http://kids.britannica.com/comptons/article-200192/energy

In the early 19th century the Industrial Revolution was well underway. The many newly invented machines of the time were powered by the burning of fuel. These machines provided scientists with a great deal of information about how heat energy could be converted to other types of energy, how other types of energy could be converted to heat energy, and how heat energy could do work. Some of these observations were condensed into the laws of thermodynamics. (Thermodynamics is the branch of physics that studies relationships between heat energy, other forms of energy, and work.)

The first law of thermodynamics is a mathematical statement of the conservation of energy. This law says that the amount of heat added to a system exactly equals any change in energy of the system plus all the work done by the system. The equations derived from the first law of thermodynamics describe three variables: the internal energy of a system, the heat energy added to the system, and the work done by the system on its surroundings.

The practical importance of the first law of thermodynamics is that it shows that the addition of heat to a system enables it to do work. This, by definition, means that heat is a form of energy. When the first law was proposed, many people found it difficult to accept because they did not believe that heat was a form of energy. They thought of it as a mysterious fluid. But the first law did describe the action of heat engines and of many other kinds of heat interactions, so it came to be accepted as valid.

The first law says that the total energy of the universe remains constant. It does not say what kinds of energy can be changed into what other kinds of energy. After many false starts, a principle—the second law of thermodynamics—was worked out that described the kinds of energy conversions that are possible. This law states that conditions within any system tend to change to a condition of maximum disorder. (The amount of disorder in a system is called entropy.) Work must therefore be done from outside the system to impose more order on the system—that is, to decrease its entropy.

The second law of thermodynamics may seem surprising, yet it does describe many common experiences. For example, when someone kicks off his shoes, it is far more likely that they will land not in the closet where they belong but somewhere else. To get them where they belong the person must exert work. He must pick them up, carry them to the closet, and place them in their proper location.

Heat energy is the most disordered form of energy. (The individual molecules in an object move in random directions.) Therefore, according to the second law only a fraction of the heat energy available can be converted to useful work. Heat engines can transform some but not all of the heat energy available to them into mechanical energy. The rest remains as heat energy whether or not it is needed, wanted, or welcome.

Mechanical energy, on the other hand, can be completely converted to heat energy. This is a significant asymmetry. In both conversions the total amount of energy is conserved. But the second law of thermodynamics describes a restriction in the direction in which the conversions of energy can take place.

An automobile engine changes the chemical energy of gasoline into heat energy. The heat energy causes the gas to expand and push on a piston, thereby changing the heat energy partially to mechanical energy. Much of the heat energy, however, simply heats up the engine. The mechanical energy of the pistons is transferred to the tires, which push against the road’s surface and move the car forward. But some of the energy in the tires is changed to heat energy by friction. In this and in all other processes involving conversions of heat energy to mechanical energy, much of the original heat energy remains.

To illustrate the difference between the second law of thermodynamics and the first, consider a pan of water that is heated by a burner. The first law of thermodynamics would perfectly well allow the water to freeze and the flame of the burner to get hotter, just as long as the total amount of energy remained the same. The second law of thermodynamics asserts that this is impossible. The process must proceed in the direction that transfers heat from the hotter to the colder body. The general direction of all processes occurring in the observed universe is that which increases entropy.

The third law of thermodynamics concerns a temperature called absolute zero. Absolute zero occurs at about –273° C (–460° F). At absolute zero all substances theoretically would possess the minimum possible amount of energy, and some substances would possess zero entropy (be completely ordered). The third law states that, while absolute zero may be approached more and more closely, it is impossible actually to reach it (see cryogenics).