12.1 Effects of Heat • When a substance is heated, it increases in size. • We say the substance expands. • This process is called expansion. • When a substance is cooled, it decreases in size. • We say the substance contracts. • This process is called contraction.
Expansion and contraction • Solids • Usually solids expand when heated, and contract when cooled. • Some solids expand very little such that we may not notice the expansion. • For example, a one metre long brass rod increases 1 mm in length when its temperature increases by 100 C.
Experiment 1: Expansion of solids Predict what happen when we heat the metal ball to high temperature! Why? Ans: Expansion and contraction metal ring metal ball 1. Before heating, the metal ball is able to go through the ring.
Experiment 1: Expansion of solids Ans: After heating, the metal ball expands and is unable to go through the ring.
Predict what happen when we heat the metal ring to high temperature! Why? Ans: Experiment 1: Expansion of solids metal ring metal ball 1. Before heating, the metal ball is able to go through the ring.
nut iron bar steel rod 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. • The steel rod was heated before it is fixed in position by tightening the nut. • The steel rod is then allows to cool in the set-up. • After some time, the iron bar breaks into two pieces.
bicycle spoke light pointer drinking straw clamp weight 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. 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.
Expansion and contraction • Liquids • When a pot or kettle is filled to the brim with water, the water will spill out when it boils. • This is because water expands when heated. • In general, most liquids expand when heated, and contract when cooled.
coloured water hot water 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.
liquid level has dropped ice water Experiment 3: Expansion and contraction of 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.
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! Expansion of cake
Yeast is added to the dough Later bake in the oven at high temperature. Why? Expansion of cake
Expansion and contraction • Gases • 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. • In general, gases expand when heated, and contract when cooled.
glass tube drop of coloured water has dropped air iced water 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.
Problems caused by expansion • Roads and pathways • Too much expansion on very hot days can cause roads or pavements to crack, or tile to pop out. • To solve this problem, the tiles on pavements and walls are laid with expansion gaps.
How long is a railway track? Can you guess what happens to railway track during a very hot day? Problems caused by expansion
Problems caused by expansion Solution: Introduce expansion gap Why?
Problems caused by expansion • Bridges and metal girders • Bridges and metal girders on buildings have expansion gaps or a free end resting on rollers to allow for expansion. • This will avoid them from being damaged in very hot weather. allowance for expansion
Problems caused by expansion • Water pipes • Water pipes and pipelines can expand during hot weather, and may burst. • Expansion bends are made in this pipes to allow them to expand without any damage.
Problems caused by expansion • Telephone wires and cables • Telephone wires and cables raised on poles contract in cold weather and become taut. They may even snap. • To prevent this, they are strung loosely from pole to pole to allow for contraction.
Uses of expansion • Rivets • Rivets are used to join 2 steel plates together 1. When red-hot, the rivet is put into position.
Rivets 2. It is then hammered into a head, and allowed to cool.
Rivets 3. The contraction on cooling helps to pull the steel plates together.
rivets steel rivets Uses of expansion • If we rivet a strip of brass and a strip of steel together, we have what is called a bimetallic strip.
brass steel heated 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.
brass steel chilled 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.
Uses of bimetallic strips • Thermostat • A thermostat keeps temperature constant. • 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.
Uses of bimetallic strips • Bimetallic thermometer • A bimetallic strip can be made into a long, nearly flat coil. • It has one end fixed and the other carries a pointer. • The coil tightens or unwinds as the temperature goes up and down. • The pointer then moves over a scale marked in temperature degrees.
Miscellaneous • http://en.wikipedia.org/wiki/Thermal_expansion • http://www.translatorscafe.com/cafe/units-converter/thermal-expansion/ • http://en.wikipedia.org/wiki/Expansion_joint • http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/bimet.html • http://home.howstuffworks.com/home-thermostat.htm