What quantities appear in the first law of thermodynamics?

Question

BlackTom AI · Accepted Answer

The energy balance relation ΔU = Q − W (or equivalently ΔU = Q + W with the opposite sign convention for work) identifies the three quantities that appear in the first law: internal energy (U), heat (Q), and work (W).

1. Relevant knowledge points and concepts
- Internal energy (U): the total microscopic energy stored in a system (kinetic + potential of molecules).  
- Heat (Q): energy transfer across the system boundary due to a temperature difference.  
- Work (W): energy transfer across the boundary due to macroscopic forces (e.g., pressure–volume work).  
- First law of thermodynamics: statement of energy conservation for thermodynamic systems, relating changes in internal energy to heat and work transfers.

2. Step-by-step reasoning
- The first law expresses conservation of energy for a closed system: any change in the system’s internal energy equals energy added as heat minus energy leaving as work (ΔU = Q − W under the convention that W is work done by the system).  
- Therefore the law explicitly involves the three quantities U, Q, and W.

3. Calculational steps (symbolic)
- Start with the law: ΔU = Q − W.  
- Each symbol corresponds to one of the listed physical quantities: ΔU (internal energy change), Q (heat added), W (work done by the system).

4. Why the correct answer is correct
- Option b (work, heat, internal energy) lists exactly the three quantities in the first-law equation. Those are the fundamental terms that appear in the law.

5. Why the other options are incorrect
- a (work, heat, entropy): entropy is central to the second law, not the first; entropy does not appear in the first-law energy balance.  
- c (force, mass, acceleration): these are Newtonian mechanics quantities, not the thermodynamic energy terms in the first law.  
- d (enthalpy, entropy, heat): enthalpy and entropy are useful thermodynamic functions, but enthalpy = U + PV and entropy belongs to the second law; they are not the three terms of the first-law balance.  
- e (inertia, torque, angular momentum): rotational mechanics terms, unrelated to the first-law statement.

6. Conclusion
The correct choice is b: work, heat, internal energy, which matches the provided answer.

What quantities appear in the first law of thermodynamics?单项选择题

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