Before we start on the subject of nuclear physics there are a few fundamentals that we have to understand:

Energy

Without energy nothing would happen. For example energy is needed to make things move and to make chemical reactions happen.

I am not going to tell you what energy is because it is rather a difficult question and since energy is so fundimental to physics it is more of a metaphysical question than a physics one. Anyway it is quite useful to understand how energy behaves.

Energy is neither created or destroyed but it spreads out if it can

It often seems that energy disappears. For example the hot tea that I have next to me will gradually loose its heat energy and go cold. However, that energy does not disappear but spreads out and heats the room up.

If we put a mug of boiling of boiling water into a bath of water at room temperature (19°C) then we might be able to see the heat energy (measured by the temperature) flow from the hot mug to the surrounding water.

If we drew the temperature distribution at different times we might see something like this:

• Initially (blue) there is one part of the system in the centre that is at 100°C. The rest of the water is at 19°C.
• After a certain length of time (red) some of this heat has transferred to the surrounding water.
• A little later the heat has spread even further (green, blue and then black).
• Notice how eventually the overall temperature of the water has risen.

Part of a system that has high energy will lose energy if it can. However the energy of the system as a whole remains unchanged.

Energy differences are important not the energy

After the energy has spread out into an even distribution nothing seems to happen. Although on a very small scale there are very small variations these average out on a larger scale.

Lets do a little hypothetical thought experiment even though it is not at all possible. Imagine that we are on the surface of the sun – it is really hot and there is lots of energy. Now we want to use this energy to do something useful. If we had some water with us we could use the heat of the sun to boil the water and move a piston. Then what? Either we have to get some more cold water or we have to cool the steam that we have generated to turn it back into water to carry out the process again.

It is the difference between the energy content of the hot surface of the sun and the cold water that is important.

It is possible to run an engine using room temperature and a low temperature. In this case dry ice:

http://www.youtube.com/watch?v=si1iGuigAiQ

What Stops Energy Spreading?

I said that energy spreads out when it can so we now need to look at what can stop it spreading.

Let us think of gravitational energy – a ball rolling down a hill. When the ball is at the top of the hill it has gravitational potential energy. As it rolls down this energy is converted to kinetic energy i.e. the energy associated with the movement of the ball.

You can see in the diagram above that the ball will roll down one side or the other.

Now let us consider what happens if there is a slight dip at the top of the hill:

The ball is trapped in what is called a potential well. We could give the ball a little push to get it over one of the humps. If we give the ball this little bit of energy it can then roll down the hill as before.

If the potential well was a lot deeper we would have to give it a larger amount of energy to overcome the potential well:

Although we have used gravitational energy as an example this is applicable in many other situations. For example natural gas (methane CH₄) is not stable in air and could release energy by combining with oxygen to create water and carbon dioxide:

CH₄ + 2O₂ -> CO₂ + 2H₂O

However, first of all we must put a bit of energy in to overcome a potential well. We do this with a spark.

Energy ‘Generation’

We have already said that energy can neither be created or destroyed. So how do we ‘generate’ energy – for example by burning gas or in a nuclear power plant.

What we are in fact doing is changing the form of the energy. With burning gas the methane (CH₄) and the oxygen (O₂) have greater chemical energy than the carbon dioxide (CO₂) and water (H₂O) they produce. This excess energy comes out as heat and light.

Similarly when there is radioactive decay or nuclear fission the decrease in energy of the different isotopes comes out as heat and radiation.