Discussion
Consider the following statements about the general energy balance for an open system.
1. It reduces to the closed system first law when no mass crosses the boundary.
2. It contains an accumulation term that vanishes at steady state.
3. It applies only to ideal gases.
Answer: C
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: with no mass crossing the boundary the flow terms vanish and the balance becomes the closed system first law, and the accumulation term is exactly what distinguishes unsteady from steady operation. Statement 3 is wrong - the balance is a statement of energy conservation and holds for any substance in any phase. It is the *property data* that depend on the substance, not the balance.
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