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# Thermodynamics

1.

Mass and energy are conserved quantities, but entropy is generally not conserved.

This is a basic fact about entropy.

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2.

A hot metal piece is cooled rapidly to 25°C, removing 1000 kJ from the metal. Calculate the change of entropy if energy is absorbed by vaporizing liquid nitrogen at 101.3 kPa pressure.

Nitrogen boiling at 101.3 kPa; m = 1Q2 /h(fg) = 1000/198.842 = 5.029 kg
∆S(N2) = ms(fg) = 5.029(2.5708) = 12.929 kJ/K.

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3.

Which of the following is a type of energy?

These are two types in which the sources of energy can be divided into.

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4.

A piston cylinder contains 0.5 kg of air at 500 kPa and 500 K. The air expands in a process so pressure is linearly decreasing with volume to a final state of 100 kPa and 300 K. Find the work in the process.

Work = ⌠ PdV = (1/2)(P1 + P2)(V2 – V1)
V1 = mR T1/ P1 = 0.5 × 0.287 × (500/500) = 0.1435 m^3
V2 = mR T2/ P2 = 0.5 × 0.287 × (300/100) = 0.4305 m^3
W = (1/2)(500 + 100)(0.4305 – 0.1435) = 86.1 kJ.

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5.

At equilibrium, the isolated system exists at the peak of the entropy-hill and

At equilibrium, the entropy becomes maximum and hence change in entropy is zero.

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6.

If the path A causes more entropy generation than path B, then

The amount of entropy generation quantifies the intrinsic irreversibility of the process.

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7.

A piston-cylinder device initially contains air at 150 kPa and 27°C. At this state, the volume is 400 litre. The mass of the piston is such that a 350 kPa pressure is required to move it. The air is now heated until its volume has doubled. Determine the total heat transferred to the air.

Qin – Wout = ΔU = m(u3 –u1)
m = P1V1/RT1 = 0.697 kg
u1 = u@ 300 K = 214.36 kJ/kg
u3 = u@ 1400 K = 1113.43 kJ/kg
Therefore Qin = 767 kJ.

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8.

The proof that work done in all reversible processes is same can be done by violating Kelvin-Planck statement.

During the proof, we end up violating the Kelvin-Planck statement.

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9.

The available energy is known as ____ and the unavailable energy is known as ____

Rant was the one who coined these terms.

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10.

Which of the following is the largest unit of energy?