• The temperature of a body tells us how hot the body is.
• It is measured using a thermometer and usually in degrees Celsius
(ºC).
• The kinetic theory (Chapter 17) regards temperature as a measure
of the average kinetic energy (k.E.) of the molecules of the body.
• The greater the K.E., the faster the molecules move and the
higher the temperature of the body.
Liquid-in-glass thermometer
• In this type the liquid in a glass bulb expands up a capillary tube
when the bulb is heated.
• The liquid must be easily seen and must expand (or contract) rapidly
and by a large amount over a wide range of temperature.
• It must not stick to the inside of the tube or the reading will be too
high when the temperature is falling.
• Mercury and coloured alcohol are in common use. Mercury freezes
at −39ºC and boils at 357ºC.
• Alcohol freezes at −115ºC and boils at 78ºC and is therefore more
suitable for low temperatures
Scale of temperature
• A scale and unit of temperature are obtained by choosing two
temperatures, called the fixed points, and dividing the range
between them into a number of equal divisions or degrees.
Clinical thermometer
Thermocouple thermometer
Thermocouples are used in industry to
measure a wide range of temperatures
from −250ºC up to about 1500ºC,
especially rapidly changing temperatures
and those of small objects.
Other thermometers
Resistance thermometer uses the fact that the electrical resistance of a platinum
wire increases with temperature. A resistance thermometer can measure
temperatures accurately in the range −200ºC to 1200ºC but it is bulky and best
used for steady temperatures.
A thermistor
A thermistor can also be used but over a small range, such as −5ºC to 70ºC; its
resistance decreases with temperature.
The constant-volume gas thermometer
The constant-volume gas thermometer uses the change in pressure of a gas to
measure temperatures over a wide range. It is an accurate but bulky instrument.
Thermochromic liquids
Thermochromic liquids which change colour with temperature have a limited
range around room temperatures.