Physics Articles

Why does a room become warmer when heated? Perhaps because we increase the energy of the air in the room? Certainly not

Why do we heat our houses in winter? The answer to this question appears trivial . And indeed hardly anyone would suspect that a physicist would concern himself with this topic in the pages of "Nature" magazine. Yet the Swiss scientist Robert Emden did exactly that. Under the title "Why do we have winter heating?", he wrote: "The layman would respond to this question with : 'so that the room becomes warmer'. A student of thermodynamics might expresse themselves so : 'to supply missing energy' ". In this case, the layman turns out to be correct, not the scientist.

That was in 1938 and yet even today it excites some people to raise an objection. For who could dispute that a hot entity gives out thermal energy to a room? The amount of energy that the room loses - in the form of heat which disappears through walls, windows and doors into the cold winter world - must simply be replenished. So you might always think.

Looking at it thermodynamically every room is an open system. Through heat transport it exchanges energy with the outer world. On top of this, there is also an issue of the exchange of matter with this outer world, which we do not notice too much - it is only when sometimes a door slams unexpectedly that the pressure difference between the inside and outside makes itself known dramatically. The idea that different pressures reign on the inner side and the outer side of the house, seems to be ignored by various domestic weather 'stations'. Often houses are provided with two thermometers (one for inside, one for outside) but with only one barometer.

Were the room, on the other hand, to be closed up hermetically, warmth from the heating would increase the air pressure inside just like in a bicycle tube which becomes more and more bulging if left in the Sun. In reality however, increased interior pressure leads to air flowing outside; and vice-versa air streams to the interior as soon as the room temperature sinks. Yet it is not only air itself that is transported. It always takes energy with it - the thermal energy of the room's air is understood microscopically as the kinetic energy of the unordered, racing here and there, air particles. If these particles leave the room, they also take energy from the inside to the outside with them.

If you carry out a small calculation, then you will experience a surprise. The outflowing air caused by the increased internal pressure takes exactly as much energy with it as is provided by the heating to the room's air. Formulated in another way : although energy is supplied - someone has finally turned up the heating - the amount of energy in the room stays the same.

The opposite consideration appears almost still more surprising. I cool the room's air a little when I fetch a bottle of red wine from the cold cellar and this warms itself to room temperature. However, instead of now a minor pressure reduction because the bottle has cooled the air, exactly so much air flows into the room from the cold exterior that the pressure stays the same. And again the energy remains constant. One can thus say without exaggeration : the energy which the red wine extracts from the room's air, on becoming warmer, stems ultimately from the cold outer world!

It becomes still more curious however, for looked at exactly, the energy of the room reduces even when heated and increases by cooling. The air molecules, which are turned in or out of the room as a consequence of the energy adjustments, take with them, apart from thermal energy, their own chemical binding energy, the energy of the atomic nucleus and various other things that could be taken into account. In spite of this, naturally it does not becomes more frosty or more cosy ny virtue of these energy contributions.

In our considerations we have admittedly always argued typically physically, and thus intentionally neglected "side-issues" such as the influence of the walls or the altered air volume caused by the bottle containing red wine.

Nevertheless we have gained a central perception : if we want to warm a room up, it is in no way solely a matter of the energy. That is aslo shown by a further fundamental consideration. For although some people would suppose the opposite : in fact, energy is neither produced or destroyed. In other words, it possesses the fundamental characteristic of remaining preserved always and under all conditions. This theorem of energy conservation is also named the First Law of Thermodynamics by scientists.

Were however the First Law by itself to constitute the entire truth of the situation, there would be no energy crises and neither would it be justified to pay out good money for energy. Because then, for example, it would at least theoretically be possible for richly available thermal energy to flow into the warm room all by itself from the cold surroundings and so increase the temperature. Yet that does not happen, as experience shows us. Heat is always transferred only from warm areas to colder areas and not vice-versa. This knowledge is expressed by the Second Law of Thermodynamics. Clearly stated, it means that all processes proceeding by themselves are connected with the dissipation of energy, and thus with the transfer of heat to the surroundings.

In order to generally appreciate this state of affairs, thermodynamicists long ago introduced a new quantity : the entropy. This quantity grows when energy is dissipated . And therefore the Second Law, which is also called the Entropy Law, can be formulated thus : if a process proceeds by itself, the entropy increases, or at least remains constant. The fact that a room does not warm up all by itself, a ball does not roll upwards all by itself, and a balloon does not inflate all by itself, are indications of the truth of this law : because all these procedures would reduce the entropy.

In spite of this, it is possible for heat to flow from cold areas to warm areas - if we help something along appropriately. In this way, some heating installations use the energy of cold outside air or of cold groundwater. Although by doing this the entropy goes down, the process does not function by itself but through the application of heat pumps. Their dissipated (mostly electric) energy ultimately produces so much entropy that in the total balance the aforementioned entropy reduction is, at the very least, camcelled out.

We come back to the initial point and ask how it is possible that the temperature in the room rises due to the heating, although the energy remains constant.

The decisive factor is that thereby the number of particles in the room reduces. As the temperature is closely linked with the unordered thermal motion, more exactly the average kinetic energy of the air particles, the energy distributes itself as a consequence among fewer particles. These possess henceforth a greater average kinetic energy - and we detect a higher temperature.