How heat, work and internal energy are related in gases and engines
Two bodies in thermal contact reach thermal equilibrium when they stop exchanging heat, and the zeroth law says that if A and B are each in equilibrium with C, they are in equilibrium with each other. This is what makes thermometers possible. The first law of thermodynamics is energy conservation: ΔQ = ΔU + ΔW, where heat supplied raises internal energy and lets the system do work. In isothermal processes temperature is constant, while in adiabatic processes no heat is exchanged and PV^γ stays constant.
The second law states that heat cannot flow by itself from a colder body to a hotter one, and no engine can turn all heat into work. A Carnot engine working between temperatures T₁ and T₂ has the highest possible efficiency, η = 1 − T₂/T₁. Kinetic theory links gas pressure to molecular motion, P = ⅓ρv²rms, and the average kinetic energy of a molecule is (3/2)kT, so temperature measures molecular motion.
Why it matters
Car engines, power plants, refrigerators and air conditioners are all designed using the laws of thermodynamics, and the Carnot limit explains why no engine is perfectly efficient. Kinetic theory explains why tyres heat up and gain pressure on long drives. These ideas link physics with chemistry, meteorology and engineering.
Key terms
- Internal energy
- The total kinetic and potential energy of all the molecules in a system.
- Adiabatic process
- A change in a system during which no heat enters or leaves it.
- Carnot engine
- An ideal reversible heat engine that has the maximum possible efficiency between two temperatures.
- Root mean square speed
- The square root of the average of the squares of molecular speeds in a gas.