Chapter 16: Thermodynamics

Second Law of Thermodynamics

In any energy transfer or transformation, the total entropy of a system and its surroundings always increases.

Entropy (S)

A measure of the disorder or randomness in a system.

Heat Engine

A device that converts thermal energy into mechanical work by transferring heat from a high-temperature source to a low-temperature sink.

Key Concepts

• Thermal energy flows spontaneously from hot to cold regions.
• Entropy increases in natural processes unless work is done to reverse it.
• No heat engine can operate with 100% efficiency due to inevitable energy loss as waste heat.

Efficiency of Heat Engines

Formula

$$\eta =\frac{W}{Q_{\mathrm{in}}}\times 100\%$$

Variables

• W: Work done by the engine (J).
• Q_in: Heat absorbed from the high-temperature source (J).

The Carnot Cycle

• Proposed by Sadi Carnot, it is an idealized heat engine with maximum efficiency.
• Efficiency depends on the temperature difference between the heat source and sink.
• Carnot Efficiency Formula: $\mathrm{Efficiency}=(1-\frac{T_{\mathrm{cold}}}{T_{\mathrm{hot}}})\times 100\%$

Questions for Students

1. State the Second Law of Thermodynamics in your own words.
2. What is entropy, and how does it change in natural processes?
3. Explain why no heat engine can be 100% efficient.
4. Define the Carnot Cycle and its significance in thermodynamics.
5. Calculate the efficiency of a Carnot engine with T_hot = 500 K and T_cold = 300 K.