# Radioactivity – problems and solutions

1. Based on the figure below, radioactive activity after decay for 13.86 hours is …

Known : Half-life (T1/2) = 6.93 hours

Time-lapse (t) = 13.86 hours

Solution : A = radioactive activity, λ = the decay constant, Nt = The number of radioactive atoms after decaying during a certain time interval, T1/2 = half-life

The decay constant : The number of radioactive atoms after decaying during a certain time interval :  Read :  Ohm's law - problems and solutions

2. Based on the figure below, after the radioactive substance decays for 15 minutes, then the remaining radioactive substance is …

Solution :  No = The initial amount of the radioactive atoms

Nt = The final amount of the radioactive atoms after decaying during a certain time interval (t)

t = time-lapse

T 1/2 = half-life

Calculate the Half-life :

Known :

No = 8 grams

Nt = 2 grams

t = 6 minutes

Wanted: half-life (T 1/2)

Solution : Calculate the remaining radioactive material :

Known :

The initial amount of the radioactive atoms (No) = 8 grams

Time-lapse (t) = 15 minutes

Half-life (T 1/2) = 3 minutes

Wanted: the remaining radioactive material (Nt)

Solution : Read :  Kepler's law – problems and solutions 