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A piston–cylinder device contains 1.2 kg of saturated water vapor at 200∘C. Heat is now transferred to steam, and steam expands reversibly and isothermally to a final pressure of 800 kPa. Determine the heat transferred and the work done during this process.
In these kinds of problems-isothermal reversible - what is the detailed reason behind pressure increase?
The pressure doesn't increase. It decreases. Saturated steam at 200 C has a pressure of 15.54 bars. The final pressure of 800 kPa is only 8 bars. So, at constant temperature, the pressure is about halved and the volume is about doubled. So the gas does work on the surroundings and heat has to be added to hold the temperature constant.
In your question here, the temperature remains constant because it's an isothermal process. Therefore there is no increase in pressure. But what happens here is that, when the piston is raised then the volume gets increased therefore the container takes in heat from the surroundings to maintain constant temperature.
In the question, as the heat is transferred to steam, the kinetic energy of the water molecules inside the piston-cylinder increase, due to which the temperature of the gas inside too increases i.e., this increase helps in maintaining the temperature constant.
