Unit Overview

Nuclear physics

CIE A-Level Physics· 5 min read 📊 n/a

1. Unit at a glance

This unit follows a logical flow from nuclear stability to decay behavior to energy-releasing reactions and practical safety. We start by connecting mass differences between nucleons and whole nuclei to binding energy, the force that holds nuclei together. Next we explore the random nature of radioactive decay, derive the decay law, and learn how half-life describes decay rate. We then explain how the binding energy per nucleon curve drives energy release in fission and fusion, ending with practical radiation safety guidance.

This unit covers the following sub-topics:

Mass defect and binding energy

Learn how mass defect arises, calculate binding energy, and relate binding energy per nucleon to nuclear stability.

★★★⏱ 8 min

Radioactive decay law

Derive and apply the exponential decay law to calculate activity and undecayed nuclei at any point in time.

★★★⏱ 7 min

Half-life

Understand half-life definitions, solve problems for decay over multiple half-lives, and calculate half-life from raw data.

★★⏱ 6 min

Nuclear fission and fusion

Explain energy release in fission and fusion using the binding energy curve, and describe applications for power generation.

★★★★⏱ 9 min

Radiation safety

Learn about biological effects of ionizing radiation and standard safety precautions for working with radioactive sources.

★⏱ 5 min

2. Common Pitfalls

Wrong move:

Confusing mass defect with binding energy directly

Why:

Mass defect is a mass difference, while binding energy is the energy equivalent of that mass difference

Correct move:

Always convert mass defect to binding energy using $E_{b} = Δ m c^{2}$

Wrong move:

Treating half-life as a fixed decay time for individual nuclei

Why:

Radioactive decay is random, so half-life is a statistical property of large groups of nuclei

Correct move:

Use half-life only to predict the proportion of a large sample that will decay over a given time

Wrong move:

Forgetting energy is released when products have higher binding energy per nucleon

Why:

Students often mix up the direction of energy flow in fission and fusion calculations

Correct move:

Confirm that products sit higher on the binding energy per nucleon curve for energy to be released

3. Quick Reference Cheatsheet

Concept	Key Formula / Rule
Mass defect	$Δ m = Z m_{p} + (A - Z) m_{n} - m_{n u c l e u s}$
Binding energy	$E_{b} = Δ m c^{2}$
Radioactive decay law (nuclei)	$N = N_{0} e^{- λ t}$
Activity of a source	$A = A_{0} e^{- λ t} = λ N$
Half-life relation	$t_{1/2} = \frac{l n 2}{λ}$
Energy release condition	Products have higher binding energy per nucleon than reactants
Radiation penetration in air	$α$ : ~5 cm, $β$ : ~1 m, $γ$ : ~100s of m

What's Next

We recommend working through the sub-topics of this unit in the order listed above, starting with the first sub-topic to build your foundational understanding of nuclear stability. After completing all sub-topics in this unit, you will be ready to progress to the next unit on fundamental particles in the CIE A-Level Physics syllabus.