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The First Law of Thermodynamics

The First Law of Thermodynamics

The Second Law of Thermodynamics and Heat Engines

To understand the significance of the Carnot cycle, you need to have both the First and Second Laws of Thermodynamics in your analytical toolkit. Each stage of the Carnot cycle should be assessed in terms of both laws.

The First Law of Thermodynamics allows us to describe each step in terms of heat flow, work performed, and internal energy change. The Second Law enables us to discuss how the changes in entropy in the system and surroundings are balancing out at every stage. Every stage of the Carnot cycle is microscopically reversible, and the entropy of the universe does not increase.

However, heat still must be expelled at the cold sink, even though entropy is not increasing. Otherwise, entropy in the universe would actually decrease. By being reversible, the Carnot cycle finds the limit of heat that can be turned into work without decreaseing the entropy of the universe. In this sense, the Carnot cycle represents the ideal (most efficient) thermodynamic cycle possible. Compared to the Carnot cycle, all real cycles would result in even more heat being lost to the cold sink.

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