Nuclear Fusion & Fission

Revise Nuclear Fusion & Fission for Physics 5 (IAL) WPH15 (A2 Level) — revision notes and instant AI marking.

📖 Revision notes · preview

Edexcel IAL Physics · Nuclear Physics

Nuclear Fusion & Fission

💡 The Big Idea: Nuclei are held together tighter than the sum of their parts — and whenever you rearrange nucleons to make them more tightly bound (either by fusing small nuclei together or splitting huge ones apart), the "missing mass" turns straight into a burst of energy.

📋 Summary — What This Chapter Covers

Mass defect (Δm): a nucleus always weighs LESS than its individual protons and neutrons added up separately.
Binding energy (ΔE): the energy you'd need to put IN to rip a nucleus apart into free nucleons — calculated from the missing mass via E = mc².
Mass–energy equivalence: Einstein's idea that mass and energy are two forms of the same thing, linked by ΔE = Δmc².
Atomic mass unit (u): a handy mass unit for tiny particles, where 1 u = 1.66 × 10⁻²⁷ kg = 931.5 MeV.
Binding energy per nucleon graph: the single most important graph in nuclear physics — it tells you which nuclei are stable, and predicts fusion (light elements) vs fission (heavy elements).
Iron-56 sits at the peak of the graph — the most stable nucleus that exists.
Nuclear fusion: small nuclei joining to form a bigger, more stable nucleus, releasing energy — powers stars and is the goal of D-T fusion reactors on Earth.
Conditions for fusion: nuclei need huge kinetic energy (extreme heat/density) to overcome electrostatic repulsion and get close enough for the strong nuclear force to bind them.
The proton–proton chain: the 5-stage process by which stars fuse 4 hydrogen nuclei into 1 helium nucleus.

1. Nuclear Binding Energy & Mass Deficit

Here's something that sounds impossible at first: if you carefully weigh a nucleus, and then separately weigh all the protons and neutrons that make it up, the nucleus weighs less than the sum of its parts. Not a rounding error — a real, measurable difference. This missing mass is called the mass defect (symbol Δm).

BEFORE (separated) AFTER (bound nucleus) ⚫🔴 🔴⚫ ⚫🔴 🔴⚫ ┌─────────────┐ 6 protons + 6 neutrons ──► │ ⚫🔴⚫🔴⚫🔴 │ Carbon-12 nucleus (all separate, far apart) │ 🔴⚫🔴⚫🔴⚫ │ └─────────────┘ MASS = bigger MASS = smaller (the "missing" mass became released energy — ΔE = Δmc²)

Why does this happen? Nucleons are held together by the strong nuclear force. Pulling them together into a bound nucleus is like a ball rolling down into a valley — it releases energy as it settles into that lower-energy, more stable state. Since mass and energy are equivalent (thanks, Einstein), releasing energy means losing a tiny bit of mass too. That lost mass IS the mass defect.

Mass Defect Formula

Δm = Zm_p + (A − Z)m_n − m_total

In plain words: take the number of protons (Z) × mass of a proton, add the number of neutrons (A − Z) × mass of a neutron, then subtract the actual measured mass of the nucleus. Whatever's left over is the mass defect.

Z = proton number · A = nucleon (mass) number · m_p = proton mass (kg) · m_n = neutron mass (kg) · m_total = measured nucleus mass (kg)