Topic Cluster

Thermodynamics

Thermodynamics: heat, temperature, entropy and energy transfer — the science of engines, refrigerators and the universe’s arrow of time.

Thermodynamics — the core equations of thermodynamics, and every article on the topic
The equations this topic keeps coming back to.

What Is Thermodynamics?

Thermodynamics is the physics of heat, temperature and energy transfer in systems made of enormous numbers of particles — where entropy emerges from the statistics of countless tiny collisions.

Core equations in thermodynamics
RelationshipEquationWhat it tells you
Specific heatQ = m·c·ΔTEnergy to change a temperature
Latent heatQ = m·LEnergy to change a state
First lawΔU = Q − WEnergy conservation for a gas
Ideal gas lawP·V = n·R·TState of an ideal gas
Carnot efficiencyη = 1 − Tc / ThBest possible heat-engine efficiency
Entropy changeΔS = Q / TDisorder gained in a reversible step
Energy Conversion Examples: Where the Joules Actually Go
Thermodynamics

Energy Conversion Examples: Where the Joules Actually Go

Energy conversion is the same joules changing form without changing total, and this guide traces fourteen of the commonest conversions from one end to the other. It covers the one bookkeeping line every conversion obeys, how to write a conversion chain and why every chain is really a tree, five everyday examples from a torch to a phone, the four routes a power station can take to the same electricity, and where the energy that looks lost has actually gone, with two original diagrams, seven worked problems and an interactive lab.

21 min read
Isothermal Process: What It Is and How to Find the Work
Thermodynamics

Isothermal Process: What It Is and How to Find the Work

An isothermal process holds the temperature still while the volume changes, so an ideal gas travels the curve PV = nRT and the work it does is the area underneath: W = nRT ln(V2/V1). One mole at 300 K doubling from 10.0 L to 20.0 L does 1728.8 J of work and draws in the same 1728.8 J as heat; let it expand freely into a vacuum between the same two states and it does none of it, while the entropy change stays at 5.763 J/K. Eight worked problems, three original diagrams, an interactive lab and a calculator.

23 min read
Thermal Diffusivity: How Fast Heat Really Moves Through a Material
Thermodynamics

Thermal Diffusivity: How Fast Heat Really Moves Through a Material

Thermal diffusivity is how fast a temperature change travels through a material, alpha = k divided by rho c, and it answers the question conductivity cannot: how long. A wall twice as thick takes four times as long to warm right through, still air outruns stainless steel, and a third quantity built from the same three properties decides why metal feels colder than wood. Eight worked problems, three original diagrams, an interactive lab and a calculator.

20 min read
The Combined Gas Law: When Pressure and Temperature Both Move
Thermodynamics

The Combined Gas Law: When Pressure and Temperature Both Move

The combined gas law reads PV = nRT twice - once before a change and once after - with the amount of gas held fixed, which leaves P1V1/T1 = P2V2/T2. Boyle, Charles and Gay-Lussac each hold one quantity still; this relation holds none of them. Eight worked problems, three original diagrams, an interactive simulator, and the two input mistakes that quietly produce plausible wrong answers: Celsius fed into a ratio, and gauge pressure.

21 min read