Lesson 2 of 2
The First and Second Laws
Understand conservation of energy and the direction of natural processes.
The First Law
The First Law of Thermodynamics is conservation of energy. For a closed system:
ΔU = Q - W
The change in internal energy equals the heat added to the system minus the work done by the system. Energy is never created or destroyed, only transformed.
The Second Law
The Second Law introduces entropy and direction. It states that the total entropy of an isolated system never decreases:
ΔS_universe ≥ 0
Practical consequences:
- Heat flows spontaneously from hot to cold, never the reverse.
- No heat engine can be 100% efficient.
- Some energy is always "lost" to disorder.
The Carnot limit
The maximum possible efficiency of a heat engine operating between two temperatures is:
η_max = 1 - (T_cold / T_hot)
with temperatures in Kelvin. Real engines fall short of this ideal, but it sets the ceiling every designer aims for.
You now have the two laws that govern all energy systems. Well done!