5.1 Thermodynamic systems & direction of processes
- Spontaneous process: Occurs naturally (e.g. heat flows hot → cold).
- Irreversible process: Cannot be reversed (e.g. gas mixing, friction).
- Reversible process: Ideal, infinitely slow, no energy loss.
Q. Why does heat flow from hot to cold?
👉 Due to natural direction of thermodynamic processes.
5.2 Second law of thermodynamics
- Kelvin-Planck statement: Impossible to make a cyclic process that converts all heat into work.
- Clausius statement: Heat cannot flow from cold body to hot body without external work.
Q. State the 2nd law in simple words.
👉 Heat cannot fully be converted into work, and it flows hot → cold naturally.
5.3 Heat Engines
- Heat engine: Device that converts heat → work.
- Efficiency (η):
. Define heat engine efficiency.
👉 Ratio of work output to heat input.
5.4 Internal combustion engines & Carnot cycle
- Otto cycle: Petrol engine (spark ignition).
- Diesel cycle: Diesel engine (compression ignition).
- Carnot cycle: Ideal cycle, max efficiency:
Q. Which engine has highest efficiency?
👉 Carnot engine (ideal).
5.5 Refrigerator
- Refrigerator: Transfers heat from cold → hot by doing work.
- Coefficient of performance (COP):
Q. What is COP?
👉 Ratio of heat removed (Qc) to work done.
5.6 Entropy & disorder (Intro)
- Entropy (S): Measure of disorder/randomness.
- Second law (entropy form): Entropy of an isolated system never decreases.
Q. What does entropy mean?
👉 Degree of randomness/disorder in system.
✅ Exam Quick Picks:
- Heat flows hot → cold (not reverse).
- Efficiency of Carnot cycle depends on temperature only.
- COP of refrigerator always > 1.
- Entropy always increases in natural processes.
Important Questions and Answers in Short
Q1. What is the direction of thermodynamic process?
→ Heat always flows from hot body → cold body (not reverse, unless work is done).
Q2. State the Second Law of Thermodynamics.
→ It is impossible for heat to flow from cold body to hot body without external work.
→ No heat engine can be 100% efficient.
Q3. What is a Heat Engine?
→ Device that converts heat energy into work.
Works between:
- Hot reservoir (T₁) → gives heat Q₁
- Cold reservoir (T₂) → rejects heat Q₂
- Work output = Q₁ − Q₂
Q4. Otto Cycle (Petrol engine)?
- Process: 2 Adiabatic + 2 Isochoric
Q5. Diesel Cycle (Diesel engine)?
- Process: 2 Adiabatic + 1 Isochoric + 1 Isobaric
Q6. Carnot Engine?
- Ideal engine, 2 isothermal + 2 adiabatic.
- Efficiency:
Q7. What is Refrigerator?
→ Device that transfers heat from cold body to hot body using external work.
Q8. What is Entropy?
→ Measure of disorder/randomness.
✅ Formula Sheet (Quick Revision)
