The most easily observed examples of thermal expansion are size changes of materials as they are heated or cooled. Almost all materials (solids, liquids, and gases) expand when they are heated, and contract when they are cooled. Increased temperature increases the frequency and magnitude of the molecular motion of the material and produces more energetic collisions. Increasing the energy of the collisions forces the molecules further apart and causes the material to expand.
Different materials expand or contract at different rates. In general, gases expand more than liquids, and liquids expand more than solids. Observation of thermal expansion in a solid object requires careful scrutiny. Several everyday examples are: 1) The sag in outdoor electrical lines is much larger on hot summer days than it is on cold winter days. 2) The rails for trains are installed during warm weather and have small gaps between the ends to allow for further expansion during very hot summer days. 3) Because the metal expands more than glass a stuck metal lid on a glass container can be loosened by running hot water over the joint between the lid and the container.
Liquids generally expand by larger amounts than solids. This difference in expansion rate is sometimes observed when the gas tank of a car is filled on a hot day. Gasoline pumped from the underground container is cold and it gradually heats to the temperature of the car as it sits in the gas tank. The gasoline expands in volume faster than the gas tank and overflows onto the ground.
Gases expand even more than liquids when heated. The expansion difference between a gas and a solid can be observed by filling a plastic air mattress in a cool room and then using it on a hot beach. The difference in thermal expansion between the container and the gas could unexpectedly over inflate the mattress and blow a hole in the plastic.
Sometimes man's ingenuity has led him to find practical applications for these differences in thermal expansion between different materials. In other cases, he has developed technologies or applications that overcome the problems caused by the difference in thermal expansion between different materials.
The thermally induced change in the length of a thin strip of metal differs for each material. For example, when heated, a strip of steel would expand by half as much as an equal length piece of aluminum. Welding together a thin piece of each of these materials produces a bimetallic strip.
The difference in expansion causes the bimetallic strip to bend when the temperature is changed. This movement has many common uses including: thermostats to control temperature, oven thermometers to measure temperature, and switches to regulate toasters. Some practical solutions to everyday thermal expansion problems in solids are: 1) The material developed for filling teeth has the same expansion as the natural enamel of the tooth. 2) The steel developed to reinforce concrete has the same expansion as the concrete. 3) Concrete roads are poured with expansion joints between the slabs to allow for thermal expansion (these joints are the cause of the thumping noise commonly experienced when traveling on a concrete highway).
The manufacture of mercury and alcohol thermometers is based upon the expansion difference between solids and liquids. Thermometer fabrication consists of capturing a small amount of liquid (mercury or alcohol) inside an empty tube made of glass or clear plastic.
Because the liquid expands at a faster rate than the tube, it rises as the temperature increases and drops as the temperature decreases. The first step in producing a thermometer scale is to record the height of the liquid at two known temperatures (i.e. the boiling point and freezing point of water). The difference in fluid height between these point is divided into equal increments to indicate the temperature at heights between these extremes.
Automobile engine coolant systems provide a practical example of a liquid-thermal expansion problem. If the radiator is filled with coolant when the engine is cold, it will overflow when the engine heats during operation. In older car models, the excess fluid produced by the hot temperatures was released onto the ground. Periodic replacement was required to avoid overheating. Newer cars have an overflow container that collects the released fluid during thermal expansion and returns it to the radiator as the engine cools after operation. This improvement in the coolant system reduces the number of times the coolant fluid level must be checked and avoids the expense of replacing costly antifreeze material mixed with the radiator fluid.
Hot-air balloons are an obvious example of the practical use of the thermal expansion difference between a gas and a solid. Because the hot air inside the balloon bag increases in size faster than the container it stretches so that it expands and displaces the colder (heavier) air outside the bag. The difference between the lower density of the air inside the bag compared to the lower density of the air outside the bag causes the balloon to rise. Cooling the air inside the bag causes the balloon to descend.
Water, like most other liquids, expands when heated and contracts when cooled, except in the temperature region between 32°F (0°C) and 39.2°F (4°C). A given mass of fresh water decreases in volume until the temperature is decreased to 39.2°F (4°C). Below this temperature, the volume per unit mass increases until the water freezes. This unusual behavior is important to freshwater plants and animals that exist in climates where water freezes in the colder seasons of the year. As the water surface cools to 39.2°F (4°C), it becomes more dense and sinks to the bottom, pushing the warmer water to the surface.
This mixing action continues until all of the water has reached this temperature. The upper layer of water then becomes colder and less compact and stays near the surface where it freezes. When an ice layer forms, it provides an insulation barrier that retards cooling of the remaining water. Without this process, fresh water animal and plant life could not survive the winter.
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