1 Isobaric Process (Isobaric change)
It is a physical change of a system where the pressure of the system remains constant.
If the working substance (gas) is taken in an expanding chamber in which the pressure is kept constant, the process is isobaric process.The Thermodynamic Processes are indicated in the following diagram (Isobaric Process)
In this process the gas either expands or contracts to maintain the pressure constant and a net amount of work is done by or on the system.
The graph A B is the indicator diagram of an isobaric process. The work done by the gas when its volume increases from V1 to V2,
W = P (V2 - V1) = Area of the graph A B B' A'
2 Isochoric Process (Isochoric change)
It is a physical change of a system where the volume of the system remains constant.
Since there is no change in the volume, the work done during the isochoric change is zero.
A B is an indicator diagram for an isochoric change. The graph A B is a straight line parallel to the pressure axis.The Thermodynamic Processes are indicated in the following diagram (Isochoric Process).
3 Isothermal process (Isothermal change)
When a system suffers a physical change under the condition that its temperature remains constant throughout the process, it is called an isothermal process.'
The essential condition for an isothermal process is that the system must be contained in a perfectly conducting chamber and the changes should be slow enough to ensure free exchange of heat with the surroundings. This ensures constancy of temperature. Since there is no change in internal energy, ΔU = 0.
From first law,
ΔU = ΔQ — ΔW
0= ΔQ — ΔW
ΔQ = ΔW
Thus in an isothermal process heat absorbed by the system is equal to the work done
Examples:
(a) Melting
(b) Boiling.
(c) Consider a gas inside a conducting cylinder fitted with a smooth conducting piston, expanding very slowly. Then the temperature of the gas remains constant. When the gas expands, work is done by the gas at the expense of its internal energy and hence its temperature falls. But, since the expansion is very slow and the cylinder and the piston are conducting, the gas absorbs heat from the surrounding to keep its temperature constant. So the expansion of the gas is an isothermal change, an isothermal expansion.
Similarly when the gas is compressed slowly, work is done on the gas. Its internal energy increases. But heat is transmitted to surrounding, and hence the temperature of the gas remains constant. The compression of the gas is an isothermal change, isothermal compression.
4 Adiabatic process (Adiabatic change)
When a system undergoes a physical change under a condition that no heat enters or leaves the system, the change is said to be adiabatic.
For an adiabatic process, the system must be perfectly insulated from the surroundings and the change must be rapid. When work is done on the system the temperature increases and when work is done by the system, the temperature decreases.
Since no heat enters or leaves the system, ΔQ = 0. Applying first law,
ΔU = ΔQ — ΔW
Therefore, ΔU = - ΔW
Thus in an adiabatic process the external work is done at the expense of internal energy. Hence when a gas expands adiabatically, its internal energy decreases. Consequently the temperature falls.
Examples:
(a) When a motor car tyre bursts, the sudden expansion of air into the atmosphere is adiabatic and hence the air is cooled.
(b) Consider a gas enclosed inside a perfectly non-conducting cylinder fitted with a perfectly non-conducting piston. When the gas expands, the gas does work at the expense of its internal energy. Since the gas is perfectly insulated from the surrounding, if the expansion is quick, the temperature of the gas falls. The expansion of the gas is adiabatic, an adiabatic expansion because no heat is absorbed from the surroundings or transmitted to the surroundings.
Similarly, when the gas is compressed quickly, as no heat is transmitted to or absorbed from the surroundings, the compression is an adiabatic compression.
Note:
(a) When water vapour from the surfaces of river, sea etc rises up, if it reach a low pressure region in the atmosphere it expands adiabatically. This produces cooling and vapour condensed into small droplets causing the rain fall.
(b) When sun shines, the denser air over the valley of a mountain gets heated up. The hot air rises up. When it reaches the higher altitude, the top of the mountain, it expands adiabatically because the pressure of air is very low at higher altitude. Due to adiabatic expansion cooling is produced. Hence the mountain tops are cooler.