Question

1. Which of these can be statements of II law of TD? Check all that apply.

(a) Heat can flow only from cold body to hot body with no work input

(b) Heat can flow only from hot body to cold body with no work input

(c) The entropy generation in a (system+ surroundings) CV is always positive

(d) A power cycle cannot have 100% efficiency

(e) All the heat input from a heat source to a power cycle can be converted to work.

2. The meaning of perfect gas assumption is (check all that apply)

(a) The gas obeys ideal gas equation

(b) The specific heats of the gas are independent of temperature

(c) Most gases can be considered perfect gases at high temperature and low pressure, far away from the critical point of the dome shape curve on the T-v diagram. With additional assumption that the CP and Cv don’t change with temperature, it becomes a perfect gas.

(d) All of the above

3. Which of the following processes are irreversible (check all that apply)

(a) Free expansion of gas into vacuum

(b) Deceleration of a block sliding on a flat ramp.

(c) Heat transfer from a hot body to cold body

(d) Melting of ice dropped in boiling water

Answer 1

Ans 1

Option A is false

Heat can never flow only from cold body to hot body with no work input

Option B is true

Heat can flow only from hot body to cold body with no work input

Clausius statement from Second law of thermodynamics

Heat can never pass from a colder body to warmer body without any external work.

Option C - True

The entropy generation in a (system+ surroundings) CV is always positive.

According to Second law of thermodynamics entropy of a control volume or closed system always increase or constant because the entropy goes to thermodynamic equilibrium. The entropy would remain the same or would increase. In practical, it never decreases.

dS > dQ/T

Option D is true

A power cycle cannot have 100% efficiency.

Only cyclic reversible power cycle have 100% efficiency i.e. Carnot engine which is ideal or fictitious.

Option E is false

All the heat input from a heat source to a power cycle can not be converted to work.

Kelvin statement from Second law of thermodynamics

All heat that drains cannot be converted completely into work without any external work.