Thermodynamics is the science of heat and temperature. It is part of physics (and physical chemistry) that was developed at the time of the industrial revolution. It helped us understand how engines work using heat from burning coal or oil to create energy in a form that can do work.
The study of thermodynamics is easiest to understand in the context of Newtonian mechanics. Newtonian mechanics describes the effect of forces on objects. Forces acting over a distance do work: they transfer energy.
Thermodynamics describes the effect of heat transfer on objects. When heat is transferred, the temperature of an object changes. Temperature and heat are also intimately related to energy. A hot gas in a piston has a high pressure and it can do mechanical work by applying a force to a piston. By Newtonian mechanics the work is directly related to a transfer of energy.
The laws of Newtonian mechanics are simplest to describe using the abstract concept of a point object (a particle) with mass but no internal structure. The analogous abstraction for thermodynamic laws are materials that are in equilibrium and (even better) are homogeneous. It turns out that even the description of the equilibrium properties of materials is so rich and varied that this is still a primary focus of active research today.
Related concepts: statistical mechanics, Newtonian mechanics, separation of scales.
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