ThermodynamicsIntroduction
A foggy January morning at Lucknow station. Aman rubs his palms together hard, and they warm up: the work he does turns into warmth. At the far end of the platform, the heritage steam engine does the reverse. Its fire boils water, and the hot steam pushes the pistons that turn the wheels: heat turns into work.
This chapter is about the laws behind those two trades. For a long time people pictured heat as an invisible fluid, called caloric, hiding in the pores of a body. Put a hot body against a cold one and caloric was supposed to flow across, the way water flows through a pipe joining two tanks until the levels match.
In 1798 Benjamin Thompson, better known as Count Rumford, was in charge of boring brass cannons in Munich. Horses turned a drill against the metal, and the cannon grew hot enough to boil the water around it. If heat were a fluid squeezed out of the metal, a sharp drill that cuts more brass should set free more of it. Run Rumford's experiment and see what the heat really follows. Each press makes the horses do 0.4 MJ of work; the cannon sits in 10 kg of water at 20 °C.
The blunt drill cut a tenth as much brass, yet its rings fall on the same amber line as the sharp drill's dots. The heat depends on the work done by the horses, not on how much metal comes off. The caloric picture cannot explain that; the simple explanation is that heat is a form of energy, and boring the cannon converted work into heat.
Thermodynamics is the branch of physics that deals with heat and temperature, and with the conversion of heat into other forms of energy and back. It is a macroscopic science: it describes a gas by a handful of quantities you can measure in the lab, such as pressure, volume, temperature, mass and composition, and never by the positions and speeds of its molecules. Its laws were worked out in the nineteenth century, before anyone was sure that molecules existed at all.
A Bullet Is Not Hot Because It Is Fast
Mechanics and thermodynamics look at the same objects in different ways. Mechanics follows the motion of a body as a whole under forces and torques. Thermodynamics ignores that motion and asks about the internal state of the body: how hot it is, what its pressure and volume are.
A bullet leaves a rifle at 400 m s⁻¹. Its mechanical state has changed enormously: it carries a lot of kinetic energy. Its temperature has hardly changed. Then it buries itself in a wooden post and stops, and now the bullet and the wood around it are hot. Where did that warmth come from?
Temperature is tied to the energy of the internal, disordered motion of the molecules, not to the motion of the body as a whole. In flight, every molecule of the bullet shares the same forward velocity: that is ordered motion, and mechanics deals with it. When the bullet stops, that kinetic energy is shared out as random jiggling among the molecules of the bullet and the wood, and both get hotter. That is the kind of energy thermodynamics deals with.