Is Your Universe Disordered? Just Wait!
What is Entropy?
Entropy is a measure of disorder in a system. Since it takes work to increase the orderliness inside a closed system, an increase in order corresponds to a decrease of entropy. Hence, the second law of thermodynamics can also be expressed in terms of entropy: A decrease in the entropy of a system requires an input of work into that system.
Entropy is a quantitative measure of disorder.
The second law tells us that when we convert energy from one form to another, there is a spontaneous increase in entropy (disorder). There cannot be a spontaneous decrease in entropy, making order from chaos requires work (energy).
An electric wire with no current running through it has a certain level of order due to its chemical composition, but the atoms within the wire are randomly ordered. Add energy and the atoms align in an orderly fashion and their electrons, the charged particles which give electricity its energy properties, flow and may be used for work. Shut off the source of electricity and the atoms of the wire move to a more disordered state again.
Functionally, what this means to us is that without added energy, there is a tendency for systems to run down.
One of my least favorite activities is housework. Without an input of energy to clean and tidy my house, it inevitably gets messy and dirty. I come home and toss my hat and coat on a chair, and my boots on the floor by the door. Never, no matter how much I wish for it, does my coat spontaneously end up on the hook, or my hat on the shelf. The only way my house moves from dusty and disorganized to neat and orderly is when I put energy into cleaning it up. To move from order to disorder (i.e., to decrease the entropy of the system) requires an expenditure of energy, thus leaving less energy remaining for use in other ways.
It takes the input of energy to build any material system from its raw materials and even more energy for it to continue to function.
All systems need a constant source of energy. Without energy, they run down, meaning they are moving from more to less order. This is true of both natural and human-made systems. Without a constant input of energy, you or an ecosystem or a car or a light bulb quickly loses the ability to function.
The Second Law of Thermodynamics tells us that energy conversions are not 100% efficient. Therefore, when we convert high-quality energy for our uses, a certain percentage does not get converted to useful energy and is left over. This degraded energy is most often in the form of heat.
The human body converts food (potential energy) to the kinetic energy needed to run our metabolism. Like any other energy conversion, waste heat is produced. When functioning normally, that heat measures 98.6°F. Without food, the system (our body) quickly starts to break down, leading ultimately to death. It is the energy in food that keeps the body healthy and running (metabolizing). Countering entropy is the physical reason that we need a constant supply of energy.
What is the value of a particular source of energy to humans?
Understanding the second law of thermodynamics reveals part of the answer. The denser an energy source, the more energy is available. Density is not the only factor in energy quality, transportability and cleanliness also contribute to our view of energy quality. If we were to look at a history of human adoption of different energy sources, we find a tendency toward denser fuels tempered by the advantages of transportability.
Humans’ first fuel was wood, which has an energy density of 13 MJ/L. We next discovered coal which has about 38 MJ/L. This denser fuel powered the beginning of the Industrial Revolution, but was quickly replaced by oil which has a slightly lower energy density when processed into gasoline (34.2 MJ/L), but oil has the tremendous advantage of being liquid and easily transportable. Similarly, the ease of transportability and relative cleanliness and abundance of liquid natural gas has recently made it very popular despite its 22.2 MJ/L energy density. Uranium-235 used in nuclear fission has just under 80,000 MJ/L. Each of these steps was towards a denser fuel.
The maximum temperature that can be achieved from a particular source of energy ultimately determines the quality to humans of that energy source. Air at room temperature contains energy, but in a much less dense form than in oil and is clearly more difficult for humans to make use of. There is heat in water in a pond or the ocean, but it is of little use for doing work. Water heated to steam at 212 degrees F has a lot more energy than the heat in the atmosphere water at room temperature.
|An alternate statement of the second law of thermodynamics is that processes that generate useful work (for example, propelling electrons down a wire to provide electricity or moving an automobile) also transfer some heat to a colder surrounding. In other words, because heat and work are both forms of energy, some of the energy in the system is lost to heat rather than useful work. By contrast, a heat pump, requires the input of a large amount of energy (work) to concentrate diffuse heat energy from a cold surrounding to a warmer house. It turns out that when one grinds through the theoretical underpinnings of thermodynamics, one arrives at the conclusion that the efficiency with which heat is converted to work depends only on the temperature difference between the hot process and the colder surroundings. Therefore, it is much easier to derive useful work from steam at 212 degrees F than from water vapor at room temperature.
(In a later section, we will discuss efficient ways of using this lower quality energy with air and water heat pumps.)
|Energy Efficiency And the Second Law
The second law requires an addition of work (energy) to move heat from a colder to warmer place. An engine basically converts energy to heat. Some engines do more work than others for the same amount of energy input.Nicholas Carnot, a French physicist in the 1800’s, determined the theoretical maximum efficiency of an engine. By comparing the actual ability of an engine with the Carnot efficiency, we can get a measure of the actual efficiency of a motor.
What are the circumstances surrounding thermodynamics?
The laws of thermodynamics are simple, but rigid and they have been written about extensively both in the scientific and popular culture. Here is a smattering of those writings:
“A theory is the more impressive the greater the simplicity of its premises, the more different kinds of things it relates, and the more extended its area of applicability. Therefore, the deep impression that classical thermodynamics made upon me. It is the only physical theory of universal content which I am convinced will never be overthrown, within the framework of applicability of its basic concepts”
Albert Einstein (author), Paul Arthur, Schilpp (editor). Autobiographical Notes. A Centennial Edition. Open Court Publishing Company. 1979. p. 31 [As quoted by Don Howard, John Stachel. Einstein: The Formative Years, 1879-1909 (Einstein Studies, vol. 8). Birkhäuser Boston. 2000. p. 1]
The three laws of thermodynamics were humorously summed up by the late British scientist and author, C.P. Snow:
- You cannot win (you cannot get something for nothing because matter and energy are conserved).
- You cannot break even (you cannot return to the same energy state because entropy always increases).
- You cannot get out of the game (because absolute zero is not attainable).
C. P. Snow, 1959 Rede Lecture entitled “The Two Cultures and the Scientific Revolution”.
The renowned cartoon physicist, Homer Simpson, weighed in on the subject, too. In one episode, Lisa built a perpetual motion machine which gained energy over time. Homer said, “In this house, we obey the laws of thermodynamics.”
In our house too, we have no choice.
Like gravity, the laws of thermodynamics are among the ground rules that we have learned from the scientific method, and like it or not, we must play by these rules.
In the next blog, we’ll discuss how solar energy drives matter cycles in nature which among other things converts solar energy into a usable energy for living things.
York, Maine, USA