Mechanical Engineering - Thermodynamics - Discussion

The average velocity and molecular activities are directly proportional to the temperature of the gas of the system and as a result of the system leads to higher internal energy. So there are no changes in internal energy in an isothermal process, what internal energy referred here in thermodynamics is (microscopic forms of energy example; electron spin, nuclear spin, molecular vibrations), so microscopic interactions of molecular activities occur only if there is a change in temperatures.

Option B is correct.

Internal energy is a function of temperature only for the Ideal gas.

The logic behind this falls deep in the molecular level. Internal Energy is a function of temperature, temperature is a function of the Average Kinetic energy of the ideal gas molecules, and internal energy is a function of the average K.E and P.E at microscopic level, now, if the system is one component and does not involve a phase change (and can be approximated as ideal gas) , then the isothermal process will not change the internal energy because the temperature doesn't change and so are the kinetic energies, which is due to the fact that the energy intake to the system is changed into another form of energy, say for example piston cylinder, if the cylinder is getting heat energy from the surrounding, it will be turned into the work energy.

But if the system involves a phase change, it is no longer approximated as an ideal gas, and so the C.P.E of the molecules changes which ultimately expands the or contracts the system and so the internal energy changes (because remember Internal energy is a function of K.E as well as the Chemical Potential Energy).

delta U = MC delta T.

Internal energy directly depends on temperature.

Internal energy is the function of temperature so, temperature is the constants until internal energy is the constants.

Then how 100 celsius. Water changes to 100 degree vapour? Please clarify?

This answer is only correct for ideal gases. The question needs to clarify this.

In fact the answer is wrong.

The phase change process is isothermal but the internal energy varies.

Internal energy is function of temperature.

So no change in internal energy.