Thermodynamics: Second Law
The Second Law introduces entropy and explains why processes have direction and why heat engines cannot be 100% efficient.
1Introduction
The Second Law explains why certain processes are impossible and sets limits on energy conversion efficiency.
A hot cup of coffee cools down to room temperature. Heat spontaneously flows from hot to cold, never the reverse. This is entropy in action!
2Key Concepts
Entropy (S) always increases for isolated systems
Heat Engine
Converts heat to work. η = W/Q_H = 1 - Q_C/Q_H
Refrigerator
Uses work to move heat from cold to hot. COP = Q_L/W_in
3Carnot Cycle
η_Carnot = 1 - T_C/T_H
Maximum theoretical efficiency
4Entropy
Entropy is a measure of disorder or energy unavailability. ΔS ≥ ∫δQ/T for any process.
5Applications
- Power plants and steam turbines
- Refrigeration and air conditioning
- Heat pumps for heating
- Internal combustion engines
Test your understanding — select the correct answer for each question.
1.The Second Law of Thermodynamics states that:
2.Which device operates on a reversed heat engine cycle?
3.The Carnot efficiency depends on:
4.Entropy is a measure of:
5.A reversible adiabatic process is also:
6.COP of a refrigerator equals:
7.Which statement about heat engines is true?
8.The Clausius statement says:
9.A perpetual motion machine of the second kind would:
10.In an isentropic process: