Wikijunior:Particles/Expand or contract?

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Float or sink? Expand or contract? Final quiz

Have you ever noticed that there are tiny gaps in rails and flyovers? They are designed that way to prevent the matter from bending or breaking in summer when it expands. Matter always expands, or becomes bigger, when it is heated. This is called thermal expansion. Matter also contracts, or becomes smaller, when it is cooled. This is called thermal contraction. Thermal expansion and contraction greatly affects our daily life and can be explained with the particle model.


Do you remember, from the last chapter, that as heat made the particles move more vigorously, the volume of the air inside the balloon expanded and density of the air decreased? This is due to thermal expansion. When particles move more vigorously, they take up more space because they need more space for movement. See the diagram below.

Particles expansion contraction.svg

The bimetallic stripEdit

Before heating (above), the two strips are straight. However, after heating (below), the yellow metal expands more, so the strip bends.

A bimetallic strip is made of two separate strips of metal stuck together. These two strips expand to a different extent when heated. So, when the strip is heated, one strip will be longer than the other. Therefore, the bimetallic strip will bend. An example is iron and brass.

How a thermostat works.

Bimetallic strips are used in daily life for heat detection, such as that in fire alarms and thermostats. A thermostat is a small gadget used in various household appliances to automatically switch off the appliance when it is overheating. In such applications, the bimetallic strip bends in order to complete or break a circuit. A circuit means the path on which electricity flows from its source and back. In thermostats, the circuit is broken so that the appliance it is used in can no longer operate. In fire alarms this is the opposite: the circuit is completed so that the fire bell can ring and everyone can escape.


  1. Try to draw a diagram that illustrates how a fire alarm works.
  2. True or false?
    1. When some home appliances overheat, the bimetallic strip inside them will contract.
    2. Thermal expansion is limited to liquid and solid only.
    3. Liquids are not as dense as solids.

  1. Suggested diagram:
    Fire alarm.svg
    A legend is recommended, though not necessary.
  1. Answers to "True or false?"
    1. False
    2. False. Thermal expansion applies to all matter.
    3. True. As particles become hotter and move more, the spaces between them become greater so they are less dense. Water is an exception though; liquid water is denser than ice.