First law of thermodynamics: judge the correctness of the statement Statement: “In the first law of thermodynamics, the total energy of the system remains constant.” Choose whether the statement is correct or incorrect.
Mechanical Engineering
Thermodynamics
Difficulty: Easy
Choose an option
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AAgree (the statement is correct)
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BDisagree (the statement is incorrect)
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CCorrect only for isolated systems
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DCorrect only for cyclic processes
Answer
Correct Answer: Disagree (the statement is incorrect)
Explanation
Concept/Approach
- The first law is an energy balance: the change in a system's total energy equals heat added minus work done by the system.
- It does not say the system's energy is always constant; only an isolated system (Q = W = 0) keeps constant energy.
Key relationΔE = Q − WE includes U (internal), KE, and PE.
ExampleAt constant volume heating: W = 0 ⇒ ΔE = Q > 0, so energy increases.
Final AnswerDisagree — the system's energy need not remain constant; it changes per ΔE = Q − W.