Albert Teen

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Smoke Alarms

# Smoke Alarms

### Smoke Alarms

Radiation has many everyday uses, some of which can save lives such as detecting fires in a smoke alarm.

Smoke alarms use ionising radiation to create an electric current. Which form of radiation is the most ionising?

So smoke alarms make use of alpha particles, because they are highly ionising and weakly penetrating. What does that mean?

1

How do smoke alarms actually work?

The outside of the smoke alarm consists of a metal or plastic casing with holes in it.

2

Why do you think the holes are necessary?

A) So it won't overheat. B) So you can look inside. C) So air and smoke can enter.

1

This shows the components inside a smoke detector

The 3 basic components are the alarm bell, the battery, and the ionising chamber.

2

The ionising chamber has a top plate and a bottom plate with air and alpha particles in-between. Which terminal of the battery is the top plate connected to?

A) The positive terminal B) The negative terminal

3

There is a full electric circuit running through the smoke alarm

The rest of this lesson explains how the current is created inside the ionising chamber to complete the circuit.

1

This is a drawing of what goes on inside the ionising chamber. The chamber is mainly filled with ______.

2

The ionising chamber also contains a tiny alpha source. What element does the alpha source consist of?

3

The alpha source of americium-241 emits lots of alpha particles

These alpha particles then start to ionise the air molecules in the chamber. When the air molecules are ionised, they produce free electrons and positive ions.

4

Which plate are the electrons going to be attracted to?

A) The top plate B) The bottom plate

5

Which plate will the positively charged ions be attracted to, then?

A) Also the top plate B) The bottom plate

1

So the free electrons and the positive ions are attracted to opposite plates within the chamber

The electrons move towards the positive plate. The ions move towards the negative plate.

2

The movement of charged particles, like this, creates a _______.

3

So when the ionising chamber is full of air, a current passes through it. That means that the electric circuit inside the fire alarm is ________.

A) complete B) incomplete

4

What do you think happens if smoke gets into the chamber?

A) The flow of particles increases, so the current increases B) Smoke blocks the flow of particles, reducing the current C) Smoke completely blocks the flow of particles, stopping the current

5

When smoke drifts into the chamber, it fills the space between the plates

This blocks the flow of electrons to the top plate and reduces the electric current. The smoke alarm detects this drop and sounds the alarm!

1

Summary! Smoke alarms make use of alpha radiation

Alpha radiation is highly ionising, but very weakly penetrating, so the radiation inside a smoke alarm won't cause any harm to people living in the house.

2

The ionising chamber inside a fire alarm is mostly full of air

It also has a tiny alpha source of americium-241, that emits lots of alpha particles and ionises the air molecules.

3

When the air molecules are ionised, they produce free electrons and positive ions

The electrons move towards the positive plate. The ions move towards the negative plate.

4

This movement of electrons creates a current inside the ionising chamber

If smoke gets near the alarm, it blocks the flow of electrons and reduces the electric current. The smoke alarm detects this drop and sounds the alarm!