2nd Year Liquids and Vapours/Entropy/Second Law

Question 1

Compressed Liquid =

Answer 1

Liquid below Tsat

Question 2

Superheated Liquid =

Answer 2

Gas above Tsat

Question 3

Wet Vapour

Answer 3

Mix of gas and liquid at Tsat

Question 4

Dryness fraction

Answer 4

Quality can be found using interpolation between “Sat g” and “Sat f” values

Question 5

V =

Answer 5

mv

Useful in questions

Question 6

Can I use the perfect Gas Laws?

Answer 6

HELLA NO!

Question 7

Second Law:

Answer 7

Entropy will always increase

Question 8

Thermal efficiency =

Answer 8

W net / Qin
or
1 - Q(out)/Q(in)

Will always be less than 1

Question 9

Calculte Entropy :

Answer 9

ΔS = Q/T

T in Kelvin!

Question 10

What is entropy?

Answer 10

Referred to as “measure of chaos”

ΔS will always be positive

Question 11

Zeroth Law:

Answer 11

If 2 systems are in equilibrium with a third, then they are also in equilibrium with eachother

Question 12

Third Law:

Answer 12

Entropy of a perfect crystal at 0 Kelvin is 0.

Question 13

Δs =

Answer 13

Cv Ln(T2/T1) + R Ln(V2/V1)

Cp Ln(T2/T1) - R ln(P2/P1)

Cp Ln(V2/V1) + Cv Ln (P2/P1)

These can be manipulated for easy equations at const T,P,V

Question 14

Expansion Process

Isentropic Efficiency =

Answer 14

Actual Work out / Ideal Work out

Work Transfer decreases as Δs increases

Question 15

Compression Process

Isentropic Efficiency =

Answer 15

Ideal Work in/ Actual Work in

Work consumed decreases as Δs increases

Question 16

How to find q? (from T and S)

Answer 16

Area under a T-s Diagram

Remember T in Kelvin!