Past exam of the medicine course of the University of Oxford / 1865 / First examination / Physics / 7

What reasons are there for supposing heat to be a form of motion ?

The strong evidence is the reproducible conversion between mechanical work and thermal changes. In Joule's paddle-wheel experiment, descending masses drive vanes in a liquid, and a measured input of mechanical energy produces a measured rise in temperature. Allowing for the vessel and thermal losses gives a quantitative mechanical equivalent. Sustained frictional heating, including Rumford's cannon-boring experiment, also challenged a simple account in which rubbing merely released a fixed stored supply of a material heat fluid.^[ref]

Electrical and compressional experiments reinforce the connection. A resistor warms under Joule heating, with electrical input $\int VIdt$. A gas compressed with little heat transfer can rise in temperature, showing that mechanical work can increase its internal energy. Conversely an expanding gas can perform work while cooling. Such experiments connect several kinds of change through a conserved energy balance.

The kinetic theory of gases supplies a microscopic model. Translational molecular motion produces momentum transfer to walls; for a dilute monatomic ideal gas,Thus increasing molecular kinetic energy is reflected in temperature and pressure changes. These formulas describe a specified model, not a proof that every substance's internal energy consists only of translational motion. Rotations, vibrations and intermolecular potential energy also matter.

In modern terminology, heat is energy transferred because of a temperature difference. The body has internal energy, and microscopic motion accounts for part of it. Calling heat “a form of motion” captures the historical rejection of a conserved material caloric, but should not obscure that distinction between stored energy and an energy-transfer process.