Karteikarten
Smoke detectors
Karten in diesem Stapel (6)
- 1Why are smoke detetctors not replaced for a long time?- Americium -241 emits alpha particles, it has a half life of about 432 years.
- The long half life means that americium decays very slowly which means the smoke detector does not have to be replaced for a long time. - 2Are alpha particles dangerous to people?- The radiation close to the occupants of a house from a domestic smoke detector is almost zero.
- The alpha particles are stopped by a few cm of air. The plastic cap will stop any alpha radiation from passing through. Also people are not tall enough to be close to the detector. - 3Why is alpha radiation used in smoke detectors as opposed to beta and gamma?- Alpha particles as opposed to beta, have high ionization so sufficient air particles will be ionized for the current to exist.
- They have low penetrative power as opposed to beta, meaning they will be stopped by the plastic of the smoke detector and/or the air. - 4The alpha particles are emitted by the americium -241. They collide with air in the detectors ionisation chamber. Explain how- They remove electrons which ionises the air. A small electric current is produced.
- When smoke enters, the smoke particles absorb alpha particles.
- The ionisation is reduced and therefore the current is interrupted and this sets off an alarm. - 5Define the term 'ionisation air molecules'When radiation strips electrons from the atoms within the air molecules. The loss of electrons leaves the air molecules with a positive charge.
- 6Americium -241 emits alpha particles, explain why an alpha emitter is an important component of an ionisation smoke detector:a particles have a strong ionising ability therefore, they are able to strip electrons from the air molecules inside the smoke detector. The ionised molecules flow between charged plates, which is detected as a current in the detector. Without the a particles there will be no ionised particles and no current will flow.