Entropy and the Second Law of Thermodynamics
It is a common experience that watches run down, bouncing balls stop bouncing, and even the most perfectly designed and crafted engine will eventually cease operating. Time marches on and all things, left to themselves, stop. This universal tendency for all objects and processes to move “with the arrow of time” in the direction of running down is the hallmark of the second law of thermodynamics. The obverse to this relentless natural tendency for things to run down is the (equally relentless) search for a technical fountain of youth: the system that will produce more energy or work than it consumes. Such machines are called perpetual motion machines and are imagined in two varieties: perpetual motion machines of the first and second kind. The first is a machine that produces more energy than it absorbs as work or heat from the surroundings and thus creates energy. This kind of device has never been found (though people attempt it every day), and the first law of thermodynamics forbids its existence. The first law permits the second type of perpetual motion machine because it draws heat from the surroundings and converts that heat into work without changing the state of the surroundings. However, it turns out that there are substantial limitations on the construction of a machine or system that can take heat and convert it into directed energy or useful work.