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Heat and Its Effects

Heat and Its Effects

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12.1 Effects of Heat

• When a substance is heated, it increases in size.o We say the substance expands.o This process is called expansion.

• When a substance is cooled, it decreases in size.o We say the substance contracts.o This process is called contraction.

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Expansion and contraction

• Solidso Usually solids expand when heated, and

contract when cooled.o Some solids expand very little such that we

may not notice the expansion. o For example, a one metre long brass rod

increases 1 mm in length when its temperature increases by 100 C.

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Expansion and contraction

Experiment 1: Expansion of solids

• Predict what happen when we heat the metal ball to high temperature!

• Why?• Ans:

1. Before heating, the metal ball is able to go through the ring.

metal ball

metal ring

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Experiment 1: Expansion of solids

Ans: After heating, the metal ball expands and is unable to go through the ring.

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Experiment 1: Expansion of solids

• Predict what happen when we heat the metal ring to high temperature!

• Why?• Ans:

1. Before heating, the metal ball is able to go through the ring.

metal ball

metal ring

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Demostrations on the Strong forces of contraction

• When a body expands or contracts, large forces are produced, which may help us or cause us problems.

1. The steel rod was heated before it is fixed in position by tightening the nut.

2. The steel rod is then allows to cool in the set-up.

3. After some time, the iron bar breaks into two pieces.

steel rodiron bar

nut

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Expansion and contraction

Experiment 2: Expansion of solids

1. A bicycle spoke is made to rest with one end on a drinking straw. A weight is attached to the spoke between the flame and the straw.

clamp

bicycle spoke

weight

light pointer

drinking straw

2. When heated with the flame, the spoke expands and rolls the straw.

3. Movement is seen when a pointer piercing the straw rotates by an angle.

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Expansion and contraction

• Liquids o When a pot or kettle is filled to the

brim with water, the water will spill out when it boils.

o This is because water expands when heated.

o In general, most liquids expand when heated, and contract when cooled.

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Experiment 3: Expansion and contraction of water

1. A glass flask containing coloured water is placed in a beaker of hot water. Soon after, the liquid level is seen to rise. This shows the coloured water expands when heated by the hot water.

coloured water hot water

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Experiment 3: Expansion and contraction of water

liquid level has dropped

ice water

2. Then, the flask is placed in a beaker of ice water. This time, the liquid level is seen to drop. This is because the coloured water in the flask contracts when it was cooled by the iced water.

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Expansion of cake

• This is a dough, made of flour.

• Yeast is mixed• Later, it will be put

into the oven at bake at a temperature of 220 degree celcius!

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Expansion of cake

• Yeast is added to the dough

• Later bake in the oven at high temperature.

• Why?

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Expansion and contraction

• Gaseso As dough is heated, air bubbles trapped

inside the dough gain heat and expand, creating holes in the cake. This causes the cake to rise.

o In general, gases expand when heated, and contract when cooled.

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Expansion and contraction

Experiment 4: Expansion and contraction of air

1. A flask containing only air is heated. The drop of coloured water is seen to rise up the tube. This is because, when the flask is heated, the air inside it expands and pushes the drop of water upwards.

glass tube

air iced water

drop of coloured water has dropped

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Problems caused by expansion

• Roads and pathwayso Too much expansion on

very hot days can cause roads or pavements to crack, or tile to pop out.

o To solve this problem, the tiles on pavements and walls are laid with expansion gaps.

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Problems caused by expansion

• How long is a railway track?

• Can you guess what happens to railway track during a very hot day?

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Problems caused by expansion

Solution: Introduce expansion gapWhy?

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What are some of the problems a long bridge faces during a hot day?

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Problems caused by expansion

• Bridges and metal girders o Bridges and metal girders on buildings have expansion

gaps or a free end resting on rollers to allow for expansion.

o This will avoid them from being damaged in very hot weather.

allowance for expansion

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Problems caused by expansion

• Water pipeso Water pipes and pipelines can expand during

hot weather, and may burst.o Expansion bends are made in this pipes to

allow them to expand without any damage.

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Problems caused by expansion

• Telephone wires and cableso Telephone wires and cables raised on poles

contract in cold weather and become taut. They may even snap.

o To prevent this, they are strung loosely from pole to pole to allow for contraction.

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Uses of expansion

1. When red-hot, the rivet is put into position.

• Rivetso Rivets are used to join 2 steel plates together

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Rivets

2. It is then hammered into a head, and allowed to cool.

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Rivets

3. The contraction on cooling helps to pull the steel plates together.

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Do you think all types of material expand at the same rate?

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• If we rivet a strip of brass and a strip of steel together, we have what is called a bimetallic strip.

Uses of expansion

rivets

rivets

steel

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Uses of expansion

• The brass expands more than the steel when heated. Thus, when heated, the strip bends with the brass on the outside of the curve.

heatedsteel

brass

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Uses of expansion

• When cooled, the brass contracts more than the steel. Thus, the strip bends with the steel on the outside of the curve.

chilled

steel

brass

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Uses of bimetallic strips• Thermostat

o A thermostat keeps temperature constant. o Found in ovens and refrigerators to keep the

temperature hot and cold respectively.

In a oven: Heating takes place when the current is on. When the temperature is too hot, the strip bends away to break the contact.

In a refrigerator: Cooling takes place when the current is on. When the temperature is too cold, the strip bends away to break the contact.

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Uses of bimetallic strips

• Bimetallic thermometero A bimetallic strip can be

made into a long, nearly flat coil.

o It has one end fixed and the other carries a pointer.

o The coil tightens or unwinds as the temperature goes up and down.

o The pointer then moves over a scale marked in temperature degrees.

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Miscellaneous

• http://en.wikipedia.org/wiki/Thermal_expansion• http://www.translatorscafe.com/cafe/units-conver

ter/thermal-expansion/• http://en.wikipedia.org/wiki/Expansion_joint• http://hyperphysics.phy-astr.gsu.edu/hbase/ther

mo/bimet.html• http://home.howstuffworks.com/home-thermostat

.htm