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

78 | Thermodynamics

FIGURE 2–52

The definition of performance is not
limited to thermodynamics only.

© Reprinted with special permission of King
Features Syndicate.

the bottom of the bowl, and moves up toward point Con the opposite side.

In the ideal case of frictionless motion, the ball will oscillate between points

Aand C. The actual motion involves the conversion of the kinetic and poten-

tial energies of the ball to each other, together with overcoming resistance to

motion due to friction (doing frictional work). The general energy balance for

any system undergoing any process is

Net energy transfer Change in internal, kinetic,

by heat, work, and mass potential, etc., energies

Then the energy balance for the ball for a process from point 1 to point 2

becomes

or

since there is no energy transfer by heat or mass and no change in the internal

energy of the ball (the heat generated by frictional heating is dissipated to the

surrounding air). The frictional work term wfrictionis often expressed as elossto

represent the loss (conversion) of mechanical energy into thermal energy.

For the idealized case of frictionless motion, the last relation reduces to

where the value of the constant is C 
gh. That is, when the frictional

effects are negligible, the sum of the kinetic and potential energies of the

ball remains constant.

Discussion This is certainly a more intuitive and convenient form of the

conservation of energy equation for this and other similar processes such as

the swinging motion of the pendulum of a wall clock.

2–7 ■ ENERGY CONVERSION EFFICIENCIES

Efficiencyis one of the most frequently used terms in thermodynamics, and

it indicates how well an energy conversion or transfer process is accom-

plished. Efficiency is also one of the most frequently misused terms in ther-

modynamics and a source of misunderstandings. This is because efficiency

is often used without being properly defined first. Next we will clarify this

further, and define some efficiencies commonly used in practice.

Performance or efficiency, in general, can be expressed in terms of the

desired output and the required input as (Fig. 2–52)

(2–41)

If you are shopping for a water heater, a knowledgeable salesperson will

tell you that the efficiency of a conventional electric water heater is about

90 percent (Fig. 2–53). You may find this confusing, since the heating ele-

ments of electric water heaters are resistance heaters, and the efficiency of

Performance

Desired output

Required output

V^21

2

gz 1 

V^22

2

gz 2 ¬or¬

V^2

2

gz
C
constant

V 12

2

gz 1 

V^22

2

gz 2 wfriction

wfriction
 1 ke 2 pe 22  1 ke 1 pe 12

EinEout¬


¬

⎭⎪⎬⎪⎫^ ¢⎭E⎪⎬system⎪⎫

SEE TUTORIAL CH. 2, SEC. 7 ON THE DVD.

INTERACTIVE

TUTORIAL