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Question8: State the Second Law of Thermodynamics in its alternative forms. Discuss the assertion of 1stand 2nd laws about heat and work energies.


Second Law of Thermodynamics can be stated in a number of mutually equivalent statements. Here we give two of them.

  1. Lord Kelvin Statement

It states that it is impossible to construct a heat engine, operating continuously in a cycle, which takes heat from a heat source at a high temperature (HTR) and converts it completely into work without having a heat sink at low temperature, (LTR).

It means that to convert heat into mechanical work, a heat engine must have a heat source at high temperature (HTR) and a heat sink at lower temperature (LTR) as its essential parts.
During the cyclic process, a heat engine takes heat from heat source, converts some of it in useful mechanical work and sends the remaining heat to the colder reservoir (sink). This means that the work done by a heat engine must be less than the amount of heat taken from heat source. So, a 100% efficient heat engine (ideal heat engine) cannot be constructed.

  1. Rudolf Clauses Statement

It is impossible to make the flow of heat from a cold body to a hot body without the expenditure of energy.

When two bodies are brought in thermal contact, heat flows from the hotter body to the colder one. According to Clauses statement, it is possible to make the flow of heat from the colder to the hotter body. However, some energy must be spent in order to make this happen. As an example, in the case of refrigerator, heat flows from the colder interior parts of the refrigerator to the hotter surrounding at the cost of some electric energy.

Assertion of 1st and 2nd Laws

First Law of Thermodynamics tells us about the equivalence of heat and work. It tells us that both heat and work are forms of energy. It informs us that heat can be converted to work but does not tell us under what circumstances the transformation takes place. Moreover, it does not tell us about the amount of input heat and performed work.
The drawbacks are removed by the Second Law of Thermodynamics which tells us categorically about the conversion of heat in mechanical work and the existence of an LTR, so that input > output.

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