What quantities appear in the first law of thermodynamics?

Question

BlackTom AI · Accepted Answer

The first law of thermodynamics is the statement of energy conservation for a thermodynamic system: ΔU = Q − W (or ΔU = Q + W depending on the sign convention). Here U is internal energy, Q is heat transferred into the system, and W is work done by the system.

1) Relevant knowledge points and concepts
- Internal energy (U): total microscopic energy of a system (kinetic + potential of molecules).
- Heat (Q): energy transfer due to temperature difference.
- Work (W): macroscopic energy transfer (e.g., pressure-volume work).
- First law: change in internal energy equals heat added minus work done by the system (conservation of energy).
- Distinction from other thermodynamic quantities: entropy (S) appears in the second law; enthalpy (H = U + PV) is a derived state function useful for constant-pressure processes but is not the primary statement of energy conservation.

2) Step-by-step reasoning
- Start with conservation: energy entering a system by heating or doing work changes its internal energy.
- Write the standard form: ΔU = Q − W. This explicitly includes Q, W, and ΔU (change in internal energy).
- Therefore the quantities appearing in the law are heat, work, and internal energy.

3) Intermediate steps / equation shown
- ΔU = Q − W (if W is work done by system). If using W as work done on the system, sign changes: ΔU = Q + W. Either way the three quantities are U, Q, and W.

4) Why the correct answer is correct
- Option a lists work, heat, internal energy — exactly the symbols and physical quantities in the first-law equation.

5) Why the other options are incorrect (brief)
- b: entropy appears in the second law, not the first.
- c: enthalpy and entropy are different state functions; enthalpy is useful in some processes but is not the core quantities of the first law.
- d: inertia, torque, angular momentum are mechanical/rotational quantities unrelated to the thermodynamic first law.
- e: force, mass, acceleration are Newtonian mechanics (F = ma), not the first law of thermodynamics.

6) Conclusion / confirmation
- Therefore the correct choice is a. 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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