Timothy Myers, age 14, of Dodge City, Kan., for his question:
WHAT IS THERMODYNAMICS?
Thermodynamics is the study of the relationship between heat and energy. It is based on certain principles, or "laws."
The first law of thermodynamics is the law of conservation of energy. It states that energy is never created or destroyed. Energy may change form such as from internal energy to mechanical motion but the total quantity of energy in any system or group of things remains the same.
The second law of thermodynamics states that all spontaneous or natural events act to increase the entropy within a system. Entropy is a measure of the amount of energy unavailable for work during a natural process.
As long as a system has not reached its maximum entropy, it can do useful work. But in doing work, the entropy of the system increases until the system can no longer perform work.
The third law of thermodynamics describes matter at absolute zero. For example, when solid argon is at absolute zero, it has no disorder.
The idea that heat is a form of energy was proved during the mid 1800s largely by three people: a German physician and physicist named Julius von Mayer, a German physicist named Hermann von Helmholtz and a British physicist named James Joule.
Mayer observed that people in warm and cold climates needed different amounts of food energy to maintain their normal body temperatures.
Helmholtz found that heat is actually a form of energy.
Joule measured the amount of mechanical energy needed to raise the temperature of a quantity of water and found the relationship between mechanical energy and heat energy.
Mechanical energy and heat energy are related. As an example, mechanical energy is changed into heat by friction between the moving parts of a machine. Heat energy, in turn, can be changed into mechanical energy by heat engines.
Heat engines can be divided into two groups: external combustion engines and internal combustion engines.
External combustion engines use heat produced outside the engine. Such engines include gas and steam turbines and reciprocating steam engines.
Internal combustion engines produce heat inside the engine from burning fuels. Such engines include diesel and gasoline engines, jet aircraft engines and rocket engines.
A good example of an external combustion engine is a steam turbine. Heat from burning fuel changes water in a boiler to steam. Pipes carry the steam into a turbine which has a series of bladed wheels attached to a shaft. The high temperature steam expands as it rushes through the turbine and so pushes on the blades and causes them to turn the shaft.
The spinning shaft can drive an electric generator, move a ship's propeller or do other useful work. Heat in a steam turbine can also come from an atomic reactor rather than burning fuel. An automobile is a good example of a machine with an internal combustion engine.