Transcript Slide 1
Heat and Its Effects 1 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. 2 Expansion and contraction • Solids o 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. 3 Expansion and contraction Experiment 1: Expansion of solids metal ring metal ball • 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. 4 Experiment 1: Expansion of solids Ans: After heating, the metal ball expands and is unable to go through the ring. 5 Experiment 1: Expansion of solids metal ring metal ball • 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. 6 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. nut iron bar steel rod 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. 7 Expansion and contraction Experiment 2: Expansion of solids bicycle spoke light pointer drinking straw clamp weight 1. 2. AMovement 3. When bicycle heated spoke is seen with is made the when flame, toa rest pointer the with spoke piercing one expands end the straw on and a drinking rotates rolls the by straw. straw. an Aangle. weight is attached to the spoke between the flame and the straw. 8 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. 9 Experiment 3: Expansion and contraction of water coloured water hot 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. 10 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. 11 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! 12 Expansion of cake • Yeast is added to the dough • Later bake in the oven at high temperature. • Why? 13 Expansion and contraction • Gases o 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. 14 Expansion and contraction Experiment 4: Expansion and contraction of air glass tube drop of coloured water has dropped air iced water 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. 15 Problems caused by expansion • Roads and pathways o 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. 16 Problems caused by expansion • How long is a railway track? • Can you guess what happens to railway track during a very hot day? 17 Problems caused by expansion Solution: Introduce expansion gap Why? 18 What are some of the problems a long bridge faces during a hot day? 19 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 20 Problems caused by expansion • Water pipes o 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. 21 Problems caused by expansion • Telephone wires and cables o 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. 22 Uses of expansion • Rivets o Rivets are used to join 2 steel plates together 1. When red-hot, the rivet is put into position. 23 Rivets 2. It is then hammered into a head, and allowed to cool. 24 Rivets 3. The contraction on cooling helps to pull the steel plates together. 25 Do you think all types of material expand at the same rate? 26 Uses of expansion • If we rivet a strip of brass and a strip of steel together, we have what is called a bimetallic strip. rivets rivets steel 27 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 heated steel 28 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. brass steel chilled 29 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 30 strip bends away to break the contact. Uses of bimetallic strips • Bimetallic thermometer o 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. 31 Miscellaneous • http://en.wikipedia.org/wiki/Thermal_expansion • http://www.translatorscafe.com/cafe/unitsconverter/thermal-expansion/ • http://en.wikipedia.org/wiki/Expansion_joint • http://hyperphysics.phyastr.gsu.edu/hbase/thermo/bimet.html • http://home.howstuffworks.com/homethermostat.htm 32