About a year ago my 15-year-old granddaughter asked me to explain something in her physics homework. The previous time she had sought my help she had wanted to know how far individual electrons in the AC mains moved backwards and forwards; this time she was less demanding. But it was more than half a century since I had been a 15 year old doing physics homework, and having spent the bulk of my career as a research physiologist, I thought I ought to find out what 15 year olds are meant to learn today.

I therefore bought and read copies of the five GCSE physics textbooks that were on the shelves of two of Cambridge’s biggest bookshops. These books are aimed at pupils in the two years before they take their GCSE exams at the age of 16. What I found may go some way to explaining why there has been a 35% decrease since 1991 in the number of pupils who go on to take A-level physics at schools in England, Wales and Northern Ireland.

Strange new order

All of the books were, as you would expect, handsomely produced – good paper, clear print, attractive diagrams and illustrations. But my initial impression of excellence faded when I began to read the text. Most surprising was the extraordinary variety in the order in which different topics were discussed. You might think that it would be natural to start with forces and motion, so introducing the concepts of mass, length and time. In fact, of the five books, only two started in this way; one started with astronomy, one with electricity, and one with light.

In the book that started with astronomy, I was amazed to find pupils being taught about how the Moon causes tides, and about the effect of gravity on the direction of motion of a satellite, before there had been any discussion of forces and motion. And how can a pupil who has only the vaguest notions about the wave theory of light understand why the redshift is evidence for the expansion of the universe?

Starting with electricity is even more difficult. We are told on the first page of this particular book that an electric current is a flow of charge, but the concept of charge is not explained until static electricity is discussed five chapters later. Early on, we learn that when charge flows around an electrical circuit, the voltage or potential difference across each component indicates how much energy it is converting. We are then introduced to the law of conservation of energy, even though we have not yet learned what physicists mean by work or energy.

By comparison, starting with light (and putting off wave theory as long as possible) is relatively trouble free. When waves are eventually introduced, though, it is confusing to be told about electromagnetic radiation before there has been any discussion of either electricity or magnetism.

Shaky explanations

Within individual fields, explanations are sometimes sloppy. There are, of course, real difficulties. It is confusing, for example, that we use the kilogram as a unit of weight in ordinary life but as a unit of mass in physics; yet only two of the five books point out this source of confusion, and it is only these two that specifically mention inertia. A middle-school textbook is probably not the appropriate place to introduce gravitational mass and inertial mass, but children do need to understand the concepts of gravity and inertia.

To do that they need to be told about Galileo’s experiment of dropping balls of different weights from the Leaning Tower of Pisa – even if the involvement of the tower may be mythical. They can then understand why astronauts on the Moon have only a sixth of their normal weight, yet the same inertia and momentum as they would have on Earth. This peculiar situation explains why those astronauts fell over so easily, why they found it impossible to walk quickly or run, and why they adopted a long, loping gait.

And coming back to Earth, though all five books discuss Newton’s laws of motion – and the concepts of kinetic energy and momentum – two of them fail to explain how those laws show that kinetic energy equals ½mv². Another of the five does not explain how it follows from Newton’s laws that momentum is conserved in a collision. These omissions are thoroughly muddling.

The concepts of energy and of the conservation of energy are also treated inadequately. Physicists have traditionally defined energy – the word comes from the Greek words for “in” and “work” – as the capacity to do mechanical work (i.e. the work that is done when an object is moved by a force). Yet four of the five books introduce the concept of energy before they have introduced the concept of mechanical work. Energy is either left undefined or is inadequately defined as what makes things move or makes changes happen. There always have been difficulties about the concept of energy, which, unlike mass, cannot be measured directly; but today’s textbooks make these difficulties worse by ignoring the traditional definition.

Often the best and the most interesting way to introduce a new concept is to describe the historical experiments that led to it, yet these books are curiously arbitrary in their use of history. All five mention Faraday’s discovery of electromagnetic induction, yet none of them mentions his discovery that electric currents in aqueous solution are carried by charged particles, which he called ions (after the Greek word for “wanderer”). All five mention electric cells, but only one discusses Volta and his electric “pile” – the first device to deliver a continuous current and still the French word for battery.

None of the books mentions concentration cells. These are electric cells in which a membrane that is selectively permeable to certain ions separates two solutions containing different concentrations of those ions. Invented in the 19th century, and subsequently found to occur naturally, concentration cells are not only easy to understand but are also the immediate source of energy for the electrical events that underlie all nervous activity. Even more surprisingly, there is no mention of Galvani and his seminal experiments on animal electricity. At a time when so many of those teaching physics in schools are biologists, these omissions seem particularly sad.

The dangers of relevance

It is fashionable nowadays (as well as sensible) to emphasize the relevance of physics to everyday life, and all five books do that. Much of what is included is interesting and worthwhile, but there are two snags to this approach. First, it does not help pupils to understand new concepts if the book is so anxious to be relevant that applications are interpolated at every opportunity. For example, it is bizarre for lasers and their uses to appear in the fourth paragraph of the first chapter of a GCSE physics textbook.

The other snag is that bright children can find it very off-putting if an application is only half understood. For example, all but one of the five books talk about the way photocopiers work (or used to work), yet none of them explains why the charge on the surface of the drum leaks away faster from areas that are illuminated than from areas that are dark. Reluctant (for good reason) to tackle the nature of photoelectric phenomena at this stage, the authors leave the crucial step in the photocopying process as a mystery.

And there are extraordinary omissions. In discussing sound, all five books relate pitch to frequency, loudness to amplitude, and timbre to waveform, but only two of them point out that raising the pitch of a note by an octave doubles the frequency; and none of them mentions that two notes sound well together only if the ratio of their frequencies can be represented by the use of small numbers. Pythagoras knew this 2500 years ago and wondered why; Helmholtz eventually provided the answer. The effect is dramatic, easily demonstrated and fundamental to musical composition, yet it seems to be thought irrelevant for children taking GCSE physics.

Even more remarkably, only one of the five books explains how an internal-combustion engine works. In our car-dominated world, the simple, ingenious, easily comprehensible, four-stroke petrol engine is largely ignored in school physics. And though horse power is still the unit of power used by the car industry, it too is mentioned in only one of the books. Horses do not graze in silicon valley.

Getting back to basics

The blame for some of these deficiencies should perhaps be directed less to the authors of the textbooks than to the peculiarities of the curriculum. In the 2005 curriculum for GCSE physics, for example, electric cells and batteries are mentioned five times, yet there is not a word about how they work. Nor is there any mention of internal-combustion engines or the physical basis of musical harmony.

More worrying than gaps in the curriculum, which can be (and often are) ignored by judicious writers of textbooks, are its insatiable demands. The sensible desire to give pupils a greater general understanding of astronomy, geology and environmental problems is in danger of elbowing out explanations of basic physics. It is, then, vital to restrict unnecessary detail in these peripheral areas. It is far more important for pupils of GCSE physics to understand Newton’s laws of motion and gravitation, the evidence for the wave theory of light, and how electric motors work than for them to become familiar with the subtleties of the movements of tectonic plates or the nature of supernovae.

Children can be put off by failing to understand things, or by being swamped with facts. The current bloated, yet inadequate, curriculum adds to both risks.