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 (oC)– Fahrenheit scale (oF)

• Relationship F = 1.8 C + 32• Example

If C =30 oC then F =84 oF

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

Different type of thermometers• liquid-in-glass thermometer

– mercury-in-glass– alcohol-in-glass

• Other materials which can vary with temperature– resistance, current, length, color, infra-red

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

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

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

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

Other types of thermometer

• Rotary thermometer• Resistance thermometer• Liquid Crystal thermometer• Thermocouple thermometer• Infra-red thermometer

Rotary thermometerthermometric property : expansion rate

Resistance thermometerthermometric property : resistance

Liquid Crystal thermometer

thermometric property : colour

Thermocouple thermometerthermometric property : current

Infra-red thermometer

thermometric property : infra-red

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