8/10/2019 Lecture 2 2014

1/82

LAWS OF THERMODYNAMICS:

Thermodynamics rests upon two fundamental laws:

First law

Second law

First law of thermodynamics

- Principle of conservation of energy

- Energy can be neither created nor destroyed

8/10/2019 Lecture 2 2014

2/82

Net heat =J (Net work)

Here, J is a proportionality factor that depends on the units

used for work and heat.

As in SI system The unit of Heat and Work are same, so the

value of J is unity.

W in Wout

Q in

Q out

Basic statement: When any closed system is taken through a cycle, the net

work delivered to the surroundings (/ done on the system ) is proportional to the net heat taken from the surroundings (/

delivered to the surroundings ).

8/10/2019 Lecture 2 2014

3/82

W in Wout

Q in

Q out

inout out inW W QQ

W Q W Q

Net heat =Net work

Aspects of the first law:With the aid of this law it is possible to calculate the amount of

heat & work which cross the boundary of a system when given

changes in properties occur.

8/10/2019 Lecture 2 2014

4/82

1st law of thermodynamics for closed system

(control mass):

------- (1)

Considering cycle 1-A-2-B-1

---------- (2)

Considering cycle 1-C-2-B-1

---------- (3)

(2)-(3)

---------- (4)

Since A & C represent arbitrary processes, betn. states (1) & (2) we conclude that

the quantity ( Q- W ) is the same for all processes betn. states (1) & (2).

W Q

B A B A W W QQ 12211221

BC BC W W QQ 12211221

C AC A W W QQ 2

1

2

1

2

1

2

1

C AW QW Q )()( 2121

8/10/2019 Lecture 2 2014

5/82

8/10/2019 Lecture 2 2014

6/82

By integrating we get,

-----------------(6)

Where, E 1 & E 2: initial & final values of the energy E of the

system: heat transferred to the system during the process from state

1 to state 2

: work done by the system during the process betn. state 1 to

state 2.

121212 W E E Q

12Q

12W

8/10/2019 Lecture 2 2014

7/82

Physical significance of E:

E represents all the energy of the system in the given state such as,

Kinetic Energy, KE

Potential energy, PE

Energy associates with the motion & position of the molecules

Internal energy,U Energy associated with the structure of the atom

Chemical energy and so on

8/10/2019 Lecture 2 2014

8/82

Since the terms comprising E are point function we can write

dE =dU + d(kE) + d(PE)

Eqn. (5) becomes;

Q = dU + d(kE) + d(PE) + W ---------------(7)

PE kE U E

8/10/2019 Lecture 2 2014

9/82

Let a system is initially at rest

The system be acted on by an external horizontal force F thatmoves the system a distance dx in the direction of force

.

SystemState-1

SystemState-2

dx

y

F

x

Here, d(PE) =0 (no change in PE) Q=0 (no heat transfer)dU =0 (no change in internal

energy)

Now from equ (7)

0=0+ d(kE)+ 0+(-Fdx) or, d(kE)=Fdx

8/10/2019 Lecture 2 2014

10/82

From Newtons 2nd law of motion;

ma F

dt dvm

dx

dv

dt

dxm

dxdv

mv

mvdvkE d )(

8/10/2019 Lecture 2 2014

11/82

8/10/2019 Lecture 2 2014

12/82

Substituting the expressions for kE & PE into equ. (7)

Q=dU + mvdv + mgdz + W

Integrating for a change of state from (1) to (2);

---------- (8)

This is the integral from of First Law of Thermodynamics.

In the absence of kE & PE

---------- non-flow energy eqn.

1212

122

2

1212 )()2( W z z mg vv

mU U Q

121212U U W Q

8/10/2019 Lecture 2 2014

13/82

Internal Energy (U):

Internal Energy is an extensive property (depends on the

size/mass of the system).

The symbol is U

u: specific internal energy (internal energy per unit mass).

It is a Intensive property

8/10/2019 Lecture 2 2014

14/82

Enthalpy:

Let us consider a control mass undergoing quasi-equilibriumconstant pressure process, as shown in figure.

8/10/2019 Lecture 2 2014

15/82

8/10/2019 Lecture 2 2014

16/82

Enthalpy:

Thermodynamic property

Extensive property

Independent of process

Combination of energy of the system due to temperature, pressure, and volume

A property of the system that measures its heat content.

8/10/2019 Lecture 2 2014

17/82

Specific heat (heat capacity):

Thermodynamic property

Intensive property

The amount of heat required per unit mass to raise thetemperature by one degree.

T

Q

m

c

1

8/10/2019 Lecture 2 2014

18/82

Specific heat at constant volume:

v

v T Q

mc

1

vvv

v T u

T U

mT Q

mc

11

8/10/2019 Lecture 2 2014

19/82

8/10/2019 Lecture 2 2014

20/82

Rate Equation of First Law:

8/10/2019 Lecture 2 2014

21/82

8/10/2019 Lecture 2 2014

22/82

8/10/2019 Lecture 2 2014

23/82

8/10/2019 Lecture 2 2014

24/82

8/10/2019 Lecture 2 2014

25/82

Steady-State, Steady-Flow Process:

8/10/2019 Lecture 2 2014

26/82

8/10/2019 Lecture 2 2014

27/82

Application to steady flow energy equation:

8/10/2019 Lecture 2 2014

28/82

8/10/2019 Lecture 2 2014

29/82

8/10/2019 Lecture 2 2014

30/82

8/10/2019 Lecture 2 2014

31/82

8/10/2019 Lecture 2 2014

32/82

8/10/2019 Lecture 2 2014

33/82

8/10/2019 Lecture 2 2014

34/82

Vapor-liquid-phase equilibrium in a pure substance:

8/10/2019 Lecture 2 2014

35/82

Vapor-liquid-phase equilibrium in a pure substance:

8/10/2019 Lecture 2 2014

36/82

Vapor-liquid-phase equilibrium in a pure substance:

8/10/2019 Lecture 2 2014

37/82

8/10/2019 Lecture 2 2014

38/82

8/10/2019 Lecture 2 2014

39/82

8/10/2019 Lecture 2 2014

40/82

Property Diagrams for Phase-Change Processes:

FIGURE : T-v diagram of a pure substance

8/10/2019 Lecture 2 2014

41/82

P-v Diagram::

8/10/2019 Lecture 2 2014

42/82

By convention, the subscript f is used to designate a property of

a saturated liquid and the subscript g a property of a saturated

vapor.Thus, a saturation condition involving part liquid and part vapor,

such as that shown in Figure, can be shown on T v coordinates, as

in Figure.

8/10/2019 Lecture 2 2014

43/82

All of the liquid present is at state f with specific volume v f

and all of the vapor present is at state g with v g.

The total volume is the sum of the liquid volume and the

vapor volume, or

The average specific volume of the system v is then

The definition of quality x = mvap /m.

8/10/2019 Lecture 2 2014

44/82

Using the definition

v can also be written as

Now the quality x can be viewed as the fraction ( v v f )/v f g

of the distance between saturated liquid and saturated vapor, as

indicated in Figure:

8/10/2019 Lecture 2 2014

45/82

8/10/2019 Lecture 2 2014

46/82

8/10/2019 Lecture 2 2014

47/82

TABLES OF THERMODYNAMIC PROPERTIES

Tables of thermodynamic properties of many substances

are available, and in general, all these tables have the same

form.

In this section we will refer to the steam tables.

8/10/2019 Lecture 2 2014

48/82

8/10/2019 Lecture 2 2014

49/82

8/10/2019 Lecture 2 2014

50/82

8/10/2019 Lecture 2 2014

51/82

8/10/2019 Lecture 2 2014

52/82

8/10/2019 Lecture 2 2014

53/82

8/10/2019 Lecture 2 2014

54/82

8/10/2019 Lecture 2 2014

55/82

8/10/2019 Lecture 2 2014

56/82

8/10/2019 Lecture 2 2014

57/82

8/10/2019 Lecture 2 2014

58/82

Problem:

8/10/2019 Lecture 2 2014

59/82

Problem:

8/10/2019 Lecture 2 2014

60/82

Problem:

8/10/2019 Lecture 2 2014

61/82

8/10/2019 Lecture 2 2014

62/82

Problem:

8/10/2019 Lecture 2 2014

63/82

8/10/2019 Lecture 2 2014

64/82

8/10/2019 Lecture 2 2014

65/82

8/10/2019 Lecture 2 2014

66/82

8/10/2019 Lecture 2 2014

67/82

8/10/2019 Lecture 2 2014

68/82

8/10/2019 Lecture 2 2014

69/82

8/10/2019 Lecture 2 2014

70/82

Problem:

8/10/2019 Lecture 2 2014

71/82

8/10/2019 Lecture 2 2014

72/82

Problem:

8/10/2019 Lecture 2 2014

73/82

8/10/2019 Lecture 2 2014

74/82

8/10/2019 Lecture 2 2014

75/82

8/10/2019 Lecture 2 2014

76/82

8/10/2019 Lecture 2 2014

77/82

8/10/2019 Lecture 2 2014

78/82

8/10/2019 Lecture 2 2014

79/82

8/10/2019 Lecture 2 2014

80/82

8/10/2019 Lecture 2 2014

81/82

8/10/2019 Lecture 2 2014

82/82