Simple Nature - Light and Matter

5.3 Entropy As a Macroscopic Quantity5.3.1 Efficiency and grades of energySome forms of energy are more convenient than others in certainsituations. You can’t run a spring-powered mechanical clock on abattery, and you can’t run a battery-powered clock with mechanicalenergy. However, there is no fundamental physical principle thatprevents you from converting 100% of the electrical energy in abattery into mechanical energy or vice-versa. More efficient motorsand generators are being designed every year. In general, the lawsof physics permit perfectly efficient conversion within a broad classof forms of energy.Heat is different. Friction tends to convert other forms of energyinto heat even in the best lubricated machines. When we slide abook on a table, friction brings it to a stop and converts all its kineticenergy into heat, but we never observe the opposite process, in whicha book spontaneously converts heat energy into mechanical energyand starts moving! Roughly speaking, heat is different because it isdisorganized. Scrambling an egg is easy. Unscrambling it is harder.We summarize these observations by saying that heat is a lowergrade of energy than other forms such as mechanical energy.Of course it is possible to convert heat into other forms of energysuch as mechanical energy, and that is what a car engine does withthe heat created by exploding the air-gasoline mixture. But a carengine is a tremendously inefficient device, and a great deal of theheat is simply wasted through the radiator and the exhaust. Engineershave never succeeded in creating a perfectly efficient device forconverting heat energy into mechanical energy, and we now knowthat this is because of a deeper physical principle that is far morebasic than the design of an engine.Section 5.3 Entropy As a Macroscopic Quantity 309