WASSCE Physics · Paper 1 style

Atomic and nuclear physics

One sample question with the full working. 9 more in the app, with instant feedback and repair sessions for the traps below.

A radioactive sample has a half-life of 5 days. If its initial count is 800, what is the count after 15 days?

Show the answer and working

Answer: B

Number of half-lives = 15/5 = 3. N = 800 × (½)^3 = 100.

Trap A Treats radioactive decay as linear: After n half-lives the amount is N₀ ÷ 2ⁿ. Halve step by step: 800 → 400 → 200 → 100.

Trap C Miscounts steps or makes a counting slip: Count the steps explicitly on paper (list each half-life, each strand), then check the total.

Trap D Half-life misconceptions: After n half-lives the fraction remaining is (½)ⁿ: 1 → ½ → ¼ → ⅛. It halves, it does not decrease linearly.

Treats radioactive decay as linear
After n half-lives the amount is N₀ ÷ 2ⁿ. Halve step by step: 800 → 400 → 200 → 100.
Miscounts steps or makes a counting slip
Count the steps explicitly on paper (list each half-life, each strand), then check the total.
Half-life misconceptions
After n half-lives the fraction remaining is (½)ⁿ: 1 → ½ → ¼ → ⅛. It halves, it does not decrease linearly.
Confuses alpha and beta decay
Alpha: A − 4, Z − 2 (a helium nucleus leaves). Beta-minus: A unchanged, Z + 1 (a neutron becomes a proton). Gamma: no change.
Confuses alpha, beta and gamma changes
α (⁴₂He): A−4, Z−2. β⁻ (⁰₋₁e): A unchanged, Z+1. γ: no change in A or Z.
Reverses a relationship or direction
Write the relationship with an arrow or proportionality sign first (E ∝ f; R ∝ L; cooling a metal → R falls). Then check the option follows the same direction.