It may be positive or negative and is not, in itself, directional. Pressure is the force per unit area ( Pa). Several temperature scales have been described, but the SI scale uses the kelvin. It is a bulk property of matter (as are pressure, concentration, density, refractive index, and melting point) and should not be applied to single particles (e.g., molecules a single particle may possess energy but does not have a temperature). Hotness is a body's ability to impart energy as heat to another colder body. Temperature is a quantitative measure of hot and cold ( K). Latent heat is the energy required to cause phase changes between liquids, gases, and solids. Sensible heat is the energy required to change the temperature of a substance with no phase change. Heat is the amount of energy flowing from one body of matter to another spontaneously due to their temperature difference or by any means other than through work or the transfer of matter. Biological systems (e.g., live people) are not in thermodynamic equilibrium as they are not static and are subject to the movement of matter and energy to and from other systems.

Thermodynamics is not to be confused with kinetics, which describes the rate of change of processes and their mechanisms and depends on the pathway chosen. Thermodynamics describes the energy content of systems at equilibrium and their transformations and is independent of the pathway chosen. Remember this, and one will not be misled by purveyors of 'too good to be true' ideas such as running cars on just water. Of great importance is that the laws of thermodynamics are fundamental laws of the Universe and cannot be circumvented. This webpage forms an elementary introduction to the terms and ideas of thermodynamics.

Absolute entropy from heat capacity