Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

252 | Thermodynamics

Thus,

and

Therefore, after 30 min there is 0.6 kg water (liquid vapor) left in the

pressure cooker.

Discussion Note that almost half of the water in the pressure cooker has

evaporated during cooking.

m 2 

V

v 2



0.006 m^3

0.01 m^3 /kg

0.6 kg

v 2 0.001 1 0.009 21 1.004
0.001 2 m^3 /kg0.010 m^3 /kg

One of the most fundamental laws in nature is the first law of thermody-

namics,also known as the conservation of energy principle,which pro-

vides a sound basis for studying the relationships among the various forms

of energy and energy interactions. It states that energy can be neither created

nor destroyed during a process; it can only change forms.

The energy content of a fixed quantity of mass (a closed system) can be

changed by two mechanisms:heat transfer Qand work transfer W. Then the

conservation of energy for a fixed quantity of mass can be expressed in rate

form as

(5–47)

where Q

.

Q

.

net,inQ

.

in
Q

.

outis the net rate of heat transfer to the system

(negative, if from the system),W

.

W

.

net,outW

.

out
W

.

inis the net power

output from the system in all forms (negative, if power input) and dEsys/dtis

the rate of change of the total energy content of the system. The overdot

stands for time rate. For simple compressible systems, total energy consists

of internal, kinetic, and potential energies, and it is expressed on a unit-mass

basis as

(5–48)

Note that total energy is a property, and its value does not change unless the

state of the system changes.

An energy interaction is heatif its driving force is a temperature differ-

ence, and it is workif it is associated with a force acting through a distance,

as explained in Chap. 2. A system may involve numerous forms of work, and

the total work can be expressed as

(5–49)

where Wshaftis the work transmitted by a rotating shaft,Wpressureis the work

done by the pressure forces on the control surface,Wviscousis the work done

WtotalWshaftWpressureWviscousWother

eukepeu

V^2

2

gz

Q

#


W

#



dEsys

dt

¬or¬Q

#


W

#



d

dt

(^)

sys
re¬dV
TOPIC OF SPECIAL INTEREST General Energy Equation
This section can be skipped without a loss in continuity.