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

between the total energy entering and the total energy leaving the system

during that process.That is,

or

This relation is often referred to as the energy balanceand is applicable to

any kind of system undergoing any kind of process. The successful use of

this relation to solve engineering problems depends on understanding the

various forms of energy and recognizing the forms of energy transfer.

Energy Change of a System, Esystem

The determination of the energy change of a system during a process

involves the evaluation of the energy of the system at the beginning and at

the end of the process, and taking their difference. That is,

or

(2–32)

Note that energy is a property, and the value of a property does not change

unless the state of the system changes. Therefore, the energy change of a

system is zero if the state of the system does not change during the process.

Also, energy can exist in numerous forms such as internal (sensible, latent,

chemical, and nuclear), kinetic, potential, electric, and magnetic, and their

sum constitutes the total energy Eof a system. In the absence of electric,

magnetic, and surface tension effects (i.e., for simple compressible sys-

tems), the change in the total energy of a system during a process is the sum

of the changes in its internal, kinetic, and potential energies and can be

expressed as

(2–33)

where

When the initial and final states are specified, the values of the specific

internal energies u 1 and u 2 can be determined directly from the property

tables or thermodynamic property relations.

Most systems encountered in practice are stationary, that is, they do not

involve any changes in their velocity or elevation during a process (Fig.

2–44). Thus, for stationary systems,the changes in kinetic and potential

energies are zero (that is,KE 
PE 
0), and the total energy change

relation in Eq. 2–33 reduces to E
Ufor such systems. Also, the energy

¢PE
mg 1 z 2 z 12

¢KE
^12 m 1 V^22 V 122

¢U
m 1 u 2 u 12

¢E
¢U¢KE¢PE

¢Esystem
EfinalEinitial
E 2 E 1

Energy change
Energy at final stateEnergy at initial state

EinEout
¢Esystem

a

Total energy

entering the system

ba

Total energy

leaving the system

b
a

Change in the total

energy of the system

b

72 | Thermodynamics

Wsh, in = 8 kJ

(Adiabatic)

∆E = 8 kJ

FIGURE 2–41

The work (shaft) done on an adiabatic

system is equal to the increase in the

energy of the system.

Wb,in = 10 kJ

(Adiabatic)

∆E = 10 kJ

FIGURE 2–42

The work (boundary) done on an

adiabatic system is equal to the

increase in the energy of the system.

Qout = 3 kJ

Qin = 15 kJ

∆E = (15 – 3) + 6

= 18 kJ

Wsh, in = 6 kJ

FIGURE 2–43

The energy change of a system during

a process is equal to the network and

heat transfer between the system and

its surroundings.