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Temperature and thermometer

Temperature and thermometer. Temperature. A basic physical quantity Measure the degree of hotness of a body Measure by an instrument called thermometer. Common Temperature Scale. 2 common types Celsius scale ( o C) Fahrenheit scale ( o F) Relationship F = 1.8 C + 32 Example

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Temperature and thermometer

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  1. Temperature and thermometer

  2. Temperature • A basic physical quantity • Measure the degree of hotness of a body • Measure by an instrument called thermometer

  3. Common Temperature Scale • 2 common types • Celsius scale (oC) • Fahrenheit scale (oF) • Relationship F = 1.8 C + 32 • Example If C =30 oC then F =84 oF

  4. Temperature Scale • 2 fixed points at temperatures (0oC and 100oC) are chosen • ice point (0 oC) • steam point (100 oC) • divide the range between ice point and steam point by 100 equal divisions

  5. Question length of mercury column at 0oC is 12 cm length of mercury column at 100oC is 22 cm What is the temperature if the length of the column is 16 cm?

  6. Answer 1 cm of mercury column represents 100 / (22 - 12 ) = 10 oC length of the mercury column is (16 - 12) cm = 4 cm temperature at length 16 cm is 4 x 10 = 40 oC

  7. 100 oC 40 0 cm 12 16 22 From the graph, the value is 40oC Graphical Method

  8. Question length of mercury column at 37oC is 12.0 cm length of mercury column at 100oC is 26.1 cm What is the length of the mercury column at temperature 0 oC?

  9. Different type of thermometers • liquid-in-glass thermometer • mercury-in-glass • alcohol-in-glass • Why not use water ??? • Other materials which can vary with temperature • resistance, current, length, color, infra-red

  10. Liquid-in-glass thermometer • Liquid-in-glass thermometer • Capillary tube ensure that a small change in volume causes a large change in length • Alcohol • range –115oC to 78oC • Mercury • range –39oC to 357oC

  11. Mercury-in-glass thermometer • Advantages • Expands evenly on heating • Responds quickly to temperature • A high boiling point, so used in hot places • It does not wet • Disadvantages • Poisonous • Expensive • A high freezing point, so not used in cold places

  12. Alcohol-in-glass thermometer • Advantages • Expands about six times of mercury • Expand evenly on heating • A low freezing point, so used in very cold places • It is safe • It is cheap • Disadvantages • It is dyed • It wet the tube • It does not respond quickly with temperature • A low boiling point, so not used in hot places

  13. Clinical thermometer • Clinical thermometer • A specialized mercury-in-glass thermometer to take body temperature • A constriction to prevent mercury from falling • range a few degrees above and below 37oC

  14. Other types of thermometer • Rotary thermometer • Resistance thermometer • Liquid Crystal thermometer • Thermistor thermometer • Thermocouple thermometer • Infra-red thermometer

  15. Rotary thermometer thermometric property : expansion rate

  16. Resistance thermometer thermometric property : resistance

  17. Liquid Crystal thermometer thermometric property : colour

  18. Thermistor thermometer thermometric property : resistance

  19. Thermocouple thermometer thermometric property : current

  20. Infra-red thermometer thermometric property : infra-red

  21. To define a temperature scale • To define a temperature scale • thermometers must have thermometric properties (e.g. length, current) which varies with temperature • the values of the property such as length are measured at 2 fixed temperatures which are easily reproduced • the interval between the 2 fixed temperatures is divided into a number of subintervals

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