What Do You Think You Are The Science of What Makes You You - Brian Clegg Page 0,27
apparent, both in oceans and in the clouds. In total there are around 1.4 billion cubic kilometres of water on Earth.*
It’s hard to relate to that, but that is around 0.185 cubic kilometres of water for each person. That’s 185,000,000,000 litres of water each. We typically need around 2 litres of water a day (of which half, in practice, comes from our food) – so it would take around 250 million years to get through that water – and that would only be if we consumed the water, but didn’t return it to the environment. In practice, an adult’s body holds around 45 litres of water and isn’t going to soak up more. The rest gets recycled.
You may hear that our water consumption, particularly in the West, is a lot higher than the volume we drink – and this is true. We each of us result in between 5,000 and 10,000 litres a day being used – in part through our activities and even more so as a result of the products we consume. It has been estimated that it takes 3,000 litres of water (fed to cattle) to make a hamburger and a remarkable 20,000 litres of water to produce 1 kg of coffee. Again, though, we have to be a little careful about this – that kilogram of coffee contains hardly any water. We are again talking about the water required by the system, the majority of which will be naturally recycled.
It seems, then, that there is a vast amount of water out there for every individual, and even the relatively small amount of it that we ‘use’ is rapidly returned. Which makes it clear that simply saying that water is a scarce commodity is just plain wrong. The problem is not a shortage of water, but that many of us live in places where water isn’t readily available, and the vast bulk of the water in the world (around 97 per cent) is salt water, rather than drinkable fresh water. Both of these problems can be overcome – but it takes energy to do so. It’s arguably energy that’s in short supply.
ENERGY – WHATEVER THAT IS
This brings us neatly around to the other requirement to making you function as a biological machine. Energy is one of those terms from physics, where we all kind of know what we mean when we use the word, but it is really difficult to describe what it actually is. The great 20th-century American physicist Richard Feynman rather depressingly said that ‘in physics today, we have no knowledge of what energy is’ – and things haven’t changed since. When we employ the word in everyday use, it tends to be a loose concept of the ‘oomph’ that makes things happen. Energy is often about motion, whether it’s the kinetic energy of a moving vehicle that can do so much damage in a collision, or heat energy, which we experience through the movement of the individual atoms and molecules that make up the matter around us.
We also speak of ‘potential energy’. This is really just energy that is stored up in a whole range of possible ways. The most familiar from school is probably the potential energy due to gravity. If we push a boulder to the top of a hill, then it takes work to get it up there – transferring energy to the boulder because of its position. Once at the top, we have stored up some of the energy we exerted (the rest will mostly have gone to heat), which can then be released when we let it go and it rolls back down.
That’s the simplest form of potential energy – but at first glance there are lots of others. For example, the electrical energy stored in a battery that keeps your phone going, or the energy stored up in a spring when it’s wound. Most importantly as far as keeping you operating, there is the energy in food. As it happens, though operating in different ways, pretty much all three of these kinds of potential energy rely on chemical energy – the energy in the bonds between atoms in chemical compounds – which in the end is a form of electromagnetic energy.†
In the battery, chemical energy in the battery’s component parts is used to drive electrons around a circuit. In the spring, the electromagnetic bonds between atoms are stretched – this takes energy to do, storing it up to later release it when the bonds return to their normal