Some Properties of Matter. Грекова О.А. - 12 стр.

                                                 12
                           III
       In imagination we can go to extremely high temperatures, but we cannot go to
unlimitedly low temperatures. There is a definite lower limit to the temperature scale, called
the absolute zero: it is 273 o C. below the freezing-point of water, or – 273o C. Modern workers
have got very close to this absolute zero in the laboratory.
       There is practically no limit to the height that temperature can reach: the highest
temperature in our hottest furnaces is extremely low compared to the temperature in the
interior of stars or that reached for a moment in the explosion of atomic bombs, when the
breaking of atoms is in question. On the other hand, the lower the temperature the less the
motion of atoms and molecules, but this motion cannot diminish to less than nothing at all.
There cannot be a temperature lower than that at which all the particles are at rest, and it is this
temperature that is the absolute zero. At the absolute zero gases cannot exist, and all the
familiar gases have solid form well before the absolute zero is reached.

                                                     IV
       Heat is a form of energy. There is no friction without the generation – that is, the birth
– of heat, and the heat generated is equivalent to the work done against friction. The
establishment of this was a most important step and the statement that heat is a form of energy
constitutes what is known as the first law of thermodynamics.
       The unit of heat is called calorie, and is the heat required to rise the temperature of 1
gramme of water by 1 o C. What is called the mechanical equivalent of heat is the amount of
work – i.e., of energy – required to produce 1 calorie. To raise the temperature of 1 gr. of water
by 1o C. takes the work required to lift 1 kg. Through 43 cm., or to raise the temperature of 1
pound of water by 1o F. needs about the work required to lift 79 pounds through 10 feet or 790
pounds through 1 foot.
       It is not only possible to convert work into heat, it is also possible to convert heat into
work, as every steam-engine demonstrates. Here the rate of exchange is the same: the heat of 1
calorie, if it can be turned into work, suffices to lift 1 kg through 43 cm. But there is a special
law that governs the conversion of heat into work, and this we must now consider.
       Heat can never be converted into work unless there is a difference of temperature. There
is a law which states that it is not possible to construct a machine which will continuously
furnish useful work by taking heat from a body no warmer than its surroundings. This is called
the second law of thermodynamics.
       To turn heat into work, then, we must have a difference of temperature, a body hotter
than its surroundings. In any steam-engine, which we may take as an example, we have a
boiler, very hot, and a condenser, relatively cold. The fraction of the heat that we can use is
fixed by the difference of temperature – the greater this is the better.
       The sum always works out right: heat taken from boiler equals heat turned into work
plus heat returned to condenser. That is the Conservation of Energy. If we could have a
condenser at the absolute zero of temperature we could, theoretically, turn all the energy of a
hot body into work, but this is, of course, not practically possible. The best we can do is to
keep the condenser at something near the temperature of the surroundings, by cooling it with
the coldest water available.
                                      Comprehension check
    1) Which temperature scales are widely used in the modern world?
    2) What is the main principle of Celsius scale? (Fahrenheit scale?)
    3) Which scale is used in Russia? (Gr. Britain, America)?
    4) What kinds of thermometers do you know? Which is the most accurate?
    5) What is absolute zero?