WASSCE Physics · Paper 1 style

Radioactivity

One sample question with the full working. 2 more in the app, with instant feedback and repair sessions for the traps below.

A sample of 80 g has a half-life of 2 days. Mass left after 4 days:

A20 g
B40 g
C0 g
D60 g

Show the answer and working

Answer: A

20 g.

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

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

Trap D Takes part of the condition as the whole answer: Check whether the option is enough on its own. Equilibrium needs zero resultant force AND zero total moment; a complete explanation names every factor in the stem.

Traps in Radioactivity

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.
Takes part of the condition as the whole answer
Check whether the option is enough on its own. Equilibrium needs zero resultant force AND zero total moment; a complete explanation names every factor in the stem.
Miscounts steps or makes a counting slip
Count the steps explicitly on paper (list each half-life, each strand), then check the total.

Practise Radioactivity (2 questions)Send to your class on WhatsApp