8.4 Thermodynamics

- Combined law of thermodynamics: In thermodynamics it is simply a mathematical formulation of the first law of thermodynamics and the second law of thermodynamics subsumed into a single concise mathematical statement as shown below:

Here, U is internal energy, T is temperature, S is entropy, p is pressure, and V is volume.

- Entropy: In physics, it is a measure of the unavailability of a system's energy to do work. Spontaneous changes, in isolated systems, occur with an increase in entropy. Spontaneous changes tend to smooth out differences in temperature, pressure, density, and chemical potential that may exist in a system, and entropy is thus a measure of how far this smoothing-out process has progressed.

- First law of thermodynamics: it is an expression of the more universal physical law of the conservation of energy.

- Kelvin: The Kelvin scale of temperature is a thermodynamic (absolute) temperature scale where absolute zero -the coldest possible temperature- is zero Kelvin (0 K).

- Phase transition or phase change: It is the transformation of a thermodynamic system from one phase to another. The distinguishing characteristic of a phase transition is an abrupt change in one or more physical properties, in particular the heat capacity, with a small change in a thermodynamic variable such as the temperature.

- Second law of thermodynamics: It is an expression of the universal law of increasing entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium. There are many versions of the second law, but they all have the same effect, which is to explain the phenomenon of irreversibility in nature.