Temperature (1) - Oxford University Press

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102 THERMAL T EFFECTS ice point 0 °C pure melting ice 5.03 � Finding the lower fixed point steam point 100 °C boiling water � Finding the upper fixed point <strong>Temperature</strong> (2) Fixing a temperature scale To create a temperature scale, two standard temperatures must be chosen against which others can be judged. These fixed points need to be defined so that they can be reproduced in laboratories anywhere in the world: On the Celsius scale: 0 degrees Celsius (0 �C) is defined as the melting point of pure ice. This is the lower fixed point, known as the ice point. 100 degrees Celsius (100 �C) is defined as the boiling point of pure water, where the water is boiling under standard atmospheric pressure (101 325 Pa). This is the upper fixed point, known as the steam point. Putting a scale on an instrument, so that it gives accurate readings, is called calibrating the instrument. The diagrams on the left show how the fixed points can be used to calibrate an unmarked thermometer. To find the 0 �C point, the unmarked thermometer is placed in pure, melting ice, as in the upper diagram on the left. The ice needs to be pure because its melting point is lowered if any impurities are present. To find the 100 �C point, the thermometer is placed in steam above boiling water, as in the lower diagram on the left. Boiling must take place under standard atmospheric pressure because a change in pressure alters the boiling point of the water. Impurities also affect the boiling point but, in most cases, they do not affect the temperature of the steam just above the water. ice point steam point 0 °C if end of ’thread‘ is half way between 0 °C and 100 °C, temperature is 50 °C 100 °C Once the 0 �C and 100 �C points have been fixed, the rest of the scale is made by dividing the distance between them into 100 equal divisions, or degrees (‘centigrade’ means ‘one hundred divisions’). The idea can can be extended to produce a scale going above 100 �C and below 0 �C, although for some thermometers, additional fixed points are used (see the top of the next page). Putting equal divisions on a thermometer defines the temperature scale for that particular type of thermometer. For example, if the end of the ‘thread’ is exactly half way between the ice points and the steam points, as in the diagram above, then by definition, the temperature is exactly half way between 0 �C and 100 �C. So the temperature is 50 �C. If a scale has equal divisions, it is described as a linear scale.
Q Additional fixed points Although the ice and steam points are sufficient to create a scale over the range required for everyday temperature measurements, there are additional fixed points for much higher and lower temperatures. You can see some examples on the right. For simplicity, the temperatures are given to the nearest degree. In practice, greater accuracy is used. Liquid-in-glass thermometers Nearly all liquids expand slightly when heated. This property is used in liquid-in-glass thermometers, which are normally filled with alcohol or mercury. Sensitivity Some thermometers are more sensitive to temperature change than others. The ‘thread’ of liquid moves further. The diagrams on the right show how tube width affects the sensitivity. The narrower the tube, the higher the sensitivity of the thermometer. Mercury expands less than alcohol (for the same volume and same temperature rise). So a mercury thermometer must have a narrower tube than an alcohol thermometer to give the same sensitivity. Range Mercury freezes at �39 �C; alcohol freezes at a much lower temperature, �115 �C. However, some mercury thermometers have an upper limit of 500 �C, which is much higher than that of any alcohol thermometer. Responsiveness Some thermometers respond more quickly to a change in temperature than others. A thermometer with a larger bulb, or thicker glass round the bulb, is less responsive because it takes longer for the alcohol or mercury to reach the temperature of the surroundings. Linearity Although mercury and alcohol thermometers must agree at the fixed points, they do not exactly agree at other temperatures. That is because the expansion of one liquid is not quite linear compared with the other. However, within the 0�100 �C range, the disagreement is very small. Thermocouple thermometer For a diagram and brief description, see the previous spread, 5.02. Compared with a liquid-in-glass thermometer, a thermocouple thermometer is robust, quick to respond to temperature change, has a wide range (�200 �C to 1100 �C), and can be linked to other electrical circuits or a computer. 1 The thermometer on the right has the ice and steam points marked on it. a On the Celsius scale, what is the temperature of i the ice point ii the steam point? b What is the temperature reading in �C, if the end of the ‘thread’ is at i point A ii point B iii point C? c Explain why reading C would not be possible with a mercury thermometer. 2 A – smaller bulb B – thicker glass round bulb C – thinner tube For a liquid-in-glass thermometer, which of the above would a increase the sensitivity? b increase the responsiveness? Related topics: standard atmospheric pressure 3.08 THERMAL EFFECTS Fixed point <strong>Temperature</strong> boiling point of liquid oxygen �183 �C freezing point of molten (liquid) zinc 420 �C freezing point of molten (liquid) silver 962 �C freezing point of molten (liquid) gold 1064 �C steam point same volume of liquid alcohol lower temperature same increase in volume of liquid alcohol higher temperature � The narrower the tube, the further the liquid moves up it when the temperature rises. ice point A B C cm 30 20 10 0 ! 103
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