Key idea
Half-life
N(t)=N₀
Start with the central idea, then test it against a concrete element or example. A scientific model is useful when we know what it explains — and where its limits are.
Half-life
The key idea
Half-life is the time required for the expected number of undecayed nuclei, or equivalently the activity of a large sample, to fall to one half of its initial value.
Why it matters
Successive half-lives produce halves, quarters, eighths and so on; the process is exponential rather than linear.
A picture to remember
Key idea
Half-life
Guided example
Half-life
Start with the central idea, then test it against a concrete element or example. A scientific model is useful when we know what it explains — and where its limits are.
Put it into practice
Half-life
Choose a concrete case linked to Half-life. Write down the known information, what you are trying to find, and the units. Then state the rule, model or trend you will use. If a calculation is involved, keep the units visible at every line; if it is conceptual, state what changes and what remains fixed.
Check your answer
A strong answer contains four things: the starting situation, the variable that changes, the direction or numerical result, and one check. The check can be a unit, an order of magnitude, a limiting case or a comparison with a familiar example.