Wednesday, January 12, 2011

What is temperature?

According to the thermometer it's hotter than it was yesterday, but it feels colder; um why? To answer this requires some delving into what temperature is and that can be answered at the same time as how it is transferred.

In high-school physics the three main types of heat transfer are listed as conduction, convection, and radiation and like so much physics learned at that time it's not quite right. There are only two - conduction and radiation; what is referred to as convection is in fact fluidic conduction which combines heat transfer and heat distribution a distinction I'll deal with later.

Radiation is the heat type we receive from the sun it requires no medium to travel in and can travel long distances; it is however not a speedy method of heat transfer. The only reason it has such a large effect on us is the sheer amount being pumped out by the sun.

The most active type of heat transfer is conduction and this requires a small explanation as to what heat is. Heat is energy, pump a molecule full of energy and it starts to jig about. Imagine an atom or molecule as a small sphere then imagine a larger sphere around it that describes its range of motion. The more energy (heat) it has the larger that sphere.

So we have an atom or molecule jigging about, now add another such also jigging about and their two range spheres may intersect. If that happens then the two atoms may bump into each other and when that happens they exchange energy; think of two snooker balls colliding. This is conduction.

Now in a solid the atoms are linked by bonds which means their range spheres are constrained, pump enough energy into them and those bonds can't contain the jigging and they become liquids, add more energy and even those tenuous bonds break and they become gases. In this state the atoms can move about more freely and hit atoms other than those that were directly sitting next to them as in a solid. Likewise this jigging about means that a volume of a hot liquid or gas within another volume contains less atoms and is thus less dense, which in turn means that it will move as an entity within the colder gas. All these added complications is why the term convection is split off from conduction - it's conduction with bells on.

Onto the next high school lie - energy transfer moves from hot to cold. This is only correct when the cold object is truly cold; that is has zero energy otherwise the phrase misses a term "net" thus net energy transfer is from hot to cold. It works like this - a hotter object will transfer more energy than a colder object of the same type thus a hot object may transfer 2° of heat into a colder object but only receive 1° in return, a net transfer of 1° of heat into the colder object.

Again hopefully it can be seen that in such closed circumstances the hotter object will get colder while the colder object becomes warmer until they both reach the same temperature and simply exchange the same energy between them.

What if the system is not closed? If I can keep providing the hot object with new colder objects to collide with then the hot object will drop in temperature while the colder objects will only warm up a partial amount before being moved away. This is the action behind fans - they don't make the air cooler they simply circulate a stream of colder air across a hotter object (often yourself). You transfer energy into the cold air and the fan acts to move it away and replace it with a fresh source of cold air. This should also explain why fans don't work when the air itself is too hot or worse can make you feel hotter. If the air is hotter than you then in still air the air loses energy to you making you hotter, but the non-moving air becomes cooler until you reach equilibrium. With a fan that still hot air is being replaced each time with hotter air which transfers its energy to you.

At this point we reach subjective temperature - the temperature you feel. As just mentioned the fan does not cool the air yet it feels cold to us because we can transfer our heat energy into it while receiving little back in return. As such our perception of heat is to do with the transfer of energy from ourselves.

Air can transfer heat at a specific rate, water can transfer heat at a different rate. So have two areas of the same temperature and fill one with dry air and the other with wet air and to us the temperature feels different because the air is transferring heat from us at different rates - the wet air is 'pulling' heat from us quicker than the dry air and thus our brains report it as being colder.

The converse is also possible. If the wet air is hotter than us it transfers energy to us quicker than dry air and thus feels hotter.

So this morning it feels colder despite being the same temperature as yesterday because it's wetter.