Science › Classical physics
Thermodynamics: energy and heat
1850· Theory
ProvenTested many times; part of working physics.
The 19th-century science of heat, work and energy, built to understand steam engines and now applied everywhere.
See it on the Science map →How it works
It rests on a few laws. Energy is never created or destroyed, only converted (first law). Heat flows on its own from hot to cold, and total disorder, called entropy, never decreases in a closed system (second law). Absolute zero, about −273 °C, can be approached but never reached (third law). The laws hold regardless of what matter is made of, which is their strength.
Why it matters
It sets hard limits on every engine, power plant, refrigerator and HVAC system: no machine can turn heat into work at 100% efficiency.
Think of it like
The first law is a balanced ledger: energy is never lost, only moved between accounts. The second law is a fee on every transfer that can never be refunded.
How we know
Joule's 1840s experiments showed work converts to heat at a fixed rate. Carnot (1824), Clausius and Kelvin (1850s) formalised the laws; no device has ever beaten them, despite countless perpetual-motion schemes.
Connections (4)
▸Thermodynamics: energy and heat is built on Conservation of energyProven
The relationship
The first law of thermodynamics is conservation of energy applied to heat: energy added to a system equals the rise in its internal energy plus the work it does. Every engine efficiency calculation starts by balancing this energy budget. Recognising heat as a form of energy, not a substance, made the law possible.
How the link was found
Mayer (1842), Joule (1843–1850) and Helmholtz (1847) established energy conservation; Clausius and Kelvin made it the first law around 1850–1851.
Why it's proven
Joule measured a fixed exchange rate between mechanical work and heat. No machine has ever put out more energy than it took in.
Open questions and mysteries
On cosmic scales, in an expanding universe, energy conservation lacks a simple global form; how to account for it is still discussed.
▸Thermodynamics: energy and heat introduced Entropy: disorder always growsProven
The relationship
Studying why no engine can turn all its heat into work, Clausius defined a quantity, entropy, that measures how spread out heat energy has become. The second law says it never decreases in an isolated system. That one rule explains why heat flows from hot to cold and sets the maximum efficiency of every engine.
How the link was found
Sadi Carnot's 1824 engine analysis laid the groundwork. Rudolf Clausius coined 'entropy' in 1865, from the Greek for transformation.
Why it's proven
Every measured heat engine, refrigerator and chemical reaction obeys it. No exception has ever been seen in a large system.
Open questions and mysteries
Thermodynamics alone says entropy grows but not why. Boltzmann's statistics explained it, but the universe's low-entropy starting point remains unexplained.
▸Thermodynamics: energy and heat led, via the glowing-oven puzzle, to Energy comes in packets (quanta)Proven
The relationship
Applying classical heat physics to radiation predicts a hot oven should glow with unlimited ultraviolet energy, later nicknamed the 'ultraviolet catastrophe'. Real ovens don't. Planck used thermodynamic reasoning about entropy and how energy is shared, and found that only if energy came in packets would high-frequency light be suppressed. A heat problem forced the quantum idea.
How the link was found
Max Planck in Berlin, October–December 1900, fitting precise oven-radiation measurements by Rubens, Kurlbaum, Lummer and Pringsheim at Germany's national standards institute.
Why it's proven
Planck's formula matches measured radiation from hot bodies at all frequencies, where classical formulas fail badly. The cosmic microwave background fits it to better than one part in ten thousand.
Open questions and mysteries
Planck did not believe the packets were real at first. Why thermodynamics proved such a reliable guide to new physics, here and later with black holes, still intrigues physicists.
▸Ludwig Boltzmann explained with atoms Thermodynamics: energy and heatProven
The relationship
Thermodynamics described heat without saying what heat is. Boltzmann explained it as atoms in motion: temperature reflects average molecular energy, pressure is molecules striking walls, and entropy counts arrangements. That rooted the thermodynamic laws in the mechanics of countless particles, the field now called statistical mechanics.
How the link was found
Maxwell began with gas theory in 1860; Boltzmann developed the full approach between 1868 and 1877; Gibbs systematised it in 1902.
Why it's proven
Einstein (1905) and Perrin (1908) confirmed atoms through Brownian motion, and statistical mechanics predicts measured material properties.
Open questions and mysteries
How time-symmetric atomic motion yields one-way thermodynamics (Loschmidt's 1876 objection) still rests on the unexplained low-entropy start of the universe.