Friday, 8 February 2013

Living in the real world

I once gave a talk to the local Rotarians on the history of the concept of energy. Not an obvious topic for an after dinner speech but it turned out to be mildly provocative; which I suppose was the point.

It began with the physical concept of mechanical work. Push a supermarket trolley and the amount of work you do depends on how hard you push it and how far you push it. That's it. Push it twice as hard, or twice as far, and you do twice as much work.

This led onto the concept of energy. If something's got energy you can get it to do some work. The amount of energy its got is simply the same as the amount of work it could do. So, use a pulley to connect a falling weight to the supermarket trolley and you can get it to move. The weight, before it falls, must have energy. In this case gravitational potential energy. You can also show that although energy can be converted from one form to another it can't be created or destroyed; the principle of the conservation of energy.

Finally, I must have got around to the concept of power which is simply the rate at which something can do work. More powerful things can simply do work more quickly.

So, if you speed something up by applying a force then you're doing work and the thing you've speeded up acquires  kinetic energy. This can then be used to do work when the thing slows down. If you apply Newton's Laws of Motion it isn't hard to show that the kinetic energy of a moving body depends on the body's mass and on the square of its speed. The classic formula is KE = 1/2 x mass x velocity^2.

Now, the first serious application of Newton's laws was to calculate the orbits of the planets around the Sun. Because these orbits aren't completely circular the planets speed up and slow down as they move closer or further from the Sun. All that's happening here is that gravitational potential energy is being swapped for kinetic energy and vice versa. So, for example, the quantity (1/2 x mass x velocity^2) had been used in calculating the details of the orbit of Mars but there wasn't anything else you could actually do with it. There wasn't any way to get Mars to do any useful work.

And here we get to the bit that was supposed to interest the Rotarians. The concept of energy might have been useful as a calculating device, but it was only when James Watt and Matthew Boulton began trying to sell their steam engines that it became something that was treated as though it had a real independent existence. Their selling technique was a bit like the present Government's New Green Deal. They demonstrated how many horses the engines would replace, or how much less coal they'd use than the earlier less efficient engines, and then offered to sell them at a price that mean the purchaser was bound to be better off. 

But to do this they needed some idea of the power output of a typical horse and decided that it was equivalent to lifting a weight of 33,000 lbs every minute through a height of 1 foot. In current units this is about 750 watts. No prizes for guessing who those are named after.  

So far so vaguely stimulating. You'll appreciate that by modern standards 750W is not a great deal of power. It may be more than can be produced by fit cyclists, and even then not for long, but a typical car produces at least 50 times as much. 

Apart from bringing the concepts of work and power down to Earth as it were, the steam engine liberated us from renewable sources of energy. Up to that point it was either biomass to feed draft animals or renewable energy in the form of water wheels, wind mills or sails.

I then pointed out that  for the last 200 years we'd been using fossil fuel resources about a million times faster than they were laid down. i.e burning up a million years worth of dead plants and animals every year. I didn't say this was wrong, I didn't have to, but one response was revealing and this was "This is all very well, but you're not living in the real world"

Let's hope that there's a real world out there where natural resources really are infinite and where we can choose whether or not the laws of physics apply. It looks like we might be needing it.

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