Heat as energy: what thermodynamics studies

Thermodynamics is about the laws that govern thermal energy: how work turns into heat and heat into work. Rubbing your palms together in winter warms them (work → heat); in a steam engine the steam's heat pushes the pistons that turn the wheels (heat → work).

The old picture treated heat as an invisible fluid, 'caloric', held in the pores of a substance and flowing from one body to another until the 'caloric levels' (temperatures) matched, much as water levels equalise in two tanks joined by a pipe.

Count Rumford (Benjamin Thomson), 1798: boring a brass cannon produced enough heat to boil water. The heat depended on the work done by the horses turning the drill, not on how sharp the drill was. A sharper drill should have released more caloric; it did not.

The natural reading: heat is a form of energy, and the cannon showed work being converted into heat. The caloric idea was dropped in favour of this.

Thermodynamics is a macroscopic science. It describes bulk systems through a few measurable variables such as pressure, volume, temperature, mass and composition, and never needs the positions and velocities of individual molecules. Its laws were set down in the nineteenth century, before the molecular picture was firmly established.

Kinetic theory sits in between: it does not track each molecule but does use the distribution of molecular velocities.

Mechanics studies the motion of a body as a whole under forces and torques; thermodynamics studies its internal state. A fired bullet gains kinetic energy, not temperature. When it stops in a block of wood, that kinetic energy becomes heat and warms the bullet and the wood around it.

Temperature is tied to the disordered internal motion of a body's molecules, not to the motion of the body as a whole.