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Thermodynamics

Question
CBSEENPH11020630
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When 1 kg of ice at 0oC  melts to water 0oC, the resulting change in its entropy, taking latent heat of ice to be 80 cal/oC, is

  • 8 x 104 cal /K

  • 80 cal/K

  • 293 cal/ K

  • 273 cal/ K

Solution

C.

293 cal/ K

Change in entropy

increment straight S space equals space ml over straight T space equals space fraction numerator 1000 space straight x space 80 over denominator 273 end fraction space equals space 293 space cal space straight K to the power of negative 1 end exponent

Question
CBSEENPH11026459
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Assertion:  In an isolated system the entropy increases. 

Reason:  The process in an isolated system is adiabatic only.

  • If both assertion and reason are true and reason is the correct explanation of assertion.

  • If both assertion and reason are true but reason is not the correct explanation of assertion.

  • If assertion is true but reason is false.

  • If both assertion and reason are false.

Solution

C.

If assertion is true but reason is false.

An isolated system may have any process such as isothermal, adiabatic, cyclic etc.

Question
CBSEENPH11026465
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Assertion: In an adiabatic process, change in internal energy of a gas is equal to work done on or by the gas in the process.

Reason: Temperature of gas remains constant in an adiabatic process.

  • If both assertion and reason are true and reason is the correct explanation of assertion.

  • If both assertion and reason are true but reason is not the correct explanation of assertion.

  • If assertion is true but reason is false.

  • If both assertion and reason are false.

Solution

C.

If assertion is true but reason is false.

In an adiabatic process, no exchange of heat is permissible

i.e                    dQ = 0.

As  dQ = dU + dW

           = 0

∴  dU = - dW.

Assertion is true but reason is false.

Question
CBSEENPH11026437
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Assertion:  The specific heat of a gas in an adiabatic process is zero and in an isothermal process is infinite.

Reason:  Specific heat of gas is directly proportional to change of heat in system and inversely proportional to change in temperature.

  • If both assertion and reason are true and reason is the correct explanation of assertion.

  • If both assertion and reason are true but reason is not the correct explanation of assertion.

  • If assertion is true but reason is false.

  • If both assertion and reason are false.

Solution

A.

If both assertion and reason are true and reason is the correct explanation of assertion.

In adiabatic process, no heat transfer takes place between gas and surrounding 

            ΔQ =  0

From definition of specific heat

            C = ∆Qm ∆T = 0

Again, for isothermal process

           ΔT =  0

∴          C = ∆Qm ∆T = ∞

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Question
CBSEENPH11026330
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1 kg of water is converted into steam at the same temperature and at 1 atm (100 kPa). The density of water and steam are 1000 kgm-3 and 0.6 kgm-3 respectively. The latent heat of vaporisation of water is 2.25 x 106 J kg-1, What will be increase in energy?

  • 3 × 105 J

  • 4 × 106 J

  • 2.08 × 106 J

  • None of these

Solution

C.

2.08 × 106 J

Given:-

Latent heat of vaporization = 2.25 × 106 J kg-1

P = 1atm = 100kPa = 1 × 105 Pa 

Mass of the water = 1 kg

The volume of 1 kg water

⇒          V = 11000 m3

and volume of 1 kg steam 

⇒         V' = 10.6 m3

The increase in volume 

⇒           ΔV = ( V' - V )

⇒          ΔV  = 10.6 m3-  11000 m3

                   = ( 1.7 - 0.001 ) m3

⇒           ΔV = 1.699

⇒           ΔV ≈ 1.7 m3

The work done by steam is

      ΔW = P ΔV

      ΔW = ( 100 kPa ) (1.7 m3 )

      ΔW = 1.7 × 105 J

The change in internal energy

      ΔU = ΔQ - ΔW

           = 2.25 × 106 J - 1.7 × 105 J

      ΔU = 2.08 × 106 J  

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