 x 10^-27 = 3.346 x 10^-27 kg.
- Use E = m x c^2 to find the total energy released: E = 3.346 x 10^-27 x (3.00 x 10^8)^2 = 3.01 x 10^-10 J.
- Divide by 2, since the energy is shared equally between two identical photons: E(photon) = 1.51 x 10^-10 J.
- Rearrange E = h x f to make f the subject: f = E / h.
- Substitute: f = 1.51 x 10^-10 / 6.63 x 10^-34.
- Final answer: f = 2.28 x 10^23 Hz.
Practice questions
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Q1State the charge, in coulombs, of a proton.Show answer
Answer: +1.60 x 10^-19 C
Q2State the quark composition of a proton.Show answer
Answer: uud (two up quarks, one down quark)
Q3A nucleus of thorium-234 decays by beta-minus emission. State the mass number and atomic number of the daughter nucleus.Show answer
Answer: Mass number 234, atomic number 91 (protactinium-234)
Q4Classify a neutrino as a hadron or a lepton, and give one reason.Show answer
Answer: Lepton; it is a fundamental particle and does not experience the strong nuclear force.
Q5A photon has frequency 5.00 x 10^14 Hz. Calculate its energy. (h = 6.63 x 10^-34 Js)Show answer
Answer: 3.32 x 10^-19 J (E = h x f = 6.63 x 10^-34 x 5.00 x 10^14)
Q6In a proposed interaction, a proton decays into a positron and a neutral pion (p -> e+ + pi0). Determine whether this is allowed, stating the conservation law that would be violated if it is not.Show answer
Answer: Not allowed; violates conservation of baryon number (baryon number 1 on the left, 0 on the right).
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
State two properties of leptons that distinguish them from hadrons.
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A muon (mass 1.88 x 10^-28 kg, charge -1.60 x 10^-19 C) decays via the weak interaction: muon -> electron + muon-neutrino + X. State the identity of particle X, needed to balance lepton number, and calculate the specific charge of the muon.
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Describe how the results of the alpha-particle scattering experiment provided evidence for the nuclear model of the atom, in preference to the earlier plum pudding model.
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Free printable worksheet
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