Temperature is a pretty weird thing if you think about it. How do you best define temperature? Let me go ahead and give you my favorite definition:
Temperature is the thing that two objects have in common when they have been in contact for a long time.
Yes, that is a good definition. Maybe now you can see why temperature is weird. Doesn’t it have something to do with energy? Well, something – yes. Let me take an example. Suppose pour some hot coffee into a paper cup (I use paper because styrofoam(TM) is trademarked). Further suppose that this is super hot coffee from McDonald’s. Can you touch the paper and not be hurt? Yes. Can you touch the coffee and not be hurt? NO! However, they have the same temperature.
Why do the coffee and the cup have different effects even though they have the same temperature. There are two reasons. First, they have different amounts of ‘thermal energy’. I think the term thermal energy is a little vague, but it is useful. I will define thermal energy as the kind of energy that something has more of when it gets hotter. You may want to think of it as the kinetic energy of the atoms the material is made of. Hot coffee atoms (coffee should be on the periodic table) are moving faster than cold coffee atoms. So, when something is hotter than it was, it increases in some type of energy.
The other reason that the coffee and paper cup have different effects is what could be call specific heat. If you want, you can think of these two reason as really just “how much energy it has”. The coffee has more energy because there is more coffee than paper and coffee can also hold more energy per piece of coffee.
The coffee and the paper are interesting to consider since they have the same temperature but don’t act the same. Here is another great example. Suppose you are in a room with a rug on top of a ceramic floor. If they have been undisturbed, they will have the same temperature. Place your hand on the rug and then on the ceramic floor. Which one feels warmer? The rug will probably feel warmer. So, people are not very good at judging temperatures. Another good example is to comparing being in a room with 70 degree F air to being in 70 degree F water (very cold).
Ok, temperature is weird. But then what is it? How about I answer a different question? Suppose I put a cold piece of metal in room temperature glass of water. What will happen? If you say “the water will get colder and the metal will get warmer until they are at the same temperature” – then you are correct.
When that happens, the water decreases in thermal energy (where I have not really defined thermal energy) and the metal increases in thermal energy. In this case, I made it so that the two materials (water and metal) had the same, but opposite change in temperature. I just did that to make it easier to see, but clearly that does not always happen (depends on the mass of the two objects). What if the following happened?
Here, a cold metal is put into water. The result is that the water gets hotter and the metal gets colder (but energy is conserved). This would be an odd event. However, it is not really against the conservation of energy (which has been shown to be very reliable). So, why does the first case happen and the second one is never seen? It turns out that the probability of the second case is extremely small. So, temperature is the state that the two systems together have the most probable outcome. In fact, this has a lot to do with entropy. I am not going to go into that, but Built on Facts has a nice little post about entropy to get you started.