# Blog Archives

## Boyle’s Law is not a principle; so does the UK Education Secretary know what a principle is?

What [students] need is a rooting in the basic scientific principles, Newton’s laws of thermodynamics and Boyle’s law. [Education Secretary Michael Gove, reported here].

He has been justly mocked for confusing Newton’s laws with the laws of thermodynamics (e.g. here and here and, by me, here). But the kind of ignorance involved in describing Boyle’s Law as a “basic scientific principle” is far more damaging.

Disclosure: I taught Boyle’s Law for over 40 years, and it gets three index entries in my book, *From Stars to Stalagmites*.

Bottom line: Boyle’s Law is *not* basic. It is a secondary consequence of the kinetic theory of gases, which *is* basic. The difference is enormous, and matters. Anyone who thinks that Boyle’s Law is a principle doesn’t know what a principle is. (So Gove doesn’t know what a principle is? That figures.)

Reasoning: Boyle’s Law states that if you double the pressure on a sample of gas, you will halve the volume. Boyle thought this was because the molecules of gas repel each other, so it takes more pressure to push them closer together, and Newton put this idea on a mathematical footing, by suggesting an inverse square law for repulsion, rather like his inverse square law for gravitational attraction. They were wrong.

Mathematically, the Law is simply stated, which may be why Mr Gove thinks it is basic: volume is inversely proportional to pressure, which gives you a nice simple equation (P x V = a constant) that even a Cabinet Minister can understand. But on its own, it is of no educational value whatsoever. It only acquires value if you put it in its context, but this involves a concept of education that seems to be beyond his understanding.

Now to what *is* basic. Boyle’s Law is now explained using the kinetic theory of gases. This describes a gas as a whole lot of molecules, of such small volume compared to their container that we can think of them as points, each wandering around doing their own thing, and, from time to time, bouncing off the walls. It is the impact of these bounces that gives rise to pressure. If you push the same number of molecules (at the same temperature) into half the volume, each area of wall will get twice as many bounces per second, and so will experience twice the pressure. Pressure x volume remains constant.

Actually, Boyle’s Law isn’t even true. Simple kinetic theory neglects the fact that gas molecules attract each other a little, making the pressure less than what the theory tells you it ought to be. And if we compress the gas into a very small volume, we can no longer ignore the volume taken up by the actual molecules themselves.

So what does teaching Boyle’s Law achieve? Firstly, a bit of elementary algebra that gives clear answers, and that can be used to bully students if, as so often happens, they meet it in science before they have been adequately prepared in their maths classes. This, I suspect, is the aspect that Gove finds particularly appealing. Secondly, some rather nice experiments involving balancing weights on top of sealed-off syringes. Thirdly, insight into how to use a mathematical model and, at a more advanced level, how to allow for the fact that real gases do not exactly meet its assumptions. Fourthly, a good example of how the practice of science depends on the technology of the society that produces it. In this case, seventeenth century improvements in glassmaking made it possible to construct tubes of uniform cross-section, which were needed to measure volumes of gas accurately. Fifthly … but that’s enough to be going on with. Further elaboration would, ironically, lead us on to introductory thermodynamics. Ironically, given the interview that started this discussion.

Educationally, context is everything, the key to understanding and to making that understanding worthwhile. A person who decries the study of context is unfit for involvement with education.

Even at Cabinet level.