The Kelvin statement:
For example, it is easy to convert mechanical work completely into thermal energy, but it is impossible to remove thermal energy from a system and convert it completely into mechanical work with no other changes. This experimental fact is one statement of the second law of thermodynamics.
“It is impossible to remove thermal energy from a system at a single temperature and convert it to mechanical work without changing the system or surroundings in some other way”.
Second Law of Thermodynamics: Kelvin Statement
The Clausius statement:
If we place an ice cube on a hot day, the ice cube will melt. From the point of view of energy, what has happened is that some of the heat from the surrounding air enters the ice cube, raising its temperature, and eventually melting it. The surrounding air subsequently cools somewhat. However, nothing from energy conversion, or Newton’s law in general, would prevent heat from leaving the ice cube, making the ice cube colder and the surrounding air warmer. Why then doesn’t this later phenomena occur?
A common example of the conversion of mechanical energy into thermal energy is movement with friction. For example, when a block slides along a rough table, the initial mechanical [kinetic] energy of the block is converted into thermal energy as the block and the table are heated. The reverse process never occurs- a block and table that are warm will never spontaneously cool by converting their thermal energy into kinetic energy that sends the block sliding across the table. Thus there is a lack of symmetry in the roles played by heat and work. This lack of symmetry is related to the fact that some processes are irreversible. It may be mentioned that thermodynamic processes that occur in nature are all irreversible.
Let us take the case of heat conduction which is an irreversible process. If we place a hot body in contact with a cold body, heat will flow from the hot body to the cold body until they are at the same temperature. The reverse does not occur; heat does not flow from one to the other making one colder and the other warmer.
The answer lies in the Clausius statement of the 2nd law of thermodynamics, which can be written as:
“A process whose only final result is to transfer thermal energy from a cooler object to a hotter one is impossible”.
Second Law of Thermodynamics: Clausius Statement
In other words, “Heat never flows spontaneously from low temperature to high temperature”.
The Refrigerator statement:
Refrigerator:
It is impossible to make heat flow from a body at a lower temperature to a body at a higher temperature without doing external work on the working substance. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator.
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| “No cyclic process can transfer heat from a colder place to a hotter place with no input of mechanical work”. Second Law of Thermodynamics: Refrigerator Statement |
