Temperature (1) - Oxford University Press

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104 invar (metal) Pyrex glass platinum alloy glass concrete steel brass aluminium THERMAL T EFFECTS gap 5.04 Kinetic theory essentials According to the kinetic theory, solids and liquids are made up of tiny, vibrating particles (atoms or molecules) which attract each other. The higher the temperature, then on average, the faster the particles vibrate. increase in length of a 1 m bar for a 100 °C rise in temperature rollers 0.1 mm 0.3 mm 0.9 mm 0.9 mm 1 mm 1 mm 2 mm 3 mm Allowing for expansion... Gaps are left at the ends of bridges to allow for expansion. One end of the bridge is often supported on rollers so that movement can take place. ! Expanding solids and liquids If a concrete or steel bar is heated, its volume will increase slightly. The effect is called ca thermal expansion. It is usually too small to notice, but unless space is le left for it, it can produce enough force to crack the concrete or buckle the steel steel. Most solids expand when heated. So do most liquids – and by more than solid solids. If a liquid is stored in a sealed container, a space must be left at the top to al allow for expansion. cold The kinetic theory explains thermal expansion as follows. When, say, a steel bar is heated, its particles speed up. Their vibrations take up more space, so the bar expands slightly in all directions. If the temperature falls, the reverse happens and the material contracts (gets smaller). Comparing expansions The chart on the left shows how much 1 metre lengths of different materials expand when their temperature goes up by 100 �C. For greater lengths and higher temperature increases, the expansion is more. When choosing materials for particular jobs, it can be important to know how much they will expand. Here are two examples: Steel rods can be used to reinforce concrete because both materials expand equally. If the expansions were different, the steel might crack the concrete on a hot day. If an ordinary glass dish is put straight into a hot oven, the outside of the glass expands before the inside and the strain cracks the glass. Pyrex expands much less than ordinary glass, so should not crack. cold day hot day ... and contraction When overhead cables are suspended from poles or pylons, they are left slack, partly to allow for the contraction that would happen on a very cold day. hot
Using expansion alcohol (or mercury) ‘thread’ narrow tube In the thermometer above, the liquid in the bulb expands when the temperature rises. The tube is made narrow so that a small increase in volume of the liquid produces a large movement along the tube, as explained in the previous spread, 5.03. bimetal strip: cold ...hot invar brass brass expands most In the bimetal strip above, thin strips of two different metals are bonded together. When heated, one metal expands more than the other, which makes the bimetal strip bend. Bimetal strips are used in some thermostats – devices for keeping a steady temperature. The thermostat shown on the right is controlling an electric heater. Water and ice* When hot water cools, it contracts. However, when water freezes it expands as it turns into ice. The force of the expansion can burst water pipes and split rocks with rainwater trapped in them. Water expands on freezing for the following reason. In liquid water, the particles (water molecules) are close together. But in ice, the molecules link up in a very open structure that actually takes up more space than in the liquid – as shown in the diagram on the right. Ice has a lower density than liquid water – in other words, each kilogram has a greater volume. Because of its lower density, ice floats on water. When liquid water is cooled, the molecules start forming into an open structure at 4 �C, just before freezing point is reached. As a result, water expands very slightly as it is cooled from 4 �C to 0 �C. It takes up least space, and therefore has its maximum density, at 4 �C. Q 1 Explain the following: a A metal bar expands when heated. b Overhead cables are hung with plenty of slack in them. c It would not be a good idea to reinforce concrete with aluminium rods. d A bimetal strip bends when heated. e* Water expands when it freezes. Related topics: density 1.04; kinetic theory and particles 5.01; thermometers 5.02 °C THERMAL EFFECTS current from supply control knob contacts � Bimetal thermostat When the temperature rises, the bimetal strip bends, the contacts separate, and the current to the heater is cut off. When the temperature falls, the bimetal strip straightens, and the current is switched on again. In this way, an approximately steady temperature is maintained. molecules in liquid water molecules in ice current to heater brass invar bimetal strip 2 This question is about the thermostat in the diagram at the top of the page. a Why does the power to the heater get cut off if the temperature rises too much? b To maintain a higher temperature, which way would you move the control knob? – to the right so that it moves towards the contacts, or to the left? Explain your answer. 105
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Temperature (1) - Oxford University Press

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