1. Chapter 2
Part 2 of 4

2. 1st law of Thermodynamics
O Closed System 𝑄 − 𝑊 = ∆𝑈 
O Open system 
Dr.Khaled S. Al-zahrani

3. EInitial Efinal
 Q 𝒎 𝒐𝒖𝒕 
Ui open Uf
KEi system KEf
PEi  W PEf
DEi 𝒎 𝒊𝒏 DEf
Total Initial Energy Ei = Ui + K𝐸 𝑖 + PEi + DEi
Total Final Energy Ef = Uf + K𝐸 𝑓 + PEf + DEf
Conservation of energy law
Ei + EE = Ef + EL
 Q  W
Dr.Khaled S. Al-zahrani

4. Conservation of energy law
Ei + EE = Ef + EL
( Ui + K𝑬 𝒊 + PEi + DEi )+ Q = ( Uf + K𝑬 𝒇+ PEf +DEf )+ W
Knowing that:
𝐷𝐸 𝑖 = 𝑃𝑖 × v 𝑖 and 𝐷𝐸 𝑓 = 𝑃 𝑓 × v 𝑓
𝑃𝐸 𝑖 = 𝑔 × 𝑍 𝑖 𝑃𝐸 𝑓 = 𝑔 × 𝑍 𝑓
𝐶2 𝑖 𝐶2 𝑓
𝐾𝐸 𝑖 = 𝐾𝐸 𝑓 =
2 2
Then:
𝐶2 𝑖 𝐶2 𝑓
( Ui + +𝑔 × 𝑍 𝑖 +𝑃𝑖 × v 𝑖 )+ Q = ( Uf + +𝑔 × 𝑍 𝑓 +𝑃 𝑓 × v 𝑓 )+ W
2 2
If the specific Enthalpy is defined as:
hi= 𝑝 𝑖 V 𝑖 + 𝑈𝑖 and 𝑕 𝑓 = 𝑝 𝑓 V 𝑓 +Uf
Then:
𝐶2 𝑖 𝐶2 𝑓
+𝑔𝑍 𝑖 +𝑕 𝑖 + Q = +𝑔𝑍 𝑓 +𝑕 𝑓 + W
2 2
Dr.Khaled S. Al-zahrani

5. Enthalpy (h)
is a measure of the total energy of a thermodynamic system.
It includes the internal energy, which is the energy required to
create a system, and the amount of energy required to make
room for it by displacing its environment and establishing its
volume and pressure.
H = U + pV
Where
H is the enthalpy of the system (in joules).
U is the internal energy of the system (in joules).
p is the pressure at the boundary of the system (in Pascal).
V is the volume of the system, (in cubic meters).
Dr.Khaled S. Al-zahrani

6. 𝐶2 𝑖 𝐶2 𝑓
+𝑔𝑍 𝑖 +𝑕 𝑖 + Q = +𝑔𝑍 𝑓 +𝑕 𝑓 + W
2 2
Usually the affect of Potential Energy is small enough to be ignored:
𝐶2 𝑖 𝐶2 𝑓
+𝑕 𝑖 + Q = +𝑕 𝑓 + W
2 2
OR
2
𝐶 𝑓 −𝐶 𝑖
Q-W= + (𝑕𝑓 −𝑕 𝑖)
2
Dr.Khaled S. Al-zahrani

7. Continuity of mass flow
𝑘𝑔 𝑚3 𝑚3
𝑉= 𝑚 𝑣 = × =
𝑠 𝑘𝑔 𝑠
Also
𝑚 𝑚3
𝑉 = 𝐴 𝐶 = 𝑚2 × =
𝑠 𝑠
From both:
𝑉= 𝑚 𝑣= 𝐴 𝐶
Then:
𝐴 𝐶
𝑚= = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝑣
If we have an Open system with constant flow
𝑚1 = 𝑚2
Or
𝐴1 𝐶1 𝐴2 𝐶2
=
𝑣1 𝑣2
Dr.Khaled S. Al-zahrani

8. Dr.Khaled S. Al-zahrani

9. Information manual: pages 83 & 84
Questions: 1 , 2 , 3 , 4 , 5 , 6 , 9, 10 and 11
Dr.Khaled S. Al-zahrani