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Second Law, Heat Engines and Entropy

concepthard~45 min study9 MCQ

No engine can convert heat wholly into work; the Carnot efficiency 1 - Tc/Th is the upper bound for any engine working between two temperatures.

Practice 9 questionsBack to syllabus~14 min

What is Second Law, Heat Engines and Entropy?

No engine can convert heat wholly into work; the Carnot efficiency 1 - Tc/Th is the upper bound for any engine working between two temperatures.

Key formula / rule: Carnot efficiency = 1 - Tc/Th with absolute temperatures.

Key points

  • Compute Carnot efficiency and refrigerator coefficient of performance.
  • Evaluate a claimed engine performance against the second law.

Common exam trap

Using Celsius temperatures in the efficiency formula.

Definitions

Term

Second Law, Heat Engines and Entropy

Meaning

No engine can convert heat wholly into work; the Carnot efficiency 1 - Tc/Th is the upper bound for any engine working between two temperatures.

Term

Second Law, Heat Engines and Entropy — explanation

Meaning

The second law introduces direction into thermodynamics. Entropy increases for any real process, and the Carnot cycle is the reversible ideal that no real engine can beat.

Learning objectives

  • Compute Carnot efficiency and refrigerator coefficient of performance.

  • Evaluate a claimed engine performance against the second law.

Formulae

Key point

Carnot efficiency = 1 - Tc/Th with absolute temperatures.

Key point

Refrigerator COP = Tc/(Th - Tc).

Key point

Entropy of an isolated system never decreases.

Prerequisites

  • PHY-U3-THR-T2-S1-C2

Common mistakes

  • Using Celsius temperatures in the efficiency formula.

  • Claiming 100 percent efficiency is achievable with better engineering.

Keywords

  • Second

  • Engines

  • Entropy

Practice preview

  • Which of the following statements is a direct consequence of the Second Law of Thermodynamics regarding heat engines?…

    easy

  • A heat engine operates between a source at 500 K and a sink at 300 K. What is the maximum possible efficiency of this engine?…

    easy

  • In a Carnot engine, the working substance is an ideal gas. During which process is the entropy of the working substance decreased?…

    easy

Practice 9 questions