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A-LEVEL 9702 · A2 · TOPIC 23
Nuclear physics
The chain of the topic: a nucleus weighs less than its parts, so that mass defect is locked-up binding energy through E = mc²; the binding-energy-per-nucleon curve then explains fusion and fission. Separately, decay is random and spontaneous, which forces an exponential decay law with a fixed half-life. Around the hexagon are the four ideas; above is what it builds on, below is where it leads.
TOPIC 23: NUCLEAR PHYSICS
CAMBRIDGE A-LEVEL PHYSICS 9702 · PATHWAYS
TheLucidSTEM · thelucidstem.com
BUILDS ON
T11 Nuclides & decay T22 Photon energy E = hf T5 Energy conservation 23.1
23.1
23.2
23.2
TOPIC 23
NUCLEAR
PHYSICS
1 · MASS-ENERGY & NUCLEAR EQUATIONS
Mass and energy are one currency.
Einstein: a change of mass Δm carries energy.
Balance nuclide equations: conserve nucleon
number A and proton number Z.
1 u = 931 MeV; 1 eV = 1.60 × 10−19 J.
E = m c²
ΔE = c² Δm
+
energy out
reactants
product
lost mass Δm reappears as released energy
2 · BINDING ENERGY PER NUCLEON
A nucleus weighs less than its parts.
Mass defect: nucleus mass < sum of nucleons.
Binding energy = energy to pull it fully apart.
Peak near iron-56: the most stable nuclei.
Fusion (light) and fission (heavy) climb the peak.
binding energy per nucleon = E / A
E/A
A
Fe-56
fusion
fission
light
3 · RANDOM DECAY & ACTIVITY
Each nucleus decays at random.
Decay is spontaneous and random: unaffected by
temperature, pressure or chemical state.
Decay constant λ: probability per unit time.
Activity A is the number of decays per second,
measured in becquerel (Bq).
A = λ N
more nuclei N means more decays
sample of N nuclei
emitted α, β, γ
at random times
4 · EXPONENTIAL DECAY & HALF-LIFE
Randomness forces an exponential law.
Rate of loss is proportional to N, so the count
falls exponentially with time.
Half-life t½: time for N (or A) to halve.
N, A and received count all follow the same law.
x = x₀ e−λt
λ t½ = ln 2
N
t
N₀/2
N₀/4
t½
2t½
each t½
halves N
LEADS TO
T24 Medical physics: tracers decay, beams attenuate as e−μx T25 Astronomy: fusion in stars releases binding energy Each reuses one idea: mass becomes energy through E = mc², and unstable nuclei decay exponentially with a fixed half-life.
← Builds on IGCSE: Radioactivity