Expand or contract?
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.
Explanation
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.
The bimetallic strip
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.
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.
Quiz
Try to draw a diagram that illustrates how a fire alarm works.
True or false?
When some home appliances overheat, the bimetallic strip inside them will contract.
Thermal expansion is limited to liquid and solid only.
Liquids are not as dense as solids.
Suggested diagram:
:
:A legend is recommended, though not necessary. Answers to “True or false?” False False. Thermal expansion applies to all matter. 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.
Written for younger readers
Expand or contract?, in simpler words
This version comes from Wikijunior, a set of books written for children aged 8 to 11. It is shorter and uses plainer language than the article above.
From Wikijunior: Particles
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.
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.
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.
Try to draw a diagram that illustrates how a fire alarm works.
True or false?
When some home appliances overheat, the bimetallic strip inside them will contract.
Thermal expansion is limited to liquid and solid only.
Liquids are not as dense as solids.
Suggested diagram:
:
:A legend is recommended, though not necessary. Answers to “True or false?” False False. Thermal expansion applies to all matter. 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.
Where this page comes from
The article above is adapted from “Expand or contract?” on Simple English Wikipedia, by its contributors. We removed reference markers, navigation boxes and tables, expanded measurement templates into readable numbers, and kept the prose otherwise intact. The simpler version is adapted from Wikijunior on Wikibooks.
Both sources are published under CC BY-SA 4.0, so this page is published under the same licence. You may share and adapt it, including commercially, as long as you credit the original and keep the same licence.
Worksheets, answer keys and printable layouts elsewhere on K5Print are our own work and are not covered by this licence.