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This content is taken from the University of Liverpool's online course, Energy: Thermodynamics in Everyday Life. Join the course to learn more.

Skip to 0 minutes and 3 seconds In the opening video of this course, I quoted Matthew Boulton from the back of a British £50 note. “I sell here, sir, what all the world desires to have - power.” Nearly 200 years later, we prefer to talk about energy in this context. Power is rate of energy supplied, that’s energy per second. They sell energy here too. But how much energy do I get when I buy fuel for my car? Oil companies, or energy companies as many prefer to be called today, don’t price their product in terms of energy. We don’t buy diesel at four pence per megajoule. Perhaps this is because many of us might feel cheated if we did.

Skip to 0 minutes and 53 seconds At 70 miles an hour, my car uses about a gallon of diesel every 56 miles. Or in other words, it’s doing 56 miles per gallon. If you’d like that in metric, or SI units, it’s equivalent to about five litres every 100 kilometres. Sounds pretty good, doesn’t it? Until you think about it in terms of energy. The calorific value of a litre of diesel is about 30 megajoules. So five litres per 100 kilometres is about 150 megajoules per 100 kilometres, or 1.5 megajoule per kilometre. The drag force on my car at 70 miles per hour, is about 180 newtons. If you want to work out the drag force on your car, there’s a nice Wikipedia site that will help you.

Skip to 1 minute and 44 seconds Just search for “automobile drag coefficient.” Now, the work done to move my car one kilometre is the force times the distance. That is, 180 newtons times 1,000 metres, which is 180,000 joules or 0.18 megajoules. But my car uses 1.5 megajoules. What happened to the rest of the energy? Did the oil company dilute the diesel? No. The Second Law of Thermodynamics stepped in. William Thompson, who became Lord Kelvin, is credited with stating the Second Law as “no process is possible, the sole result of which, is a body is cooled and work is done.” In other words, no engine can completely convert heat into work. Some energy will always be lost, often quite a lot.

Skip to 2 minutes and 36 seconds In my car, the heat is generated by combustion of the fuel. Some of it is converted into motion of the pistons, crankshaft, and then the wheels. But a lot is lost as heat directly from the engine, up the exhaust pipe, and friction in the power train, and so on. Kelvin was born in 1824, in Belfast. He was studying the efficiency of steam engines when he identified the Second Law. Rudolf Clausius was born in Poland in the same decade and independently developed a slightly different but equivalent statement of the Second Law. Now, watch the second video to learn about the Clausius statement of the Second Law.

What all the world desires: power

If we didn’t know about the second law of thermodynamics we might think that the oil companies were diluting the fuel they sell us! Watch Eann explain more, as he quotes from the back of a £50 banknote at a petrol station and then discusses the efficiency of his car while driving along the motorway.

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This video is from the free online course:

Energy: Thermodynamics in Everyday Life

University of Liverpool