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FURTHER THOUGHTS ON ENERGY 33

than anyone else’s, and smells nicer. Few people seem to bother reading these bits.
But in most countries, the law says the label on the can should also gives details of
the can’s contents, both in terms of the net mass of air freshener it contains and also
perhaps a few details concerning its chemical composition. Finally, a few words of
instruction say how to dispose safely of the can. In this context, the
usual phrase printed on the can is, ‘Do not incinerate, even when CFC stands for chlo-
empty’. But why? rofluorocarbon. Most
It is common for the can to contain a propellant in addition to the CFCs have now been
actual components of the air freshener mixture. Commonly, butane banned because of
or propane are chosen for this purpose, although CFCs were the their ability to dam-
favoured choice in the recent past. age the ozone layer in
Such a can is thrown away when it contains no more air fresh- the upper atmosphere.
ener, although it certainly still contains much propellant. Inciner-
ation of the can leads to an increase in the kinetic energy of the
remaining propellant molecules, causing them to move faster and
Pressure increases with
faster. And as their kinetic energy increases, so the frequency with increasing temperature
which they strike the internal walls of the can increases. The force because the collisions
of each collision also increases. In fact, we rediscover the ideal gas between the gas parti-
equation, Equation (1.13), and say that the pressure of the gas (in cles and the container
a constant-volume system) increases in proportion to any increase wall are more ener-
in its temperature. In consequence, we should not incinerate an old getic and occur more
can of air freshener because the internal pressure of any residual frequently.
propellant increases hugely and the can explodes. Also note the
additional scope for injury afforded by propane’s flammability.

1.4 Further thoughts on energy

Why is the room warm?

The energy of room temperature

Imagine coming into a nice, warm room after walking outside in the snow. We
instantly feel warmer, because the room is warmer. But what exactly is the energy
content of the room? Stated another way, how much energy do we get from the air
in the room by virtue of it being at its own particular temperature?
For simplicity, we will consider only the molecules of gas. Each molecule of gas
will have kinetic energy (the energy of movement) unless the temperature is absolute
zero. This energy may be transferred through inelastic molecules collisions. But how
much kinetic energy does the gas have?
3
At a temperature T , 1 mol of gas has a kinetic energy of RT , where T is the ther-
2
modynamic temperature and R is the gas constant. This energy is directly proportional
to the thermodynamic temperature, explaining why we occasionally call the kinetic
energy ‘thermal motion energy’. This simple relationship says that temperature is
merely a measure of the average kinetic energy of gas molecules moving chaotically.