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objectives
objectives
Fixing a Temperature Scale
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 die 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 right 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 diagram on the right.
- Bailing 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.
- 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 be extended to produce a scale going above 100 °C and below 0 0C, although for some thermometers, additional fixed points are used (see the top of the next slide).
- 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 0C. If a scale has equal divisions, it is described as a linear scale.
Liquid in-glass Thermometers
Liquid in-glass Thermometers
- Range Mercury freezes at -39 0C; 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 around 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.
- The narrower the tube, the further the liquid moves up it when the temperature rises.
Temperature 2 Worksheet
Temperature 2 Worksheet
5.03---Temperature-2.docxPage updated
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