PRESENTED BY: -
ZAIN INAM KHAN.
DATE:-24 nov 09
Adiabatic law of Compression and Expansion.
Compression of a gas.
Expansion of a gas.
It is a thermodynamic process in which no heat is
transferred to or from the system.
Any process that occurs within a container which is a
good thermal insulator is adiabatic.
Strictly speaking adiabatic processes do not
exist, since one can not provide absolute
In this sense adiabatic process is an
idealization. However, there are situations
when it is a good approximation to treat a
process as adiabatic. These situations are
The system is fairly well thermally isolated.
The process is so fast that heat has no time to escape
or to enter the system.
Neither the system is isolated, nor the process is very
fast, but the system is very large.
LAW OF ADIABATIC COMPRESSION
“Any gas will cool that is allowed to expand freely from
higher pressure to lower pressure without transfer of
external energy to it. Similarly a gas will heat if
compressed from lower to higher pressure in the
absence of transfer of energy from gas”.
ADIABATIC EXPANSION AND
COMPRESSION OF AIR .
Rising air experiences a drop in temperature .
The decrease in temperature is due to decrease in pressure at higher altitudes.
If the pressure of surrounding air is reduced air parcel will expand.
When air expands molecules of air do work this will affect air parcels temperature
Energy can only be utilized to do work for expansion or to maintain temperature but
cannot be used for both.
ADIABATIC COMPRESSION OF AIR
Total amount of energy remains same as none is added or
If air is compressed air volume decreases and it will heat
If air parcel is forced to descend then it will heat up
without taking heat from outside.
Reversible Isothermal Expansion (process 1-2):
Heat transfer between the heat source and the cylinder occurs with an
infinitesimal temperature difference. Hence, it is a reversible heat
transfer process. Gas in the cylinder expands slowly, does work to its
surroundings, and remains at a constant temperature TH. The total
amount of heat transferred to the gas during this process is QH.
Reversible adiabatic expansion (process 2-3):
The heat source is removed, and the gas expands in an adiabatic
manner. Gas in the cylinder continues to expand slowly, do work to its
surroundings till the temperature of the gas drops from TH to TL.
Assuming the piston moves frictionless and the process to be quasi-
equilibrium, the process is reversible as well as adiabatic.
Reversible isothermal compression (process 3-4):
The cylinder is brought into contact with a heat sink at temperature TL.
The piston is pushed by an external force and which does work on the
gas. During the compression, the gas temperature maintains at TL and
the process is a reversible heat transfer process. The total amount of
heat rejected to the heat sink from the gas during this process is QL.
Reversible adiabatic compression (process 4-1):
The heat sink is removed and the gas is compressed in an adiabatic
manner. Gas in the cylinder continues to be compressed slowly,
accepting work from its surroundings till the temperature of the gas
rises from TL to TH. The gas returns to its initial state, which completes