Temperature
You might hear something like "the temperature today will be
26°C" and this gives you some idea of what to expect from
past experience. In scientific terms, temperature is a measure of
"hotness" or average kinetic energy and is usually measured
with a thermometer. Knowing the temperature of an object will tell
us something about the way it interacts with its surroundings. If
an object is hotter than its surroundings, heat energy will flow from
the object to its colder surrounds. If it is colder, then heat energy
will flow from the surrounds to the object.
Temperature and heat
Think about how you can get warm quickly by getting into a hot bath.
Your temperature may be 37°C and the water 45°C. The water,
being hotter, transfers some of its energy into your body until both
you and the bath are the same temperature. Now think about getting
into a cold bath. Which way does the energy go now?
Thermometers and temperature scales
You may already know that there are at least two different temperature
scales used in everyday life, but how did they come about?
Thermometers use the principle that gases and liquids expand as they
get hotter and contract as they get colder. This expansion and contraction
is caused by the particles moving faster when heated and slower when
cooled. As matter is made up of an enormous number of particles the
combined result of all this movement is that the liquid or gas expands
when heated and contracts when cooled.
The first thermometer was made by Galileo in 1593. It was not
very accurate, but it did show changes in temperature. It wasn't until
1714 that a German physicist, Gabriel Fahrenheit, made a thermometer
with mercury in a small bore glass tube, that an accurate thermometer
with a scale was made. He chose to set his scale using "fixed
points". One point was the lowest temperature he could reach
by freezing water saturated with salt - he called this 0° . The
upper fixed point was the human body temperature which he set (slightly
incorrectly as it happens) at 100°. Having done this, the freezing
point of water became 32°F and the boiling point of water 212°F
- both rather odd numbers!
A much neater scale was developed in 1742 by a Swedish astronomer
named Anders Celsius. He set the freezing point of pure water at 0°
and the boiling point of pure water, at sea level, as 100°. The
scale was divided into 100 equal divisions, which is why it has been
called Centigrade (divided into 100 parts).
How low can you get?
The usual temperature scales are fine for everyday use as they cover
our normal range of weather, cooking, heating and so on. However,
how cold is it possible to get?
Think back to what temperature is; it is a measure of the movement
of particles. As the temperature gets lower, the particles move less
quickly. At some point they should (theoretically!) stop their wholesale
movement. This lowest possible temperature was established by a British
physicist named William Kelvin (Lord Kelvin) in the 1800s.
It is called absolute zero and is -273°C. At this very low
temperature unusual things happen, for example some metals become
super conductive to electricity which has opened up all sorts of possibilities
for inventions. On the Kelvin scale, the degrees sign is not used
so the freezing point of water is 273 K.
Note:- Modern quantum physics requires that the subatomic particles
in atoms must still move even at absolute zero, but the atoms themselves
do not move.
Other thermometers
Not all thermometers are made of liquid in glass. With the advent
of electronics, probes are made that give a digital read out. These
are much safer for reading body temperatures as there is no risk of
glass breaking and toxic mercury getting into the patient's mouth.
In furnaces, pyrometers measure temperature. These make use of the
fact that very hot steel, for example, emits light, red hot or even
white hot. The colour and intensity of the light is an indication of
temperature.
Finally, just for interest, a little known temperature scale is that
of Reaumur, where the freezing point of water is 0°R and the boiling
point is 80°R!