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KVPY2018PhysicsThermodynamics

Graphs below show the entropy versus energy U of two systems 1 and 2 at constant volume. The initial energies of the systems are indicated by U 1 , i and U 2 , i , respectively. Graphs are drawn to the same scale. The systems are then brought into thermal contact with each other. Assume that, at all times the combined energy of the two systems remains constant. Choose the most appropriate option indicating the energi

Options

  1. AU 1 increases and U 2 decreases and the total entropy remains the same
  2. BU 1 decreases and U 2 increases and the total entropy remains the same
  3. CU 1 increases and U 2 decreases and the total entropy increases
  4. DU 1 decreases and U 2 increases and the total entropy increases

Correct answer

C. U 1 increases and U 2 decreases and the total entropy increases

Step-by-step solution

By second law of thermodynamics, entropy of a system either increases or remains constant (only in reversible quasistatic processes). Also, from graphs given S 2 > S 1 ⇒ Q T 2 > Q T 1 where, S is entropy, Q is heat and T is the temperature. or T 1 < T 2 Hence, temperature of system 1 is less than system 2 . So, heat flows from system 2 to system 1 . Hence, U 1 increases and U 2 decreases.

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