From physick.wikispaces.com
By Carrie MacLeod
Half-Lives are used to label substances that are undergoing exponential decay. In physics 30, half-lives are studied using nuclear decay. The time that it takes for these samples to decay to exactly half of the initial amount is called the Half-Life of a substance. (Note that the half-life for a given isotope does not change.) Below is an example of a radioactive decay graph.
Where: Nt = The amount of sample left after a certain period of time
N0=the initial amount of the sample
t=Amount of time passed
t1/2=Half-life of a sample

The variable N can be used in three different ways:
  • N= number of nuclei present
  • N= mass of radioactive material (kg)
  • N= activity of radioactive material (Bq)
Students will know which one to use based on the information given in the question. For example, if the question gives you an initial mass(kg) and wants you to solve for Nt, the answer would be a mass in kg.


  1. http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html
  2. http://physics.info/half-life/
  3. http://ldindustries.ca/LDIndustries/P30/LangdalePhysics30.html