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© Copyright 2000, Jim Loy
We saw in the Law of Conservation of Energy, that energy is not gained or lost. The total energy in a closed system (where energy is not coming in or going out) remains the same. In physics, we study the ways in which energy changes forms, from kinetic energy to heat for example. And energy can move from one place to another or from one object to another.
The Second Law of Thermodynamics says that (again in a closed system) energy goes from a usable form to a less usable form. Things run downhill, flywheels slow to a stop, useful kinetic energy becomes useless heat. No physical process, no machine, is ever 100% efficient. Energy is not lost, but for all practical purposes, it is lost, as it is no longer usable.
To confuse this issue somewhat, physicists have come up with a measure of this loss of usable energy, called entropy. Another way of stating the Second Law is that entropy (unusable energy) tends to increase. Things run down, entropy goes up.
The Second Law also applies to communications and information theory. In these fields, information tends to be lost, and order tends to decrease. Entropy is then a measure of the loss of information and order. This loss of order also applies to physical systems, but can be confusing. A snowflake seems to have much more order than the moist air that it came from, even though it has lost energy in becoming a snowflake. It turns out that the order was there all along. The water molecules had to freeze together for us to see that order.
In order to help you remember these two laws, physicists often joke that the Law of Conservation of Energy says that you cannot come out ahead; and the Second Law of Thermodynamics says that you cannot break even. These two laws are the most tested and verified laws in all of science.
See Perpetual Motion.