Energy
Why is electricity so important? The answer is because it has exceptionally small entropy.This may sound like a dull answer but until you have understood entropy (which is not complicated) the world of energy will not make much sense.
First a practical example of entropy. If I boil a kettle and then leave it, it will gradually get cold and the room it is in will get slightly warmer. That flowing out of heat from a concentrated place is called an "increase in entropy". It is an increase in disorganisation if you like. The entropy - or disorganisation - of everything in the universe is always and everywhere increasing. At the start of everything we had the big bang. A spot in the universe - what scientists call a singularity - where everything that was or was to come was concentrated. It was infinitely hot. Since then the universe has been spreading out and cooling and will continue to do so, like heat from the kettle.
Entropy can be temporarily decreased. The stars, of which our sun is one, are an example. To do this entropy must be increased elsewhere. Matter was sucked up by gravity, leaving other parts of the universe emptier, to form stars. But a star's (like a kettle's) entropy is always increasing - even the stars burn out eventually. We reduced the entropy of our hot water (made the heat more organised) when we heated it by increasing entropy elsewhere (made heat less organised). In this case at the power station.
When energy is concentrated into a place it can be made to do something useful. When it is just spread out it can't. Think of a material analogy. In a carpenters shop sawdust is useless. Only when wood is gather into a particular shape can it do something useful. All materials are gradually reducing to dust - their entropy is increasing.
Just so with any form of energy. When it has low entropy it can be used to do something useful. Indeed entropy (or amount of disorganisation) is often defined as the amount of energy in a system that cannot do useful work. So at the big bang the singularity had zero entropy. All that energy was about to be useful - to make the universe.
In our everyday experience we measure entropy through different qualities of materials - temperature, pressure, watts. But the amount of useful work that can be done by energy depends on two entropy levels. That within the kettle and that outside.
Let's try a slightly different analogy which I hope will make things clearer. Imagine a tidal barrage. A dam that can be opened to let water in when the tide is rising and then closed when the tide falls. Once the tide has fallen far enough the water is let out to drive a turbine to generate power. Well it is clear from this illustration that at the moment that the gates are closed, at the top of the tide, the water inside the barrage is at the same height as the water outside the barrage. The relative entropies of the two bodies of water are the same and so no useful work can be done. However, once the tide has fallen the entropy outside the barrage has increased but the entropy inside has stayed the same. Now useful work can be performed by allowing the entropy states to equalise.
This notion of difference in entropy is important because it is what really defines the usefulness of a fuel. Heat is the classic example of this. Heat has a fully entropic state called absolute zero - when atoms come to rest. However, our world is warm so most of the time the entropy of atoms is a long way above absolute zero - their ground state.
It turns out that to convert one form of energy to another if you cannot get to the ground state then you will lose that part of the energy above the ground state. So if I drove my car in an world that was at -273C, I would get all the chemical energy from the petrol and would be able to drive 4 times as far on a single tank. However, because the world is at about 20C (273+20 = 293 called 293 Kelvin) a lot of the power is lost as "waste" heat.
If we go back to the barrage analogy we can see that if we have a neap tide (the ones where the water doesn't go out very far) then the amount of power we will be able to extract will be much smaller than if it went right out. The difference in entropy levels is small.
The interesting thing about electricity is that the ground state (neither positive nor negative charge) is all around us - we call it "earth" - and it is easily accessible everywhere, we just have to stick a wire in the ground.
The maximum conversion efficiency that is possible is proportionate to the difference between the high and low entropic state - i.e. the temperature difference between the kettle and the room and also, crucially, the height the room temperature is above the ground state - i.e. absolute zero. So if the room was at absolute zero (-273C) and the kettle at -173C (100C hotter) then in principle all the energy in the kettle could be converted to some other useful form. But if the room was at 0C (273 Kelvin) and the kettle had just boiled at 100C (373K) then as a maximum only 15% of the energy in the kettle could be extracted, the rest would be waste heat - i.e. end up the same temperature as the room. This is called the Carnot efficiency.
For electricity, our surroundings are always at the ground state. This makes it easy to convert electricity into other forms of energy very efficiently. Electric motors are around 95% efficient. By contrast when fuel is burnt in a car, the chemical energy is converted to motion at about 25% efficiency (i.e. 75% is lost as waste heat), and even the biggest power stations are only 50% efficient. This is not because they are bad machines but because the world we live in is warm. If it were at absolute zero we could get enormously efficient conversion - but it would be a bit chilly.
