Thermodynamics is the study of the conversion of energy between heat
and other forms, mechanical in particular. There are many processes that
convert energy from one form to another. For example burning wood converts
chemical energy (in the wood) to heat; turning a hydroelectric generator
converts the kinetic (motion) energy of the water into electrical energy.
There are two laws which we can use to help understand processes like
these: The first law of thermodynamics says that energy is conserved,
it is neither created nor destroyed but can change form. The second law
of thermodynamics says that systems always tend to states of greater disorder
-- this is another way to say that the entropy always increases. In terms
of energy conversions this means that they can never be 100% efficient.
Some portion of the energy involved in a conversion will inevitably be
lost to the surroundings as heat.
The first concept which must be understood in applying thermodynamics is the necessity to begin with the definition of what is called a "system". In thermodynamics this is any region completely enclosed within a well defined boundary. Everything outside the system is then defined as the surroundings. Although it is possible to speak of the subject matter of thermodynamics in a general sense, the establishment of analytical relationships among heat, work, and thermodynamic properties requires that they be related to a particular system.
We must always distinguish clearly between energy changes taking place within a system and energy transferred across the system boundary. We must likewise distinguish between properties of material within a system and properties of its surroundings. In accordance with their definition, thermodynamic properties apply to systems which must contain a very large number of ultimate particles. Other than this there are no fundamental restrictions on the definition of a system. The boundary may be either rigid or movable. It can be completely impermeable or it can allow energy or mass to be transported through it. In any given situation a system may be defined in several ways; although with some definitions the computations to be performed are quite simple, with others they are difficult or even impossible.
For example, it is often impossible by means of thermodynamic methods alone to make heat transfer calculations if a system is defined so that both heat transfer and diffusional mass transfer occur simultaneously through the same area on the boundary of the system. For processes in which mass transfer takes place only by bulk stream flow this problem can be avoided easily by a proper definition of the system. In a flow process of this type the system is defined so that it is enclosed by moveable boundaries with no stream flows across them. Heat transfer then always occurs across a boundary not crossed by mass.