By Robert M. Hazen, Ph.D., George Mason University
The second law of thermodynamics is incredibly pervasive; it applies to a whole host of physical situations. There are several different methods of expressing the fundamental principle of this law, for example, heat flows from hot to cold, but another, more subtle statement of the second law is that steam engines can’t be designed to convert heat 100 percent to work.
Nicolas Carnot and the Development of Steam Engines
Thermodynamics is of immense importance to the designing of steam engines because they put the focus on transferring heat and utilizing energy in various ways. Hence, in designing steam engines, the main concern is about transferring energy efficiently. So it’s not surprising that some of these insights came from the study of the design and development of steam engines.
Nicolas Léonard Sadi Carnot came closest to deriving the second law of thermodynamics from a study of work and heat and the nature of steam engines. Sadi Carnot was born in Paris in 1796. He was the son of Lazare Carnot, who was a famous military tactician, one of Napoleon’s great generals, and a minister of war under Napoleon.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
The Forgotten Book of Nicolas Carnot
The younger Carnot was educated to be a military engineer in his father’s footsteps, and he devoted much of his effort to understanding the operation of steam engines for military purposes. His most important findings were summarized in a little book called Reflections on the Motive Power of Fire.
It appeared in 1824, but it was completely ignored in his day. It was ignored for almost 20 years, long after his death. Carnot died in 1832. Carnot’s great gift was the ability to see generalizations in very specific situations. For example, working with the mechanics of a steam engine, he could see general principles. He saw steam engines as just a special case of a much more general kind of problem in thermodynamics.
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The Relationship between Work and Heat
Carnot considered two different sides of this relationship between work and heat. On the one hand, he recognized that work can be converted to heat with 100-percent efficiency. For example, a dropping ball has gravitational potential energy, it lands on the ground, and the work is converted completely to heat energy on the floor.
Imagine the same situation with an asteroid coming in from outer space and crashing into the Earth’s surface. That asteroid basically is disintegrated/obliterated, and an enormous amount of heat is released. All the energy that’s represented in that system can be converted to heat. It’s the same phenomenon with a piece of coal. When it burns, its energy is efficiently converted into heat.
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The Conversion of Heat into Work
Converting heat into work is more difficult. And this is true for at least three reasons. First and most simply, some of the heat that is introduced into the engine has to wind up heating the engine itself, or escaping into the environment, so some of the heat is wasted. For example, some of the food energy that people eat ends up just being radiated away, because they radiate 100 watts, just like a light bulb.
The second reason that some energy is always squandered, is that an engine operates in a kind of cycle. For example, a piston pushes on a rod to drive the machine, and then the piston has to be pulled back in order to operate it again.
So here is the continuous operation, and some of the energy is gone to perform the work, but then some energy has to be utilized to reset the piston in order for the engine to continue working.
Hence, any engine that operates in a cycle, has this kind of problem. Even a pumping heart, has the same kind of problem because it has to reset before it can pump again.
But there’s a third reason, a more subtle one, and this is the one that Carnot really hit on. Heat energy flows through the system from a hot reservoir, the burning fuel, to the cold reservoir, the surroundings.
Some of the energy is wasted because some of the energy that the engine started with still remains in the low-temperature reservoir that heat flows out of. Everything, even ice water, has a lot of heat energy still in it. Whatever energy any object starts with, it has to flow to the outside of its system.
Common Questions about the Second Law of Thermodynamics and the Development of Steam Engines
Nicolas Carnot, who contributed to the development of steam engines, considered two different sides of the relationship between work and heat. On the one hand, he recognized that work can be converted to heat with 100-percent efficiency, whereas converting heat into work is more difficult.
When he was working on the development of steam engines, Carnot showed that there are at least three reasons for the difficulty of converting heat into work with 100-percent efficiency. These include heat escaping into the environment and the cyclical nature of the engine.
Nicolas Carnot showed that while heat energy flows through the system from a hot reservoir, the burning fuel, to the cold reservoir, the surroundings, some of the energy is wasted because some of the energy that the engine started with still remains in the low-temperature reservoir that heat flows out of.
