By ``closed'', we mean a system that is completely cut-off, or insulated from its surroundings, so that no material or energy enters or leaves.
Also as discussed in the last chapter, heat, being a form of energy, can be transformed into work and other forms of energy, and vice versa. However, this transformation of heat energy is subject to a very important restriction, called the 2nd law of thermodynamics. This law is in fact necessary to resolve the following apparent paradox:
The answer lies in the statement of the 2nd law of thermodynamics, which can be given in three equivalent forms:
When phrased in a precise mathematical language, one can show that any one
of the forms of the 2nd law imply the other two.
We shall discuss each of these three forms in turn.