## Exponential decay

Teaching Guidance for 14-16

#### Highlighting a new form of change

Radioactive decay may well be the first example that students meet of an exponential change. It is therefore important to draw attention to this new form of change:

Teacher: So, if we look at the activity curve for the radioactive source, what can we say about its shape?

Abdul: It goes down.

Teacher: Yes, it goes down! How does it go down?

Nicki: Steeply at first and then more gently.

Teacher: Exactly right! Who can explain this pattern? Think about the activity we did with the dice.

Azul: At first there are lots of radioactive atoms… so you are more likely to get a six!

Teacher: Well, that's right. At first there are lots of unstable atoms, that decay with a fixed chance, and as time goes by the remaining stockpile of atoms gets smaller and so the rate of decay (or the number of sixes) gets smaller.