PPT about the introduction to thermodynamics-First law-Reference is P K NAG TEXTBOOK

1. First Law of Thermodynamics
-ppt-4
Presented by,
Lalitha P
Assistant Professor
Mechanical Engineering
RGUKT Basar
Engg Thermodynamics (ME2102)

2. Introduction
• Energy which enters the system as work may leave the system as heat, or energy may enters
the system as heat may leave the system as work.
First law for a “closed system” undergoing a cycle

4. • On this process, work 𝑾𝟏−𝟐is always proportional to the heat 𝑸𝟐−𝟏, and the constant of
proportionality is called, Joule’s equivalent or the mechanical equivalent of heat.
• In the given Joule’s experiment, a single quantity of heat and work are assumed to be
transferred across the system boundary to perform a cyclic process.
• In a cycle, if more quantities of heat and work energies are transferred between the s/m
and surroundings, same result will be found.
• Algebraically,
( 𝑊) 𝑐𝑦𝑐𝑙𝑒
= 𝐽( 𝑄) 𝑐𝑦𝑐𝑙𝑒
or 𝑑𝑊 = 𝐽 𝑑𝑄
Where, J is the Joule’s equivalent. Here, is the representation of a cyclic process.
• This is the first law of thermodynamics undergoing a cycle.
• As work (W) and heat(Q) is in Joules units, J will be unity as J = 1Nm/J.

5. Statement of first law undergoing a cycle:
• When a closed system executes a complete cycle the sum of heat interactions
is equal to the sum of work interactions.
( 𝑊)
𝑐𝑦𝑐𝑙𝑒
= 𝐽( 𝑄)
𝑐𝑦𝑐𝑙𝑒
Or,
• When a closed system undergoes a cycle the cyclic integral of heat is equal to
the cyclic integral ofwork.
𝑑𝑊 = 𝐽 𝑑𝑄
• In other words for a two processcycle
QA1-2+QB2-1=WA1-2+WB2-1

6. First law for a “closed system” undergoing a change of state
• Therefore, Q-W = ∆𝐸
where, ∆𝐸 is the increase in the energy of the system.
Or, Q = W+∆𝐸
• Here, Q, W and ∆𝐸 are in Joules.

7. • If there are more energy transfer quantities involved in the process, the first law becomes
𝑄2 + 𝑄3 − 𝑄1 = ∆𝐸 + (−𝑊1 + 𝑊2 + 𝑊3 − 𝑊4)
• Thus, energy is conserved.
• First law of thermodynamics is the formulation of principle of energy
conservation.

8. Energy – a property of the system
• Consider a s/m which changes its state from state 1 to
state 2 by following the path A, and returns from state 2 to
state 1 by following the path B. So the system undergoes
a cycle.
• First law of path A : 𝑄𝐴 = ∆𝐸𝐴 + 𝑊𝐴 ∴ 𝑄𝐴 − 𝑊𝐴 = ∆𝐸𝐴
For path B : 𝑄𝐵 = ∆𝐸𝐵 + 𝑊𝐵 ∴ 𝑄𝐵 − 𝑊𝐵 = ∆𝐸𝐵
• The processes A and B together constitute a cycle, for which
It yields, ∆𝐸𝐴 = -∆𝐸𝐵 (i)

9. • Similarly, had the system returned from state 2 to state 1 by following the
path C instead of path B, gives,
∆𝐸𝐴 = -∆𝐸𝐶 (ii)
• From (i) & (ii),
∆𝑬𝑩 = ∆𝑬𝑪

10. i.e., for a process, 𝑑𝑄 = 𝑑𝐸 + 𝑑𝑊
For cycles, 𝑑𝑄 = 𝑑𝐸 + 𝑑𝑊, here as 𝑑𝐸 = 0 , and the equation becomes
𝑑𝑄 = 𝑑𝑊.

11. Different forms of internal energy
Total energy (E) stored in the system
• Thus, the total internal energy form is
E = (𝐸𝐾 + 𝐸𝑃)+U
Macroscopic energy (𝐸𝐾 + 𝐸𝑃) Microscopic energy/ intermolecular energy (U)
Kinetic energy Potential energy Translational
and
Rotational
K.E.
Surface
tension
energy
Vibrational
and
magnetic
energy
Electronic
And
electrical
energy
Chemical
and
Nuclear
energy

12. • In the absence of motion and gravity, d(𝐸𝐾 + 𝐸𝑃) = 0, hence, total stored energy will
becomes, E= U.
• U is generally called as internal energy or molecular internal energy.
• Therefore, the first law for a closed system will be, Q = ∆U+W or 𝑑𝑄 = 𝑑𝑈 + 𝑑𝑊
• Internal energy U (unit is in ‘J’)is an extensive property of the system.
• Specific internal energy (u) is U/m with J/kg units.
• Modulations on the first law of TD is as follows:-
𝑄 = ∆𝑈 + 𝑝𝑑𝑉

13. Specific heat at constant volume (𝒄𝑽)
• 𝑐𝑉 of a substance at constant volume : the rate of change of specific internal energy w.r.t.
to temperature when the volume is held constant
𝒄𝑽 =
𝝏𝒖
𝝏𝑻 𝑽
• Since u, T, and v are properties, 𝑐𝑉 is a properties of the s/m.
• For a constant volume process, (∆𝒖)𝑽= 𝑻𝟏
𝑻𝟐
𝒄𝑽. 𝒅𝑻
• The first law may be written for a closed stationary system composed of a unit mass of a
pure substance
Q = ∆𝑼+W or
• For a process in the absence of work other than pdV work,

14. • When the volume is held constant, pdV = 0, then,
• Internal energy change is equal to the heat transferred in a constant volume process
involving no work other than pdV work.
• Heat transferred at constant volume increases the internal energy of the system.
• If the specific heat of a substance is defined in terms of heat transfer, then
For an ideal gas there are no intermolecular forces of
attraction and repulsion, and the internal energy depends
only on temperature. I.e., U= f(T) only.

15. Enthalpy
• Heat capacity of the system.
• It is a property of a system
• It is a summation of internal energy and pressure energy , i.e.,
H = U+pV (unit: kJ)
• Specific enthalpy, h = H/m or h = u+pv (unit: kJ/kg)
• For a closed system,

16. • i.e., heat transferred at constant pressure increases the enthalpy of a system.
• For an ideal gas, pv = RT , the enthalpy becomes
h = u+RT
• since, the internal energy depends only on the temperature (U = f(T)), the enthalpy of An
ideal gas also depends on the temperature only. i.e.,
h = f(T) only.

17. Specific heat at constant pressure(𝒄𝒑)
• The sp. Heat at constant pressure is defined as the rate of change of enthalpy w.r.t. to
temperature when the pressure is held constant.
𝒄𝒑 =
𝝏𝒉
𝝏𝑻 𝒑
its also a system property.

18. Energy of an isolated system
• An isolated system is one in which there is no interaction of the system with the
surroundings.
• For an isolated system, dQ = 0, dW = 0
• The first law gives,
dE = 0 or
E = constant
• The energy of an isolated system is always constant.

19. First law: Energy is neither created nor destroyed, but only gets transformed from one
form to another.
Perpectual Motion Machine – I (PMM – I)
There can be no machine which would continuously supply mechanical work without
some other form of energy disappearing simultaneously. Such a fictitious machine is
called PMM – I.
• Impossible machine
• Converse of PMM- I : there can be no machine which would continuously consume work
without some other form of energy appearing simultaneously.

20. Numerical Problems:-
Eg:1

22. Eg:2

25. Home Work : 1

26. Home Work : 2

27. First law – for an open system/ control volume
Control volume
• For any system and in any process, the first law is where E represents all
forms of energy stored in the system. i.e.,
• For a system is at stationary position and no mass transfer, the change in K.E. and P.E. will
be neglected. So that stored energy is the residual energy stored in the molecular structure
of the substance. i.e., E = U
• When there is mass transfer across the system boundary, the system is called an open
system.
• For a system having a particular mass of substance, and is free to move from place to place,
then the energy equation becomes,
• In an open system instead of concentrating attention upon a certain quantity of fluid, which
constitutes a moving system in flow process, attention is focused upon a certain fixed
region in space called a control volume through which the moving substance flows.
• The surface/ boundary of the control volume is known as the control surface.

28. First law applied to an open s/m

29. • Continuous flow (in and out)process w.r.t. time – thermodynamics properties vary w.r.t.
time.
• One need to convert it to steady flow ( means that the rate of flow of mass and energy
across the control surface are constant )process for the analysis because if the rate of flow
of mass and energy through the control surface change with time, the mass and energy
within the control volume also would change.
• .i.e., at the steady state of a system, any thermodynamic property will have a fixed value
at a particular location, and will not alter with time.
Mass balance:
• By the conservation of mass, if there is no accumulation of mass within the control
volume, the mass flow rate entering must equal the mass flow leaving, or,

30. Energy balance :
• In a flow process, the work transfer may be of two types: external work and flow work.
• The external work refers to all the work transfer across the control surface (i.e., shaft
work WX).
• Flow work refers to the displacement work done by the fluid of mass dm1 at the inlet
section 1 (-p1v1dm1)and that of mass dm2 at the exit section 2 (+p2v2dm2).
• Therefore, the total work transfer is given by

31. • In other words, internal energy of the stored in fluid mass stream is
Here, pV is the pressure energy or fluid displacement work energy or flow work.
• Stored energy per mass, e = E/m
• Or,
• So we can write the rate of energy of incoming fluid to the control volume as
similarly for the fluid energy leaving the control volume is .
• Since there is no accumulation of energy, by the conversation of energy, the total rate of
flow of all energy streams entering the control volume must equal to the total rate of
flow of all energy streams leaving the control volume.
since,

32. • Or, net energy entering into the C.V. = net energy leaving the C.V.
• ……..(A)
-First law of TD of an open system ( Flow energy equation)
• To convert it to the steady state relation, divide the above equation by (dm/dt).
• We get,
…………… (B)
- Steady flow energy equation (SFEE)

33. • Since, h = u+pv , modify the above equation as follows:-
• ……………(C)
• If the change in K.E. and P.E. are negligible, then,
Or,
………… (D) -another form of SFEE.

35. Some examples of steady flow processes
1. Nozzle and diffusor
• A nozzle is a device which increases the K.E. of a fluid at the expense of its pressure
drop,
• Whereas a diffusor increases the pressure of a fluid at the expense of ts K.E.

36. • SFEE of the control surface gives
• Here, change in P.E. is zero [i.e., aprrox. g(Z1-Z2 )= 0] and also no heat and work transfer
across the boundary is involved. (i.e., ).

37. 2. Turbine and compressor
• Turbines and engines give +ve output (+ve devices), whereas compressors and pumps
require power input (-ve devices).

38. • For a turbine which is well insulated, the flow velocities are often small, and the K.E.
terms can be neglected. The SFEE becomes,

39. 3. Heat Exchanger
• A heat exchanger is a device in which heat is transferred from one fluid to another.
• Fig. shows a steam condenser, where cooling water flows inside the steam condenser’s
tubes which cools down the temperature of the steam flows around the tubes and
hence the steam will be condensed.

40. • Here, change in K.E. and P.E. terms are considered small, no external work done and
there is no external heat interaction(energy exchange in the form of heat is confined
only between the two fluids).
• The SFEE gives,

41. 4. Throttling device
• When a fluid flows through a constricted passage, like a partially opened valve, an
orifice , or a porous plug, there is an appreciable drop in pressure, and the flow is said
to be throttled.
• Fig. shows the process of throttling by a partially opened valve on a fluid flowing in an
insulated pipe.

42. • Here, the changes in P.E. are very small and ignored and also no HT and WT terms are
involved.
• Hence the SFEE gives,

43. Variable flow processes
• Many flow processes, such as filling up and evacuating gas cylinders, are not steady.
• Such processes can be analysed by the control volume technique.
• Consider a device through which a fluid is flowing under non-steady state conditions.
The rate at which the mass of fluid within the control volume is accumulated as equal to
the net rate of mass flow across the control surface as given below,
• Where, mV is the mass of fluid within the control volume at any instant.

44. • Over any period of time,