The document discusses the First Law of Thermodynamics. Some key points: - The First Law is based on the understanding that heat and work can be interconverted within a system and its surroundings. It states that the total energy of an isolated system is constant. - For a closed system, the change in the system's internal energy is equal to the heat added to the system plus the work done on the system. The change in the surroundings is equal to the negative of this amount. - The internal energy of a system depends only on its current state, not the path taken to reach that state. So the total change in internal energy will be the same regardless of the process between two states.

1. The First Law of Thermodynamics
1

2. Development of Thermodynamics
 Based on general understanding of interconversion between heat and work, first and
second laws were developed
 They are postulates and no contrary experience observed so far
 These laws were later developed into a network of equations
 Have wide ranging application in all branches of engineering
2

3. System and Surroundings
 System: Body of matter that is of interest
 Surroundings: Everything else
 Universe: System + Surroundings
3
System
Surroundings

4. Size of a system
 Size of the system: Expressed as mass (m), moles (n) or total
volume (Vt)
 𝑛 =
𝑚
𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡
 Molecular weight or molar mass
Molar mass of water : 18
4

5. Density and Volume
 𝑴𝒐𝒍𝒂𝒓 𝒗𝒐𝒍𝒖𝒎𝒆 𝑽 =
𝑽𝒕
𝒏
(SI Units: 𝒎𝟑
𝒌𝒎𝒐𝒍−𝟏
)
 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑣𝑜𝑙𝑢𝑚𝑒 𝑉 =
𝑉𝑡
𝑚
(SI Units: 𝒎𝟑𝒌𝒈−𝟏)
 𝑴𝒐𝒍𝒂𝒓 𝒅𝒆𝒏𝒔𝒊𝒕𝒚 𝝆 =
𝟏
𝒎𝒐𝒍𝒂𝒓 𝒗𝒐𝒍𝒖𝒎𝒆
(SI Units: 𝒌𝒎𝒐𝒍 𝒎−𝟑 )
 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝜌 =
1
𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑣𝑜𝑙𝑢𝑚𝑒
(SI Units: 𝒌𝒈 𝒎−𝟑 )
5

6. Extensive and Intensive Properties
 Extensive Properties depend on the size (quantity) of the system
 Intensive properties: Independent of the size
 What is molar volume, V for the cases above?
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n = 5 mol
Vt = 100 lit n = 10 mol
Vt = ?
n = 10 mol
Vt = 200 lit
n = 10 mol
Vt = 200 lit
V=20 lit/mol
n = 5 mol
Vt = 100 lit
V=20 lit/mol

7. Extensive and Intensive Properties
 Specific volume (m3/kg), molar or specific density are also intensive
 Applicable only for systems where all physical properties are uniform throughout
(homogeneous system)
 In a two-phase system containing vapor and liquid, each phase will have different
values of molar volumes (or densities).
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8. Temperature
 Galileo (Early 1600’s): Thermoscope
 Romer (early 1700’s) : Developed numerical scale
 Farenheit (used Hg, 4 divisions per degree)
8

9. Temperature
 Celsius (1744) : Celsius scale
 Amontons (early 1700’s) provided approximate estimate of absolute zero by
observing PVT relationship of various gases
 At this temperature, P → 0 (corresponding to the intercept)
 Absolute temperature scales (Kelvin and Rankine)
9

10. Temperature
 Celsius (1744) : Celsius scale
 Amontons (early 1700’s) provided approximate estimate of absolute zero by
observing PVT relationship of various gases
 At this temperature, P → 0 (corresponding to the intercept)
 Absolute temperature scales (Kelvin)
10

11. Temperature
 Absolute temperature scale (Rankine)
 Relationship between temperatures
t ℉ = 1.8 t ℃ + 32
𝐓 °𝐑 = t ℉ + 459.67 𝐓 𝐊 = t ℃ + 273.15
𝐓 °𝐑 = 1.8 𝐓 𝐊
 DIY: Convert 80 ℉ into the other three temperature scales
11

12. Absolute Pressure
 Pressure: Absolute and Gauge pressure
𝑃𝑎𝑏𝑠 = 𝑃
𝑔𝑎𝑢𝑔𝑒 + 𝑃𝑎𝑡𝑚
 Recall conversions between various units for pressure
Pa, psi, bar, atm, mmHg etc.
12

13. Work
 Work done on the system: Positive sign
 When we compress a gas is work positive or negative?
 During compression volume change is negative
 Work and volume change have opposite signs!!
 𝑑𝑊 = 𝐹 × 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = −𝑃 𝐴
𝑑𝑉𝑡
𝐴
= −𝑃 𝑑𝑉𝑡
(applicable only for special processes)
13

14. Work depends on the path!
𝑊 = −
𝑉1
𝑡
𝑉2
𝑡
𝑃 𝑑𝑉𝑡
14
 Work is a path variable

15. Energy
15
 ∆𝐾. 𝐸. = ∆
𝑚 𝑢2
2
 ∆𝑃. 𝐸. = ∆ 𝑚𝑔ℎ this is potential energy due to gravity
 other forms such as potential energy due to configuration (example a compressed
spring) are also possible
 Energy resides in the system
 When work is done by the system on the surroundings (or vice-versa) energy is
transferred

16. Joule’s Experiments
16
 Relationship between heat and work
W
∆𝑇 > 0

17. Internal Energy
17
 Energy Stored in fluid
 Does NOT include macroscopic K.E and P.E
 Energy of molecules internal to the substances
 Total internal energy Ut is in kJ
 Molar internal energy U (kJ/kmol)
 U is an intensive property, Ut is an extensive property

18. Internal Energy
18
U
Molecular K.E
Translation, rotation, vibration of molecules
Molecular P.E
Intermolecular forces
Sub-molecular
bonding, electrons, nuclei etc.
 Can not be measured directly
 Change in U i.e. ∆𝑈 manifests in other forms
 ∆𝑈 is sufficient for thermodynamic analysis

19. Development of First Law
19
 Recall the example of conversion of work to heat due to
stirring
 Energy disappearing in one form appears in another
 Total of all energies is constant
∆ Energy of system + ∆ Energy of surroundings = constant
W
∆𝑇 > 0

20. First Law for Closed System
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 Closed system: No transfer of matter across the boundary
System
Surroundings
 Energy transfers as heat or
work
 Q = heat added to the system
 W = work done on the system
 ∆ Energy of system = Q + W
W
Q
 ∆ Energy of surroundings = −Q − W

21. First Law for Closed System
21
System
Surroundings
For a closed system
∆ Energy of system = ∆𝑈𝑡
d𝑈𝑡
= dQ + dW
𝒅 𝒏𝑼 = 𝐝𝐐 + 𝐝𝐖
W
Q
∆ Energy of system = Q + W

22. U does not depend on path!
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P1 : 1 bar
T1 : 27 oC
V1 : 10 lit
P2 : 0.5 bar
T2 : 327 oC
V1 : 10 lit
P1 : 1 bar
T2 : 327 oC
V3 : 5 lit
Isochoric
Isothermal
∆𝑄𝐴 , ∆𝑊𝐴
∆𝑄𝐵 , ∆𝑊𝐵
Isobaric
∆𝑄𝐶 , ∆𝑊𝐶
∆𝑄𝐴+ ∆𝑄𝐵 ≠ ∆𝑄𝐶 ∆𝑊𝐴+ ∆𝑊𝐵 ≠ ∆𝑊𝐶
∆𝑄𝐴+ ∆𝑊𝐴 + ∆𝑄𝐵+ ∆𝑊𝐵 = ∆𝑄𝐶+ ∆𝑊𝐶
∆𝑈𝐴+ ∆𝑈𝐵 = ∆𝑈𝐶 U depends only on the state!!