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Physics in Plain English

You are here: Physics → Thermodynamics → Temperature

What is Temperature?

And for that matter, what is heat? (It's not the same thing as temperature). I will answer these questions. I promise. But in order to do so, I must first deal with a slightly "different" topic. Energy.

What is Energy, then?

You may have previously learned that there are different types of energy in the world. For example, when an obsolete petrol (gasoline) driven car drives down a road, chemical energy stored in the fuel gets converted to thermal energy (heat), and sound energy; when the fuel is ignited. The thermal energy turns into kinetic energy (motion) when it causes a piston to rise and fall; and this motion drives the car down the road.

If you learned this then, I'm afraid, it's not entirely true. There's only one type of energy. Kinetic energy. All the rest are just kinetic energy in disguise.

I've already said that kinetic energy is motion. In fact, kinetic energy of a moving object is defined as follows:

Ekinetic = ½ × mv2,    where Ekinetic ≡  Kinetic energy of object,
m ≡  mass of object,
and v ≡  total speed (magnitude of velocity) of the object.

So, kinetic energy is just mass in motion. It's proportional to mass. (That's fair enough, our moving object is made of atoms, which are themselves made of smaller particles, each of which is moving at around the same average speed as the rest of the object, and thus has its own kinetic energy. Multiplying by the object's mass is just like adding up all the kinetic energies of the individual particles.)

Now, when it comes to energy, there's a very important rule - probably the single most important law of physics in the universe. This is the law of conservation of energy:

ENERGY CANNOT BE CREATED OR DESTROYED;
IT CAN ONLY BE TRANSFERRED FROM ONE BODY TO ANOTHER.

Imagine you have a set of billiard balls on a table. When one ball strikes another, it slows down because the energy gets transferred into the other ball, which in turn speeds up. When a ball rebounds off the edge of the table, some of the molecules that make up the ball strike some the particles that make up the table. These particles in turn rebound off other particles in the table, so just like tightly packed billiard balls; the shock vibrates through the wood of the table. It then keeps moving around through the wood until at the outer edge of the table, the wood particles repeatedly strike air molecules; again, just like billiard balls, causing air vibrations, which we hear as sound. The energy never disappears totally - it just keeps getting spread out in this fashion until it ends up as thermal energy, widely labeled the lowest form of energy.

And yes. Thermal energy is also kinetic energy.

Temperature explained

Imagine a room full of air. The air is made of individual particles (molecules, to be precise). These particles don't stand still - they all have their own share of kinetic energy accumulated from the odd billiard table, as well as sources like the Sun. Remember, they can't lose their kinetic energy unless they transfer it elsewhere, and since this residual energy is already all around, that's kind of difficult to do. So, the gas particles are all doomed to travel at their own speed around the room, bouncing off the walls (and each other) like billiard balls. Now if you look at all the gas particles in the room together, and take the average energy of each particle; there's a name for that: it's called temperature. Well, not quite. You actually have to multiply the average kinetic energy by a conversion factor to convert it into units of kelvins, but that's all you have to do.

T = 
  E  
k
   where  T ≡ temperature,
E ≡ average kinetic energy per particle,
and  k is a conversion factor, known as the Boltzmann Constant

The Boltzmann constant, which is simply a conversion factor between temperature and energy units, has the following values, depending on whether you deal with energy in units of joules (J) or electron-volts (eV):

k  = 1.3806505×10−23 J/K
= 8.617 339×10−5 eV/K

This also explains why it's impossible to get colder to a temperature colder than 0K (absolute zero). How, after all, can a particle have energy lower than zero? The very concept of temperatures lower than absolute zero simply doesn't make sense!

Heat explained

Heating is the transfer of the kinetic energy of particles from one body to another. A hotter object placed next to a cooler object will always transfer heat from itself into the cooler object, until both objects are of equal temperature.

For example, if you place a cup of water, at room temperature; into a room, at the same temperature; then for every time an air particle rebounds off and transfers energy to a water particle, a water particle will transfer an equivalent amount of energy into the air - so that on average, the temperature of the water will remain unchanged.

If, on the other hand, the air and water are of different relative temperatures, then the amount of energy transferred between the two bodies will be unbalanced, hence the cooler body heats up and the hotter body cools down.

Incidently, remember that temperature is only the average kinetic energy per molecule. So, even though a cup of water may only be a few degrees above freezing (nowhere near boiling point), the interactions between molecules mean that some molecules will be moving much faster than others. In fact, some of the molecules in water move so fast that they fly right out of the cup and into the atmosphere.
Now you know why water evaporates!

Arhh!! It burns! It Burns!

Yes, I'm sure it does. Speaking of which, have you ever wondered why the effect of skin burns is exactly the same as that of frostbite? It may seem hard to believe, but frostbite is actually a burn injury, which just like any other burn, is caused by an excess of heat.

In any environment, your skin is constantly under assault from collisions by gas molecules. But this is balanced by molecular vibrations due to the temperature inside your body, so the impacts on your skin are canceled out - the particles that make up your skin hardly move at all on average, and no damage results.

But if your skin comes into contact with something which is either much hotter or much colder than your internal body temperature, then your skin, which forms the interface between a low and high temperature zone, takes a rather one-sided battering, which is how burns occur.

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