Explain the radioactive smoke-alarm application
Connect alpha ionisation a changing current and a suitable source half-life in a sealed alarm model.
Section P5.2.4 · Half-life
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Know alpha ionisation.
- Distinguish current sensing from nuclear triggering.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the model detect when smoke enters?
What you will learn
- Connect alpha ionisation a changing current and a suitable source half-life in a sealed alarm model.
- Explain ionisation smoke sensing and justify a source half-life for steady operation.
Trace the signal
In an ionisation smoke-alarm model, alpha radiation ionises air between electrodes. The ions allow a small electric current. Smoke entering the sensing region interferes with ion movement and reduces that current; the electronics detects the change and sounds the alarm.
The alpha radiation has a short range and strong ionising effect, making it useful in this enclosed sensing model. A sufficiently long source half-life keeps its output reasonably steady over the intended working period rather than rapidly falling.
Match the model to the application
A smoke alarm detects a change from its normal operating signal; it does not need smoke to make the nucleus start emitting. Individual radioactive emissions remain spontaneous. The source and sensing system are designed together.
Not every smoke alarm uses a radioactive source: optical alarms use another sensing method. This labelled teaching model is not a construction, servicing or disassembly procedure; an actual sealed source must remain within its authorised enclosure.
Sources for this lesson
Pause and explain
Why prefer a sufficiently long half-life for this source model?
Worked example
Explain the signal change when smoke enters an ionisation alarm.
Show the worked solution
- Alpha ionises air.
- Smoke reduces ion movement and current.
- The electronics detects the changed current.
Answer The current falls and the electronics triggers an alarm.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the sequence from alpha emission to a normal sensing current.
Give me a hint
Follow ion formation and movement.
Compare my reasoning
- Alpha ionises air.
- Charged ions move between electrodes.
- Their movement gives a small current.
Alpha ionisation enables the small sensing current.
Look for these in your work
- Name air ions.
- Connect charge movement to current.
Explain the effect of smoke in this model.
Give me a hint
Compare normal and smoke-affected ion transport.
Compare my reasoning
- Smoke interferes with ion movement.
- The current is reduced.
- Electronics senses the change and gives an alarm.
A reduced current triggers the sensing response.
Look for these in your work
- State the signal direction.
- Separate sensing from decay triggering.
A proposed source loses almost all activity before the stated service interval ends. Evaluate its half-life suitability.
Give me a hint
Compare output stability with the operating period.
Compare my reasoning
- A rapidly falling source changes the normal signal.
- The application needs sustained ionisation.
- A more suitable half-life keeps output fairly steady over that period.
The proposed short-lived source is unsuitable for the stated steady-output model.
Look for these in your work
- Use the supplied period.
- Do not claim that long half-life means no decay.
Common mistakes
- Smoke does not trigger individual nuclear decay.
- Not all alarms are radioactive.
- The sealed source remains enclosed.
What can you explain now?
Which emission is strongly ionising over a short range?
Compare with the explanation
Alpha
Its dense ionisation is useful in this enclosed sensing model.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain ionisation smoke sensing and justify a source half-life for steady operation.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.