I don't have the money or the energy

"Are you feeling tired and lacking in energy?" the tabloid magazine cover will ask in garish letters, superimposed over a picture of Jennifer Anniston for the 3rd month in a row. But what is energy, in this situation? It sounds like a commodity which can be bought, bartered for or comes free in a cracker. Admittedly it is quite a difficult concept to grasp. Thinking back to my school days it was probably among the most familiar words and also one of the most complicated concepts. From friends who are teachers I gather that it is still something that they are continually looking for inventive ways to describe. So I thought I would give them some ideas.

The standard physics textbook definition of energy is

energy is the ability to do work

This is quite opaque to most people, but especially to a 10 year old, last lesson on a Friday afternoon. This is the nub of the problem; energy can only be defined in the context of what you can do with it. The really tricky part is conservation of energy, and the misnomer of 'generators'. What they generate is electrical current, not electrical energy.

Consider this definition:

money is the ability to buy things

Is that adequate to describe money? Just about and its probably a fair comparison to energy. The big difference is that with money you have coins and notes, but with energy you don't (well you could argue that you have photons as the currency of energy, but we'll leave that for the moment).

Armed with our analogy lets see if we can sort out some of the basic conservation of energy challenges.

The law of the conservation of energy states:

Energy cannot be created but can only change from one form to another

in our parallel  money-world this could be written:
Money cannot be minted but can only change from one currency to another

Obviously money can be minted, but only by the Royal Mint, but all energy in the Universe was created in the Big Bang. I'm in no way saying that the Royal Mint is like God!

When you exchange foreign money at the end of your holiday you usually lose some because the travel agent/post office/dodgy chap in the back of Marks and Spencers will work the rate so they get something for their trouble. Energy often works in a similar way, if you slap a table the kinetic energy in your hand is transferred mostly to sound, but the commission is friction which heats your hand and the table up a little bit.

Energy also has a certain amount of creative accounting, just like the banking crisis.  For a long time scientists thought the Sun was powered by thermal gas pressure alone, and were faced with a Sun that could only be a few thousand years old and would only live for another few thousand years. This is enough to make anyone grumpy, and was also in complete contradiction to the fossil record. Fortunately when Quantum Mechanics can along the problem was largely solved. This is how.

At the heart of Quantum Mechanics is the Heisenberg Uncertainty principle. This, and most of the rest of quantum mechanics, goes against "common sense" and to fully be believed you really should get your hands dirty with the maths (don't worry, I won't). The Uncertainty principle says, very generally:

You cannot determine the position and momentum of an object absolutely

The amount of uncertainty is very, very small and completely irrelevent in everyday life. It is not the cause of you not being able to find the car after doing your shopping or misjudging the first step out of the pub on a Friday night. But for atoms, the uncertainty is significant. Instead of momentum and position we could say the same thing slightly differently:

You cannot determine the energy and time that an object has that energy absolutely

This is the really interesting part because it is saying that because of this uncertainty our atom could have a very high energy for a very short time, and no-one would notice. In banking this happens all the time where huge sums of money are moved around for short periods of time, but as long as they are back by the time you check your statement or (more significantly) go to withdraw money, you don't care what happens in between. To keep you quiet, the bank might throw a couple of pennies your way as a thank you, in the form of interest.

This simple expression is how the Sun and all stars shine. Stars shine by fusion, where 2 atoms combine and release energy as photons; 2 hydrogen nuclei combine into 1 helium nucleus, and release a flash of light. This happens many billions of times a day producing heavier and heavier elements which in turn fuse and release energy in the form of light.

But how do you get the nuclei to fuse? They don't have electrons so they positively charged, and like charges repel. Excellent question! There is a mysterious force called simply "the strong nuclear force", this stops the protons in all elements from flying apart from the electrostatic force repelling them. To get the nuclei close enough so the strong force can hold onto them, takes extreme pressure and density (i.e. at the heart of star), and also a bit of quantum mechanical magic. As they get almost close enough to fuse the quantum mechanics distracts the law of conservation of energy (possibly by telling it about the final season of Lost), and for the teeniest, tiniest moment they have enough energy to get close enough, at which point the strong force takes over and they fuse. When the law of conservation of energy looks back at the nuclei, they are already in the car with the strong force and already on their way on a dirty weekend as a newly fused helium nucleus.

Unlike with unscrupulous bankers who managed to 'lose' nonillions of pounds in the last year, quantum mechanics is always very careful with your energy, a bit like a low risk investment. At least you get your money back.