Which of the following statements is true?
A. Enthalpy is defined as the difference between the internal energy (U) and the flow work term (PV).
B. In the energy balance, the work done by a flowing fluid as it enters or leaves a system is accounted for in the shaft work, if the fluid is moved by a pump.
C. If the energy balance is applied to a process that is isothermal, ?U has to be 0.
D. All of the above statements are true.
E. None of the above statements are true.
A. Incorrect. Enthalpy is the SUM of internal energy and flow work (H = U + PV)
B. Incorrect. The work done by the fluid entering or leaving is called the flow work and is incorporated into the enthalpy (H) in the energy balance. If the fluid turns a turbine, or is moved by an impeller, that is shaft work.
C. Incorrect. Internal energy can be affected by pressure. The statement is, however, true for an ideal gas. An ideal gas is a special case for which internal energy is only a function of temperature.
D. Incorrect. At least one of these statements is false.
E. Correct. All of them are provably erroneous.
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