Engines and Energy

A Little History Of Science: Engines and Energy

I sell here, Sir, what all the world desires to have – power.’ The engineer Matthew Boulton (1728–1809) knew what he was talking about. In the 1770s Boulton and other ambitious men, such as the inventor James Watt (1736–1819), were using steam engines in mining and manufacturing. They seemed to have tamed energy, or power. These men drove forward the Industrial Revolution in Britain, the first country to industrialise and to develop the factory system. It was a revolution driven by scientific advances, and relied on huge increases in power to manufacture goods at great speed and transport them far and wide. Our modern world is unimaginable without energy – lots of it. And it all started with steam.

Steam engines themselves are pretty simple. You can see the principle in action every time you boil a pan of water with a lid on: the force of the steam presses up on the lid to let the steam out and makes it rattle. Now imagine instead of a pan you have a closed cylinder with just a small hole in one end of it. Into this is fitted a moveable piston (that is, a disk that fits snugly into the cylinder, with a knob that fits snugly into the hole). The pressure of the escaping steam will force the piston up and move whatever might be attached to it: perhaps a rod with the wheels of a train attached to it.

So a steam engine changes the energy of the steam into movement: mechanical energy. This engine can do useful work, such as driving a piece of machinery or pumping lots of water out of a mine.

Neither Boulton nor Watt invented the steam engine: they had been around for more than a hundred years. But the early models were crude, unreliable and inefficient. Watt, in particular, was the brain behind the improvement of the engine. His model not only provided the power that helped Britain industrialise, it also led scientists to investigate a basic law of nature. It helped them see that heat was not a substance, as Lavoisier had thought, but a form of energy.

Among the thoughtful people who were studying engines during the Industrial Revolution, one man in particular stands out from the crowd. This was a young French engineer, Sadi Carnot (1796– 1832). The French and the British were great rivals at this time. Carnot was aware that the British had forged ahead in designing steam engines and using the power that they generated. He wanted France to catch up, and while watching steam engines do their work, he discovered a fundamental scientific principle. He was concerned with a steam engine’s efficiency.

If a steam engine is perfectly efficient, it will turn to power all the energy needed to boil the water to drive the engine. You can measure the amount of heat produced by burning coal or wood to create the steam, then measure the power, or work the piston generated. If the engine were absolutely efficient, they would be exactly the same. Alas, absolutely efficient engines are impossible to build.

All engines have what is called a heat sump, or ‘sink’, where the cooled steam and water collect after doing their work. You can measure the temperature of the steam going in and the temperature of the steam (or water) that is left at the end of each cycle. In the sump, the temperature is always lower coming out than it was going in. Carnot showed that you could use the difference between the two temperatures to calculate the efficiency of an engine. If perfect efficiency would score 1, then the actual efficiency is 1 minus the temperature in the sink (going out) divided by the temperature in the source (coming in). The only way to score the 1 of perfect efficiency would be to have the engine extracting all the heat out of the steam. Then, the ratio between out and in would be zero. That would give 1 – 0 = 1. For that to happen, one of the temperature measurements would have to be either zero or infinity: infinitely hot steam coming in or ‘absolute zero’ (the lowest temperature theoretically possible, which we will look at below) going out to the sink. Neither is possible, so efficiency is always less than perfect.

Carnot’s simple equation, aimed at measuring the efficiency of engines, also summarises a deep law of nature. It explains why ‘perpetual motion’ machines are sometimes written about in science fiction, but can never exist in the real world. We always have to use energy to produce energy – for instance, we have to burn coal or some other fuel to heat the water in the first place. In the 1840s and 1850s, other scientists were working on this basic fact of nature. One of them was a German physicist, Rudolph Clausius (1822–88), who spent much of his life looking at how heat flows in carefully controlled experimental situations. To do this, he introduced a new concept in physics: entropy. Entropy is a measure of how mixed up (disordered) the things in a system are. It is much easier to mix things up than to unmix them. If you mix white with black paint, you get grey paint. The mixing is easy, but it’s impos- sible to unmix them and get the pure black and white paints back again. If you stir milk and sugar in your tea, you can recover the sugar if you take a lot of trouble, but getting the milk back is impossible. Energy is no different: once you burn the coal, you can’t use the heat it produced to get your coal back.

For people in the nineteenth century, entropy was a depressing idea. Clausius declared that the universe is becoming more and more mixed up, because entropy is its ‘natural’ stage. Once things get mixed up, it takes more energy to unmix them, just as it takes more energy to clean up a room than to get it messy. According to Clausius, the universe is slowly running down, and the end point will be a universe in which matter and energy are evenly distributed through all space. Even our sun will eventually die, in about five billion years, and with it, life on earth. In the meantime, of course, plants and animals, and human beings and our houses and computers, defy the ultimate endpoint of Clausius’s insight. As the old saying has it, ‘make hay while the sun shines’.

While physicists and engineers were worrying about the effects of entropy, they were also looking at what, exactly, energy was. Heat is an important form of energy, so the study of energy is called thermodynamics (a word that combines the Greek words for ‘heat’ and ‘power’). In the 1840s several people came to similar conclusions about the relationships between different forms of energy. They were looking at a variety of things. What happens when water freezes or boils? How are our muscles able to lift weights? How do steam engines manage to use the hot water vapour to produce something than can do work? (The first public railway, driven by steam engines, had opened in the north of England in 1825.) Coming to the question from these different angles, they all realised that you cannot create energy out of nothing, nor can you make it completely disappear. All you can do with energy is to make it change from one form to another. Sometimes you can make this change do some work for you along the way. This became known as the principle of conservation of energy.

The Manchester physicist J.P. Joule (1818–89) wanted to under- stand the relationship between heat and work. How much energy does it take to do a certain amount of work? In a series of brilliant experiments, he showed that heat and work are directly related in ways that can be expressed mathematically. You use energy to produce work (to ride a bicycle, for instance), and heat is a common form of energy. Think about climbing to the top of a mountain. We use energy every time we move our muscles. This comes from the food that we eat and digest, using the oxygen we breathe to ‘burn’ the calories in our food. Now, there may be two paths to the mountain top, one very steep, and the other more gradual. What Joule showed is that, in terms of the energy needed, it doesn’t matter which path you take. The steep path might leave you with aching muscles, but the amount of energy that you use in moving the weight of your body from the bottom to the top is the same, whichever path you take, or whether you run or walk up. Physicists still remember Joule’s name. It is attached to several measurements, including a unit of energy, or heat.

People have long tried to measure how much heat an object contains, that is, its temperature. Galileo played around with a ‘thermoscope’, an instrument that changed as the temperature increased. A thermoscope allowed you to see that things were getting hotter or colder; a thermometer allowed you to put a number on the degree of heat. We still use two early attempts at devising a scale of temperatures. One was invented by the German physicist Daniel Gabriel Fahrenheit (1686–1736), who used thermometers containing both mercury and alcohol; in his scale, water freezes at 32 degrees, and our normal body temperature is 96 degrees. Anders Celsius (1701–44) devised his scale using the freezing and boiling points of water, with the former being set at zero degrees, and the latter at 100 degrees. His thermometer measured temperatures between these two points. These two scales are still part of our daily lives, from knowing what temperature to bake a cake at, to complaining about the weather.

The Scottish physicist William Thomson (1824–1907) invented another scale. He was especially interested in how heat and other forms of energy work in nature. He was a professor at the University of Glasgow and was later given the title Lord Kelvin. His temperature scale is known as the Kelvin or K scale. He worked out the K scale using very precise measurements and scientific principles. Compared with the K scale, Celsius and Fahrenheit turn out to be crude measures of temperature.

The K scale’s defining point is the ‘triple point of water’. This occurs when the three states of water – ice (a solid), water (a liquid) and water vapour (a gas) – are in ‘thermodynamic equilibrium’. Thermodynamic equilibrium can happen in an experimental system, when a substance is insulated from its surroundings so that temperature and pressure are fixed. Then there is no change in the state of a substance and no energy escapes or enters the system.

The triple point of water is when its solid, liquid and gas are held in perfect balance. As soon as the temperature or pressure changes then the balance or equilibrium is lost. In Celsius and Fahrenheit, temperatures go into minus when it’s very cold. You will have heard weather forecasters say ‘minus two or three degrees’. There are no negative numbers in the K scale.

Water freezes at 273.16 degrees Kelvin (as compared with 0 degrees in the Celsius scale and 32 degrees in the Fahrenheit scale). It gets a lot colder on the way down to 0 degrees Kelvin. But here 0 really means 0 or ‘absolute zero’. At this impossibly cold temperature, all motion, all energy, stops. Just like the perfectly efficient engine, we cannot quite get there.

Kelvin and others helped to explain both the science and the practical workings of all kinds of engines. As the nineteenth century progressed, the three discoveries outlined in this chapter became the first, second and third law of thermodynamics: the conservation of energy, the ‘law’ of entropy, and the absolute still- ness of atoms at absolute zero. These laws help us understand important things about energy, work and power.

The modern world could not get enough of its new-found power: to run factories, ships, trains and – towards the end of Kelvin’s life – motor cars. Trains and steam ships used the heat from coal in their furnaces to produce steam to drive the engines. But cars depended on a new kind of engine: the internal combustion engine. This needed a highly volatile fuel called petrol, or gasoline, which was discovered near the end of the nineteenth century.

Petrol would become one of the most important products of the next century. Now, in the new millennium, it is still one of the most fought over and increasingly scarce resources in the world.