Illustrated Physics
physics, drawn
Concept

The p–V diagram — where it appears

Pressure plotted against volume, on which a cycle's enclosed area is the work it does and its direction says whether that work is out or in. It makes the first law a statement about geometry: the area is the net work, the difference between the heats is the same number, and a closed loop returns the internal energy to where it began.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

A Carnot cycle on pressure–volume axes. Two isothermal steps joined by two adiabatic ones, forming a closed loop. The gas expands 9.6-fold in reaching the cold reservoir at 0.50 of the hot one, and the area enclosed is the net work done over one cycle.

The ceiling on every engine, set before it was designed

There is a maximum efficiency no heat engine can exceed, and it depends on nothing but two temperatures. Not the fuel, not the working substance, not the cleverness of the engineer.

thermodynamics · Heat engines The ceiling, inverted. How many joules of heat a perfect machine can move per joule of work, against the outside temperature, with the inside held at 21 °C. The upper curve is heating — T_h/(T_h − T_c), which is what the Carnot argument becomes when the cycle is run backwards — and the lower one is cooling the outside, T_c/(T_h − T_c). They differ by exactly one everywhere, to 1.8e-15 across the whole range as drawn, because the work put in is delivered as heat along with whatever was moved. The dashed line at one is a resistive heater, which is 100% efficient and is the worst option on the figure. At 7 °C and −7 °C the ideal coefficients are 21.0 and 10.5; a real machine reaching 25% of the ideal gets 5.3 and 2.6, which is still several times what burning the same energy would give.

The engine that pays back more than it takes

Carnot's argument puts a ceiling on how much work a flow of heat can be made to do. Run the same cycle backwards and the ceiling inverts into a floor that is greater than one — so a machine can deliver three or four joules of heat for every joule it consumes, and a perfectly efficient electric heater is the worst way to warm a room.

thermodynamics · Heat engines

Named alongside it

The objects these essays reach for when they reach for this one.

The Carnot cycleEfficiencyReversibilityThe second lawTemperatureEntropyFree energyHeat capacityIrreversibilityLatent heatPhase transitionWork

All concepts