Half-life (AQA GCSE Physics): Revision Notes
Half-life
What is half-life?
Half-life is a key concept in radioactive decay. It tells us how quickly radioactive materials break down over time.
Definition: Half-life is the time it takes for half of the unstable atoms in a radioactive sample to decay.
The activity of a radioactive source decreases over time. Activity is measured in becquerels (Bq) - this tells us how many atoms decay each second.
How radioactive decay works
Radioactive decay is a random process. We cannot predict when any single atom will decay. However, when we have large numbers of atoms, we can predict the overall pattern.
Key facts about radioactive decay:
- As atoms decay, there are fewer unstable atoms left
- This means the activity gets smaller over time
- The rate of decrease follows a predictable pattern
Understanding half-life patterns
The beauty of half-life is its consistent pattern. After each half-life period, the number of radioactive atoms halves:
Pattern Example: Half-life Reduction
- After 1 half-life: 50% of atoms remain
- After 2 half-lives: 25% of atoms remain
- After 3 half-lives: 12.5% of atoms remain
- After 4 half-lives: 6.25% of atoms remain
This pattern continues - the activity keeps halving with each half-life period, regardless of the actual time duration.
Half-life calculations
Understanding the mathematical pattern helps us calculate activity at any point in time.
Worked Example: Activity Calculation
A radioactive source has an activity of 240 Bq and a half-life of 6 hours.
Working through the half-lives:
- Start: 240 Bq
- After 6 hours (1 half-life): Bq
- After 12 hours (2 half-lives): Bq
- After 18 hours (3 half-lives): Bq
- After 24 hours (4 half-lives): Bq
Notice how we divide by 2 for each half-life period, not multiply by the time. The key is counting how many half-life periods have passed.
Reading half-life graphs
Half-life graphs show how activity changes over time. They have a distinctive curved shape that gets less steep as time goes on - this is called an exponential decay curve.
To find half-life from a graph:
- Find the starting activity value
- Calculate half of this value
- Find where this half-value crosses the curve
- Read the time - this is the half-life
Graph Reading Example
If a graph starts at 1000 Bq, find where it crosses 500 Bq. The time at this point is the half-life.
For the second half-life, find where it crosses 250 Bq, and so on.
Key Points to Remember:
- Half-life is the time for half the unstable atoms to decay
- Each half-life period halves the remaining radioactive material
- Radioactive decay is random for individual atoms but predictable for large samples
- Activity is measured in becquerels (Bq)
- Half-life graphs are curved and get less steep over time