Mechanical Engineering - Thermodynamics - Discussion

In isothermal process temperature kept constant. And internal energy function of temperature only.

An isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange. In contrast, an adiabatic process is where a system exchanges no heat with its surroundings (Q = 0). In other words, in an isothermal process, the value ΔT = 0 but Q ≠ 0, while in an adiabatic process, ΔT ≠ 0 but Q = 0.

Internal energy always zero in isothermal process. Formula.

du=Cv*dt.

Internal energy is function of temperature.

So no change in internal energy.

In fact the answer is wrong.

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

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

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

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

delta U = MC delta T.

Internal energy directly depends on temperature.

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).