1. Thermal properties of matter
Objectives :
1. describe how and why solids, liquids and gases expand when their
temperatures rise
2. explain some everyday uses and consequences of thermal
expansion
3. relate energy supplied to increase in temperature when an object
is heated
2. Thermal expansion
• Most substances – solids, liquids and gases – expand when their temperature
rises. This is called thermal expansion
• Thermal expansion happens because the particles gain energy and vibrate or
move faster, pushing each other further apart.
The expansion of solid
• Figure 10.3 shows an experiment that demonstrates that a metal ball expands
when it is heated.
• The ball, but not the ring, is heated strongly. It now will not pass through the ring.
It has expanded.
• When the ball cools down, it contracts and returns to its original size and will
once again pass through the ring.
3.
4. Uses of thermal expansion of solids
• Rivets are used in shipbuilding and other industries to join metal
plates.
• A red-hot rivet is passed through holes in two metal plates and then
hammered until the ends are rounded (Figure 10.4). As the rivet
cools, it contracts and pulls the two plates together tightly.
5. Uses of thermal expansion of solids
• A metal lid or cap may stick on a glass jar or bottle, and be hard to unscrew. Heating the
lid (for example, by running hot water over it) causes it to expand. The glass expands
much less than the metal lid, meaning that the lid loosens and can be removed.
• A steel ‘tyre’ can be fitted on to the wheel of a train while the tyre is very hot. It then
cools and contracts, so that it fits tightly on to the wheel.
• A bimetallic strip (Figure 10.6) is designed to bend as it gets hot.
• The strip is made of two metals joined firmly together. One metal expands much more
than the other. As the strip is heated, this metal expands, causing the strip to bend. The
metal that expands more is on the outside of the curve, because the outer curve is
longer than the inner one.
• These strips are used in devices such as fire alarms and thermostats. Thermostats are
used to control the temperature of devices such as ovens and irons.
6.
7. Consequences of thermal expansion of solid
• The expansion of materials can cause problems.
• For example, metal bridges and railway lines expand on hot days, and there is a
danger that they might bend.
• To avoid this, bridges are made in sections, with expansion joints between the
sections (Figure 10.7). On a hot day, the bridge expands and the gaps between
sections decrease.
• Railway lines are now usually made from a metal alloy that expands very little.
• On a concrete roadway, you may notice that the road surface is in short sections.
The gaps between are filled with soft tar, which becomes squashed as the road
expands.
8. The expansion of liquid
• Many thermometers use the expansion of a liquid to measure
temperature.
• As the temperature of the liquid rises, it expands and the level of
liquid in the tube rises.
9. The expansion of gases
• Gases expand when they are heated, just like solids and liquid
• Figure 10.9 shows some gas in a cylinder fitted with a piston.
• At first, the gas is cold and its particles press weakly on the piston. When the gas
is heated, its particles move faster.
• Now they push with greater force on the piston and push it upwards. The gas has
expanded.
10. Comparing solids, liquids and gases
• Solids expand least when they are heated. Some, such as Pyrex glass
and invar metal alloy, have been designed to expand as little as
possible.
• Liquids generally expand more than solids.
• Gases expand even more than liquids.
11.
12.
13.
14. Specific heat capacity
• Specific heat capacity of a substance is defined as the energy needed
to raise the temperature of 1 kg of the substance by 1 °C.
• It is defined by the equation:
15.
16. Specific heat capacity of water
• Water is an unusual substance.
• It has a high value of specific heat capacity (s.h.c.) compared with other
materials. This has important consequences:
1. It takes a lot of energy to heat up water
2. Hot water takes a long time to cool down
• The consequences of this can be seen in our climates. In the hot months of
summer, the land warms up quickly (low specific heat capacity) while the sea
warms up only slowly. In the winter, the sea cools gradually while the land cools
rapidly. People who live a long way from the sea (in the continental interior of
North America or Eurasia, for example) experience freezing winters and very hot
summers. People who live on islands and in coastal areas (such as western
Europe) are protected from climatic extremes because the sea acts as a store of
heat in the winter, and stays relatively cool in the summer.
17. Measuring the specific heat capacity of a metal
You will need
• a block of metal with holes for the thermometer and heater
• insulation for the block
• electric heater
• power pack
• joulemeter
• Thermometer
• access to balance
safety: The electric heater can become extremely hot. Leave it to cool inside the block
when you have finished your experiment.
18.
19. Measuring the specific heat capacity of water
You will need
• a beaker of water with a lid with holes for the thermometer and heater
• insulation for the block
• electric heater
• power pack
• joulemeter
• Thermometer
• access to balance
safety: The heater and water will get hot. Allow them to cool before clearing away your
apparatus.
20.
21.
22.
23.
24. CHANGING STATE
• Figure 10.17 shows what happens to the temperature when you take some ice
from the freezer and heat it at a steady rate.
• In a freezer, ice is at a temperature well below its freezing point, maybe as low as
−20 °C.
• From the graph, you can see that the ice warms up to 0 °C, then stays at this
temperature while it melts. As lumps of ice float in water, both are at 0 °C.
• When all of the ice has melted, the water’s temperature starts to rise again.
• At 100 °C, the boiling point of water, the temperature again remains steady. The
water is boiling to form steam. Eventually, all of the water has turned to steam.
• If you continue to heat the steam, the temperature of the steam will rise again.
25.
26. EVAPORATION
• A liquid can change state without boiling.
• After it rains, the puddles dry up even though the temperature is much lower
than 100 °C.
• The water from the puddles has evaporated. The liquid water has become a gas
called water vapour in the air. This is the process of evaporation.
• A liquid evaporates more quickly as its temperature approaches its boiling point.
That is why puddles disappear faster on a hot day than a cold day.
27. COOLING BY EVAPORATION
• If you get wet, perhaps in the rain or after swimming, you will notice that you can
quickly get cold.
• The water on your body is evaporating, and this cools you down. Why does
evaporation make things cooler?
• The particles that escape from the water are the fastest-moving ones. T
• hey are the particles with the most kinetic energy. This means that the particles
that remain are those with less energy. Now the particles of the liquid have less
energy (on average) and so the temperature of the water decreases. The water
cools down.
28. Comparing evaporation and boiling
• Boiling only happens at the boiling point of the substance. Evaporation occurs at
all temperatures.
• For a liquid to boil, it has to be heated – the kinetic energy of its particles must be
increased. Evaporation happens when the most energetic particles escape, so
evaporation takes energy from the substance.
• Boiling happens throughout the liquid. Evaporation only happens at the surface.
• A boiling liquid bubbles. A liquid can evaporate without bubbles.
29. Speeding up evaporation
Increasing the surface area
• In Figure 10.24, the liquid has a greater surface area more of the particles are close to
the surface, and so they can escape more easily. This means the liquid evaporates more
quickly.
Blowing air across the surface
A draught is moving air. When particles escape from the water, they are blown
away so that they cannot fall back in to the water. This helps the liquid evaporate
quickly.
30. Speeding up evaporation
Increasing the temperature
• Increasing the temperature of the liquid means the particles on average have
more kinetic energy. More of the particles will have enough energy to escape.
This means the liquid will evaporate more quickly.
31. EVAPORATION
• As a liquid evaporates, the remaining liquid cools. This means that thermal
energy will flow to the liquid from any objects in contact with it. When we get hot
we sweat. The sweat evaporates, causing thermal energy to flow from the skin.
This helps us cool down.
• Fridges use the cooling effect of evaporation (see Figure 10.27). A liquid is
compressed then squirted through a narrow hole so its pressure is reduced and it
evaporates. This draws thermal energy from inside the fridge into the liquid. The
liquid is then pumped out of the fridge to the pipes on the back of the fridge
where it is compressed and condenses, releasing the thermal energy to the
surroundings.
