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Heat and Thermodynamics question

2015 · Shift 0 · Q64
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Heat and Thermodynamics question

2015 · Shift 0 · Q64

JEE MainPhysicsHeat and ThermodynamicsMCQ+4 / −1
A solid body of constant heat capacity 1J/∘C1J/{}^ \circ C1J/∘C is being heated by keeping it in contact with reservoirs in two ways: (i)(i)(i) Sequentially keeping in contact with 222 reservoirs such that each reservoir         \,\,\,\,\,\,\,\, supplies same amount of heat. (ii)(ii)(ii) Sequentially keeping in contact with 888 reservoirs such that each reservoir           \,\,\,\,\,\,\,\,\,\, supplies same amount of heat. In both the cases body is brought from initial temperature 100∘C{100^ \circ }C100∘C to final temperature 200∘C{200^ \circ }C200∘C. Entropy change of the body in the two cases respectively is :
  1. A
    ln2,2ln2ln2, 2ln2ln2,2ln2
  2. B
    2ln2,8ln22ln2, 8ln22ln2,8ln2
  3. C
    ln2,4ln2ln2, 4ln2ln2,4ln2
  4. D
    ln2,ln2ln2, ln2ln2,ln2
View written solutionFree

Correct answer: EXACT CORRECT RESULT: $$\LEFT(\LN\FRAC{473}{373},\,\LN\FRAC{473}{373}\RIGHT).$$, IF FORCED TO CHOOSE FROM THE GIVEN OPTIONS, THE INTENDED OPTION IS **D** ONLY BECAUSE ENTROPY CHANGE MUST BE SAME IN BOTH CASES, BUT THE VALUE $\LN 2$ IS INCORRECT.

  1. Given data
  • Heat capacity of body: C=1 J/∘CC = 1\,\text{J}/^\circ\text{C}C=1J/∘C (constant)
  • Initial temperature: Ti=100∘CT_i = 100^\circ \text{C}Ti​=100∘C
  • Final temperature: Tf=200∘CT_f = 200^\circ \text{C}Tf​=200∘C

For entropy change, we must use absolute temperature:

Ti=100+273=373 K,Tf=200+273=473 KT_i = 100 + 273 = 373\,\text{K}, \qquad T_f = 200 + 273 = 473\,\text{K}Ti​=100+273=373K,Tf​=200+273=473K


  1. Entropy change of the body

Entropy is a state function, so it depends only on the initial and final states, not on the path or number of reservoirs.

For a body of constant heat capacity CCC,

ΔS=∫TiTfC dTT=Cln⁡(TfTi)\Delta S = \int_{T_i}^{T_f} \frac{C\,dT}{T} = C \ln\left(\frac{T_f}{T_i}\right)ΔS=∫Ti​Tf​​TCdT​=Cln(Ti​Tf​​)

Substituting C=1C=1C=1:

ΔS=ln⁡(473373)\Delta S = \ln\left(\frac{473}{373}\right)ΔS=ln(373473​)

This is the entropy change in both cases.


  1. Check the options

Numerically,

473373≈1.268\frac{473}{373} \approx 1.268373473​≈1.268

so

ΔS=ln⁡(1.268)≈0.237\Delta S = \ln(1.268) \approx 0.237ΔS=ln(1.268)≈0.237

Now compare with listed forms:

  • ln⁡2≈0.693\ln 2 \approx 0.693ln2≈0.693
  • 2ln⁡2≈1.3862\ln 2 \approx 1.3862ln2≈1.386
  • 4ln⁡2≈2.7724\ln 2 \approx 2.7724ln2≈2.772
  • 8ln⁡2≈5.5458\ln 2 \approx 5.5458ln2≈5.545

None of these equals ln⁡(473/373)\ln(473/373)ln(473/373).

So the correct physical result is:

ΔS1=ΔS2=ln⁡(473373)\Delta S_1 = \Delta S_2 = \ln\left(\frac{473}{373}\right)ΔS1​=ΔS2​=ln(373473​)


  1. Interpretation with respect to options

Since entropy change of the body is same in both processes, the pattern must be (same,same)(\text{same},\text{same})(same,same). Among the given options, only D has equal values in both cases.

However, its value ln⁡2\ln 2ln2 is not numerically correct. The exact correct value should be

(ln⁡473373, ln⁡473373)\left(\ln\frac{473}{373},\,\ln\frac{473}{373}\right)(ln373473​,ln373473​)

Thus the option structure suggests D, but the value stated is incorrect.


  1. Final conclusion
  • Derived entropy changes: ΔS1=ΔS2=ln⁡(473373)\Delta S_1 = \Delta S_2 = \ln\left(\frac{473}{373}\right)ΔS1​=ΔS2​=ln(373473​)
  • Among options, the only matching qualitative choice is D.
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