Thermodynamics

Laws of Thermodynamics

Thermodynamics studies the mutual conversion of heat, work and energy.

• First law: Q = ΔU + A. The heat supplied goes into changing the internal energy and into doing work.
• Internal energy of an ideal monatomic gas: U = (3/2)·νRT.

Work Done by a Gas

Process	Work	Note
Isobaric (p = const)	A = p·ΔV	the volume changes
Isochoric (V = const)	A = 0	the volume does not change
Adiabatic	Q = 0, A = −ΔU	there is no heat exchange

In an adiabatic process Q = 0: if the gas does work, its internal energy decreases.

Heat Engines

• Efficiency: η = (Q₁ − Q₂)/Q₁, where Q₁ is the heat received from the heater and Q₂ is the heat given to the cooler.
• For the Carnot cycle η = (T₁ − T₂)/T₁.
• In the Carnot formula the temperature is taken in kelvins, not in Celsius.

Quantity of Heat

• Heating: Q = c·m·ΔT.
• Melting: Q = λ·m.
• Vaporization: Q = L·m.

Example: 800 J of heat is supplied to a gas, and it does 300 J of work. Then ΔU = 800 − 300 = 500 J.