P5. Matter and Thermal Properties
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5.1 Thermal expansion of solids, liquids and gases
Thermal Expansion and Contraction of a Solid
A thermal expansion kit consisting of a metal ball and a metal ring, both
mounted on wooden handles is used to illustrate the idea that solids expand
when heated and contract when cooled.
Students should observe that the ball fits through the ring when both are at
room temperature, but the ball does not fit through the ring when the ball is
heated. Students should also observe that after the ball has cooled back
down to room temperature, it fits through the ring again.
Students need to infer that when the ball can no longer pass through the ring,
it is because the ball has increased in size and that when the ball cools back
to room temperature and can fit through the ring, the ball's size has
decreased, even though it may look the same to the students. A second,
unheated ball could be used to demonstrate that the ring has not changed in
size.
Thermal Expansion of a Liquid in a round-bottom flask
To appreciate that liquids expand when heated and contract when cooled,
students are shown a round-bottom flask filled with coloured water and
observe the level of water rising as the flask is heated and falling as the
flask is cooled.
Students need to infer that when the level rises, the liquid in the flask is
expanding, and when the level falls, the liquid in the flask is contracting.
Helium gas contracting and expanding in a mylar balloon
Mylar balloon is filled with helium gas at room temperature and sealed.
Mylar balloons work for this phenomenon because they do not stretch
like rubber balloons.
Students should observe that when the balloon is cooled, it looks deflated
and when the balloon returns to room temperature, it looks inflated.
Students must infer that the balloon looks deflated because the gas in it
contracted when cooled and the balloon re-inflates when it returns to
room temperature is because the gas in it expanded.
The thermal expansion of gases is much more noticeable than solids and
liquids. One liter of gas expands 34 mL for a temperature increase of
10ºC.
For all phenomena dealing with the expansion and contraction of gases,
use a pliable container to allow the gas to expand and contract safely
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Thermal Expansion - Real-life applications
Opening Jars
An everyday example of thermal expansion can be seen in the kitchen. Almost everyone
has had the experience of trying unsuccessfully to budge a tight metal lid on a glass
container, and after running hot water over the lid, finding that it gives way and opens at
last. The reason for this is that the high-temperature water causes the metal lid to
expand.
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Thermometers
In a typical mercury thermometer, mercury is placed in a long, narrow sealed tube called
a capillary. Because it expands at a much faster rate than the glass capillary, mercury
rises and falls with the temperature. A thermometer is calibrated by measuring the
difference in height between mercury at the freezing point of water, and mercury at the
boiling point of water. The interval between these two points is then divided into equal
increments in accordance with one of the well-known temperature scales.
Expansion Joints
Most large bridges include expansion joints, which look rather like two metal combs
facing one another, their teeth interlocking. When heat causes the bridge to expand
during the sunlight hours of a hot day, the two sides of the expansion joint move toward
one another; then, as the bridge cools down after dark, they begin gradually to retract.
Thus the bridge has a built-in safety zone; otherwise, it would have no room for
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expansion or contraction in response to temperature changes. As for the use of the comb
shape, this staggers the gap between the two sides of the expansion joint, thus
minimizing the bump motorists experience as they drive over it.
Railroad tracks
Most railroad tracks are built from pieces of steel supported by wooden ties, and laid with
a gap between the ends. This gap provides a buffer for thermal expansion, but there is
another matter to consider: the tracks are bolted to the wooden ties, and if the steel
expands too much, it could pull out these bolts. Hence, instead of being placed in a hole
the same size as the bolt, the bolts are fitted in slots, so that there is room for the track to
slide in place slowly when the temperature rises.
Power lines
Another example of thermal expansion in a solid is the sagging of electrical power lines
on a hot day. This happens because heat causes them to expand, and, thus, there is a
greater length of power line extending from pole to pole than under lower temperature
conditions.
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Bimetallic strips in thermostats
Diagram of a thermostat in an air-conditioning unit. A thermostat is used to regulate and
maintain temperature. The bimetallic strip below bends according to the temperature, and
becomes more bent as its heat increases. If it becomes so hot and bent that it touches
the plate on the right, it completes an electric circuit, which turns the air-conditioning unit
on. As the air-conditioning cools the surrounding air, the metallic strip also cools, causing
it to straighten and break the circuit.
The specific latent heat of vaporisation of water is 2260 KJ kg –1.The specific latent heat
of vaporisation of water is the amount of heat required to change 1 kg of water to steam
without a change in temperature.
The formula for Specific heat capacity is :
E = m x c x ΔT
we can rearrange the equation to get
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We want to find the Specific Heat capacity of water, which we know to be around
420KJKg-1°C-1
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