When I first started learning about thermodynamics, I struggled a bit. I found it more abstract than the topics in mechanics. In mechanics, you can imagine or see a ball moving, accelerating, bouncing, colliding ...

In thermodynamics, I learnt about a gas getting hot or cold, expanding and contracting, ... But I can't see the gas !?

Also, unlike kinetic energy which shows up directly like a ball moving, we cannot see the heat in the gas either.

It took me a while to realise that I have to look at the movement of the container instead - for example pushing the piston of a container, or making a balloon bigger as the gas inside gets warmer ...

But first, let us get some idea about what the word "heat" really means in physics. If you have followed through my blogs on the "mechanics" topics, you would know that simple words like like "force", "energy" and "power" have very specific meanings in physics.

For example, a search on google gives the everyday meaning of of "heat" as:

1. the quality of being hot; high temperature.
2. intensity of feeling, especially of anger or excitement.

While the first meaning is closer to our physics meaning, it is still a bit far from what it really means in physics. For example, "temperature" and "heat" have different meanings in physics.

"Temperature" in physics is about how hot or how cold something is. The everyday meaning of temperature is actually quite close to the physics meaning, because we are all familiar with reading the thermometer.

"Heat" in physics, however, has a more abstract meaning. It is the energy that goes from a hot body to a cold body in contact.

So "heat" is the same as "temperature"?

No, and that is the trickly part. Rather than trying to explain the difference directly, let me "show" the difference with an example.

Suppose I measure the time it takes to warm up a cup of water with a slow heater. Then I repeat this with the same weight of oil. I use the same heater, so the same amount of heat energy should go into the oil if I heat it for the same amount of time.

I would find that it takes only about half the time to warm up the oil to the same temperature!

So it looks like different materials need different amount of heat to warm up. In this way, we can start to see how temperature and heat have different meanings in physics.

If we are slowly enjoying a cup of coffee or tea, we would like it to stay warm for as long as possible. It so happens that for the same temperature increase, water can store a lot more heat energy compared to many other liquids and solids, maybe up to 10 times more. So our cup of tea or coffee would usually stay warm for long enough for us to finish it at our own leisure.

In physics the term "heat capacity" is used to describe this storing of the heat.

Now if we continue heating the water, it would eventually start to boil when it reaches 100 degree Celsius in temperature. Now I hope you have never been scalded by steam before. But if you had, you would know that it is a lot more painful then being scalded by hot water.

Why?

It is because for water to change to steam, it needs a lot of heat. It needs about 100 times more than the heat needed to heat the water from 0 to 100 degree Celsius. But of course, you would likely have jumped away in pain before you have the chance to experience the full effect of this heat.

Maybe because this extra amount of heat appears to be quietly "hidden" in the steam, it is called "latent heat".

We are familiar with temperature. We can feel how hot or how cold it is, and we can see the reading on a thermometer. What is less familiar is the energy changes involved.

This is not surprising, since the whole idea of "energy" in physics is kind of abstract. My descriptions in another blog on "work, energy and power" would be a useful place to know about this. As I explained in there, energy is something that can do work.

So to learn this topic on heat in physics, we usually look at the effects on a gas. And to make it simple in physics, we always start with the ideal gas. There is only one kind of energy in an ideal gas - the kinetic energies of all the particles in the gas.

The total kinetic energy of all the particles in a bottle of ideal gas - is called "internal energy".

If we bring a balloon from a cold air-conditioned room into the hot afternoon sun, the balloon would probably grow a bit bigger. We expect that the hot sun would somehow warm up the balloon, and so the balloon should expand. Why does that happen?

In an earlier article, I described how a gas (like air) is really made up of tiny molecules bouncing around randomly, and how the speeds of these molecules depend on temperature.

So in the hot sun, the air in the balloon warms up. The molecules move faster and bounce harder on the inner wall of the balloon. The effect of this is an increase in pressure the inside the balloon. This pressure pushes outward and makes the balloon get bigger.

The sum of the kinetic and potential energies of all the molcules of the gas is called "internal energy" of the air.

As the balloon expands, the inner wall of the balloon moves away from the molecules hitting the wall. The molecules lose speed as they bounce off the wall.

We can think of this as the molecules losing energy because they do work pushing out the wall.

Recall from my "physics syllabus on temperature and ideal gases" blog that the average kinetic energy of the gas molecules is related to temperature. So as the gas molecules lose kinetic energy, the gas temperature falls!

This way of looking at it is what we call a "microscopic view" or "molecular view". The opposite to that is called the "macroscopic view".

In the macroscopic view, we can think of of the expanding gas doing work as it pushes out the balloon wall. As a result, it uses up some internal energy.

So we can think of the sum total of the molecules kinetic energy as the "internal energy" of the ideal gas.

Finally, we bring in the sunlight. The hot sun brings heat energy into the air in the balloon. This increases the kinetic energies of the molecules - which is the internal energy of the air.

We can summarise the above decription like this :

heat going into the air in the balloon =
            increase in internal energy of the air     +     work done by the air

This is basically the "first law of thermodynamics". It is also works for other materials - gases, liquids or solids.

One interesting thing to note is that we have a situation here in which the kinetic energies in of molecules moving in random directions can actually be converted into kinetic energy of movement in a single direction of the expanding gas.

And this is particularly useful for driving machines, like the small pistons in car engines that run on petrol.

You can learn these concepts and more at Dr Hock's maths and physics tuition.