Question

Difference between real and ideal gas thermodynamics

Answer 1

Answer 1.....Perfect gas is one in which intermolecular forces are not considered. Most of the gases behave as perfect gases at low pressures and at very high temperatures. Perfect gas obeys ideal gas law and it has constant specific heats.

Cp, Cv has constant values. For example if you consider air as perfect gas, Cp=1.005 kJ/kg. K and Cv=0.718 kJ/kg. K at all temperatures.

So, internal energy of a perfect gas can be simply calculated from du=Cv. dT and enthalpy of perfect gas is dh=Cp.dT.

Ideal gas is a gas that obeys ideal gas law and specific heats of an ideal gas are function of temperature alone.

For example if you consider air as an ideal gas. Cp=1.005 +f(T) and similarly Cv=0.718+g(T).

So, internal energy of ideal gas can be simply calculated from du=integral{Cv. dT} and enthalpy of an ideal gas is dh=integral{Cp.dT}.

Most of the cases we can interchangeably use this both terms as both obeys ideal gas law which is the main concern while solving engineering problems.

Now, what is a real gas.

This gas does not obey the ideal gas law Pv=mRT. Because inter molecular forces and volume occupied by the gas molecules are considered here.

Instead, they follow gas laws like vanderwaals equation of state, viral equation of state etc.

For these gases Cp and Cv are assumed to be function of temperature, but change in internal energy and enthalpy are not only function of temperature, some additional terms will be there.







Answer 2.....An ideal gas is defined as one in which all collisions between atoms or molecules are perfectly eleastic and in which there are no intermolecular attractive forces. One can visualize it as a collection of perfectly hard spheres which collide but which otherwise do not interact with each other. In such a gas, all the internal energy is in the form of kinetic energy and any change in internal energy is accompanied by a change in temperature.

A perfect gas is a theoretical gas that differs from real gases in a way that makes certain calculations easier to handle. Its behavior is more simplified compared to an ideal gas (also a theoretical gas). In particular, intermolecular forces are neglected, which means that one can use the ideal gas law without restriction and neglect many complications that may arise from the Van der Waals forces.