What quantities appear in the first law of thermodynamics?

Question

BlackTom AI · Accepted Answer

ΔU = Q − W (conservation of energy for a thermodynamic system) — the first law directly relates internal energy change (ΔU) to heat added (Q) and work done (W).

1. Relevant knowledge points and concepts
- Internal energy (U): the total microscopic energy of the system (kinetic + potential of particles). The first law tracks changes ΔU.  
- Heat (Q): energy transferred across the system boundary because of temperature difference.  
- Work (W): mechanical (or other) energy transfer across the boundary (e.g., PV work).  
- Differential form: dU = δQ − δW (δ denotes inexact differentials for path-dependent Q and W).

2. Step-by-step reasoning
- Start with conservation of energy for a closed system: any energy added as heat or work changes the system’s internal energy.  
- Write the law: ΔU = Q − W. If Q > 0 (heat into system) and W > 0 (work done by system), ΔU increases by Q and decreases by W.  
- Rearranged or sign-convention variants exist (some texts use W as work done on system: ΔU = Q + W_on), but the three core quantities remain U, Q, and W.

3. Intermediate/calculation steps (illustrative)
- If a gas receives Q = 500 J and does W = 200 J on the surroundings, ΔU = 500 J − 200 J = 300 J. This shows explicit involvement of Q, W, and ΔU.

4. Why the correct answer is correct
- The first law explicitly involves heat (Q), work (W), and the change in internal energy (ΔU), so choice e (“work, heat, internal energy”) matches the law.

5. Why other options are incorrect
- a: inertia, torque, angular momentum are mechanical/rotational concepts not part of the thermodynamic energy balance.  
- b: enthalpy and entropy are state functions; entropy appears in the second law, enthalpy (H = U + PV) is a derived quantity used for constant-pressure processes but is not the fundamental three quantities in the first law.  
- c: force, mass, acceleration are Newtonian mechanics, not the first law’s quantities.  
- d: entropy belongs to the second law; the first law does not include entropy as one of its three primary terms.

Conclusion: therefore the correct choice is e. 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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