Adiabatic Process ΔQ = 0:
When a system changes in such a way that it neither gives out heat nor takes heat from the surroundings, the process is known as an adiabatic process.
To produce an adiabatic change the system is contained in a container having non-conducting walls. Moreover, if the change is carried out quickly the chances of heat exchange are further reduced. That is why sudden changes are often regarded as adiabatic processes. Other examples of adiabatic processes are- Joule-Thomson expansion, propagation of sound waves in air, bursting of a tyre, etc.
As in an adiabatic process, neither heat enters the system nor it leaves, ΔQ = 0 and the first law of thermodynamics gives-
| ΔU + ΔW = 0 ΔU = -ΔW |
i.e., the external work is done by the system at the cost of its internal energy.
Some examples of adiabatic (or nearly adiabatic) processes are given below:
(1) Vigorous shaking of a thermos containing tea. If we shake vigorously a thermos flask containing tea, then it will be an adiabatic process. The work is done on tea against viscous forces present between the various layers of the tea. The work done on the system is, therefore, positive while the work done by the system is negative. As ΔW is negative, ΔU is positive so U2 > U1. This causes a slight increase in the internal energy.
(2) The cycle pump gets heated up. If the piston of the cycle pump is suddenly moved so as to compress air, work is done on the gas which is converted into heat. As this heat is not able to go out of the barrel in such a short time, the temperature of the gas increases.
(3) Deposition of carbon dioxide particles. When a gas is suddenly expanded, the external the outside of the system. On the other hand, if any part of the cycle is irreversible, then the working substance shall not return to its original state until it causes some change in the outside of the system. However, this would require some energy. This energy could have been used for work. Thus to achieve maximum efficiency it is essential that all processes in the cycle must be reversible.
Why two isothermals? For the operation of the engine, heat is to be extracted from the source. To achieve maximum efficiency, it is necessary that the extraction of heat by the working substance must be done by a reversible process. We know that extraction of heat from the source can be reversible only if the source and the working substance are in thermal equilibrium. Hence the working substance must draw heat from the source by a reversible process carried out at the temperature T1 of the source.
For the same reasons, the working substance must reject heat at the sink by a reversible isothermal process carried out at the temperature T2 of the sink.
Why two adiabatics? The working substance extracts heat from the source by a reversible process which is carried out at the source temperature T1. Similarly, the rejection of heat to the sink is done by a reversible process carried out at the temperature T2 of the sink. This means that after expanding isothermally at T1 the working substance has to be brought down at a lower temperature at T2. The change of temperature from T1 to T2 must not involve any flow of heat because otherwise irreversibility will be introduced. Therefore, the temperature change should take place without any flow of heat or in other words the temperature change from T1 to T2 must be brought by an adiabatic expansion. Similarly, at some other point of the cycle, the temperature of the working substance must be raised from T1 to T2 by adiabatic compression.
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