Calculations based upon various considerations point to the fact that absolute zero corresponds to about -273° C.
We have just pointed out that in a perfectly efficient engine
T2 – T1
———
=
1.
T2
In order that this may be so, we must have T1 = 0, the absolute zero.
In practice it is impossible to make the temperature of the exhausted gases as low as this, and so the only way to obtain more efficient engines is to make T2 as large as possible, that is to say, the initial temperature of the gases must be high.
It is, however, just as possible to turn all the heat supplied to a heat engine into work as it is to use up all the energy of a waterfall in a turbine, because the level from which the zero of the potential of the energy water is measured is the centre of earth, which is as inaccessible as absolute zero of temperature.