Refrigerator or Heat Pump

A heat engine takes heat from a hot body, converts a part of it into work and rejects the rest to a cold body. A refrigerator also known as a heat pump, does the reverse operation. It takes an amount of heat Q₂ from a cold body, an amount of work W is done on it by the surrounding and the total energy Q₁ = Q₂ is supplied to a hot body in the form of heat.

Thus, heat is passed from the cold body to the hot body. The figure below, shows the process schematically. If the heat is taken at a single low temperature T₂, rejected at a single high temperature T₁ and all the parts of the process are carried out reversibly, we get a Carnot refrigerator. If the operating temperatures are fixed, a Carnot refrigerator needs minimum amount of work to extract a given amount of heat Q₂ from the colder body.

In this case,

This is the minimum amount of work to be done by the surrounding, if we wish to transfer heat Q₂ from the low-temperature body to the high-temperature body. This leads to another statement of second law of thermodynamics which is as follows:

It is not possible to design a refrigerator, which works in cyclic process and its only result is to transfer heat from a body to a hotter body.

This is known as the Claussius' statement of the second law.

Sub Topics

Heat engine
Heat engine is a device used for converting heat energy into mechanical energy.
Thermal efficiency of heat engine

Heat engine

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It is very easy to convert work into heat. For example, when two bodies are rubbed, heat is produced. However, for the conversion of heat into work, we require a device. This device is called heat engine.

Heat engine is a device used for converting heat energy into mechanical energy.

There are two types of heat engines: external combustion engines and internal combustion engines.

Steam engine is an example of external combustion engine. In

such an engine, the fuel is burnt outside the working portion of the engine.

A heat engine consists essentially of the following parts:

i) Source or Heat reservoir: It supplies heat energy.

ii) Sink or Cold reservoir: That heat which is not converted into work, is rejected.

iii) Working substance: It absorbs a certain quantity of heat from the source, converts a part of it into work and rejects the remaining heat to the sink. It is taken through a cycle of operations. At the end of each cycle, the working substance is restored into its original condition and there is no change in its internal energy. Since the cycle of operations can be repeated indefinitely, the heat engine gives a continuous supply of work indefinitely.

Schematic Representation of a Heat Engine

Thermal efficiency of heat engine

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It is defined as the ratio of the net external work done by the engine during one cycle to the heat absorbed from the source during that cycle. It is denoted by h.

The work W obtained in each cycle is called the output while the heat Q₁ extracted from the source in one cycle is called the input.

Since the working substance returns to its initial state after completing one cycle, there is no change in internal energy. So, applying first law of thermodynamics, we get Q₁ - Q₂ = W.

where Q₁, Q₂ and W are all measured in the same units.