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Thermodynamics question

2002 · Shift 0 · Q4
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Thermodynamics question

2002 · Shift 0 · Q4

JEE MainChemistryThermodynamicsMCQ+4 / −1
A heat engine absorbs heat Q1 at temperature T1 and heat Q2 at temperature T2. Work done by the engine is J (Q1 + Q2). This data :
  1. A
    violates 1st law of thermodynamics
  2. B
    violates 1st law of thermodynamics if Q1 is +ve
  3. C
    violates 1st law of thermodynamics if Q1 is -ve
  4. D
    does not violates 1st law of thermodynamics
View written solutionFree

Correct answer: D

  1. Apply the first law of thermodynamics to a heat engine

For a cyclic heat engine, ΔU=0\Delta U = 0ΔU=0 so by the first law, Qnet=WQ_{\text{net}} = WQnet​=W where QnetQ_{\text{net}}Qnet​ is the net heat absorbed by the engine and WWW is the work done by the engine.

  1. Interpret the given data

The engine absorbs heat Q1Q_1Q1​ at temperature T1T_1T1​ and heat Q2Q_2Q2​ at temperature T2T_2T2​. So total heat absorbed is Qnet=Q1+Q2Q_{\text{net}} = Q_1 + Q_2Qnet​=Q1​+Q2​

Given work done is W=J(Q1+Q2)W = J(Q_1 + Q_2)W=J(Q1​+Q2​) Here JJJ is the mechanical equivalent of heat, used when converting heat units to work units.

  1. Check consistency with the first law

If Q1+Q2Q_1 + Q_2Q1​+Q2​ is heat quantity and work is expressed in mechanical units, then W=J(Q1+Q2)W = J(Q_1 + Q_2)W=J(Q1​+Q2​) is exactly the proper conversion form of the first law.

Thus, this does not violate the first law.

  1. Evaluate options
  • A: violates 1st law of thermodynamics — False
  • B: violates 1st law of thermodynamics if Q1Q_1Q1​ is positive — False
  • C: violates 1st law of thermodynamics if Q1Q_1Q1​ is negative — False
  • D: does not violate 1st law of thermodynamics — True
  1. Conclusion

The given data is consistent with the first law of thermodynamics.

Therefore, the correct option is: D\boxed{D}D​

  1. Comparison with stored answer

Stored correct answer is A, but the thermodynamic relation given is consistent with the first law, so I disagree with the stored answer.

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