It is appropriate to call attention to certain thermodynamic properties of an ideal gas that are analogous to rubber-like deformation. The internal energy of an ideal gas depends on temperature only and is independent of pressure or volume. In other words, if an ideal gas is compressed and kept at constant temperature, the work done in compressing it is completely converted into heat and transferred to the surrounding heat sink. This means that work equals q which in turn equals ** f.

There is a well-known relationship between probability and entropy which states that ** f, where \q is the probability that state (i.e., volume for an ideal gas) could be reached by chance alone. this is known as conformational entropy. This conformational entropy is, in this case, equal to the usual entropy, for there are no other changes or other energies involved. Note that though the ideal gas itself contains no additional energy, the compressed gas does exert an increased pressure. The energy for any isothermal work done by the perfect gas must come as thermal energy from its surroundings.