A question of energy

Everyone has heard of energy - it forms the underpinnings of today’s society, all life forms, and indeed the universe as we know it. But do we actually understand it?

The first mention of the term ‘energy’ is thought to be by Aristotle, from the Greek which translates literally as “at work”. In fact, this translation is remarkably close to the definition given in many school textbooks: that energy is the capacity for doing work. What do we mean by ‘work’ in this context? Lifting a weight, powering a light bulb and pumping a balloon are all examples, but unfortunately there is no way to define something as being ‘work’, without making a circular argument linking back to energy.

Most of us will stop here, and stick with the intuitive notion of energy that is associated with doing work. But we can do better than that! The best way to proceed is to run with the term ‘energy’, and see if we can find any properties of this stuff. Well, it turns out that we can, and the most important of these is the law of conservation of energy.

Conservation of energy

Conservation of energy might sound more like something from a naturalist society, but in fact it’s the foundation of modern physics. The rule is that while we can do a lot of things with energy: move it around, store it in different ways, and run our computers, we can never use it up! This might sound a bit odd at first - if not just plain wrong. You might well cite the example of a light bulb, surely since we’re shovelling electricity into it, we must be consuming energy? No! The energy is being converted, as we shall see.

Energy conversion

A very interesting property of energy is that it exists in different forms, as the following example illustrates. Suppose we have a tank full of water, and suspend it above the ground. Then we connect a pipe from the bottom down to a paddle wheel, connected to an electrical generator. This is then connected to a light bulb. If we pull the plug in the tank, what do we see? The water runs from the tank, turns the wheel, generates electricity, and the bulb lights and warms up.

Since all we do is pull out the plug, which we’ll do gently, then there must be energy stored in the system. This is called potential energy, because it isn’t doing anything at the moment but “potentially could be doing something in the future”. We could even be more specific and say that it’s gravitational potential energy, since the water gets pulled down the tube by the Earth’s gravity. If we did this in deep space, it wouldn’t work! So this our first type of energy.

Next, the water pushes on the water wheel, turning it. This brings us onto another manifestation of energy - that associated with moving objects. It is termed kinetic energy, from Greek. We know now that energy is conserved, so the water must lose some of its energy, hence slowing down (kinetic energy increases with speed), whilst the wheel gains.

By now, we can see the pattern - at every stage we take one type of energy, and move it around, or convert it. So, the generator turns kinetic energy into electrical energy, then the light bulb turns electrical energy into light and heat, both of which are types of energy - so in fact we can explain the earlier dilemna! That’s really all there is to it.

Matter as energy

Probably one of the most significant and well known breakthroughs of the 20th Century was Albert Einstein’s Theory of Relativity, and the famous equation E=mc^2. Whilst this might appear complex, it is in fact nothing more than what we described above! The formula describes a form of energy (E) that is dependent on the mass of an object (m), multiplied by a constant (c^2, where c means the speed of light by convention). Thus all objects must have energy just through existence! Converting to and from this sort of energy is difficult, and is the principle behind atomic power.

Closing remarks

That really sums up the most important principles of energy. It is true there are many textbooks on the subject, complete with intimidating mathematics, but this really is not necessary to have a sound grasp of the physics behind it. However, the question of what this thing ‘energy’ is remains unanswered, as summed up by Richard Feynman:

“It is important to realise that in physics today, we have no knowledge of what energy is” - Feynman, Lectures on Physics

And with that thought, thank you for reading, and look out for more updates soon!