A7LAM CSCA ACADEMYCSCA Academy / Physics / Modern physics
CSCA Physics · Lesson 16 of 16Nuclear physics
Physique nucléaire · الفيزياء النووية
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The nucleus can change: unstable nuclei decay, and fission and fusion release huge amounts of energy.
Key ideas
- Notation ᴬ_ZX: Z protons and A − Z neutrons.
- α decay: A − 4, Z − 2. β⁻ decay: A unchanged, Z + 1. γ: no change in A or Z.
- Half-life T: after each T, half of the nuclei remain.
- A mass defect releases energy: ΔE = Δm·c².
Formulas to know
- N = N₀ (1/2)^(t/T)
- ΔE = Δmc²
- A and Z are conserved in every nuclear reaction
Worked example
80 g of an isotope has a half-life of 5 days. Mass left after 15 days?
- Count: 15 / 5 = 3 half-lives
- Halve: 80 → 40 → 20 → 10
- Result: 10 g
Answer: 10 g
Practice questions
1. ²³⁸U (Z = 92) undergoes one α decay. The new nucleus has:
A = 234, Z = 90 ✓ · A = 238, Z = 93 · A = 234, Z = 92 · A = 236, Z = 90
A − 4, Z − 2.
2. After 2 half-lives, the fraction left is:
1/2 · 1/4 ✓ · 1/8 · 0
½ × ½.
3. Which radiation is stopped by a sheet of paper?
α ✓ · β · γ · All of them
α particles are the least penetrating.