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

Solution A person switches from regular potato chips to fat-free ones. The

weight the person loses in one year is to be determined.

Assumptions Exercising and other eating habits remain the same.

Analysis The person who switches to the fat-free chips consumes 75 fewer

Calories a day. Then the annual reduction in calories consumed becomes

The metabolizable energy content of 1 kg of body fat is 33,100 kJ. There-

fore, assuming the deficit in the calorie intake is made up by burning body

fat, the person who switches to fat-free chips will lose

(about 7.6 pounds) of body fat that year.

mfat

Ereduced

Energy content of fat



27,375 Cal

33,100 kJ>kg

a

4.1868 kJ

1 Cal

b3.46 kg

Ereduced 1 75 Cal>day 21 365 day>year 2 27,375 Cal>year

Work is the energy transferred as a force acts on a system
through a distance. The most common form of mechanical
work is the boundary work, which is the work associated
with the expansion and compression of substances. On a P-V
diagram, the area under the process curve represents the
boundary work for a quasi-equilibrium process. Various
forms of boundary work are expressed as follows:

(1) General

(2) Isobaric process

(P 1 P 2 P 0 constant)

(3) Polytropic process

(PVnconstant)

(4) Isothernal process of an ideal gas

(PVmRT 0 constant)

The first law of thermodynamics is essentially an expression
of the conservation of energy principle, also called the energy
balance. The general energy balances for any systemunder-
going any processcan be expressed as

WbP 1 V 1 ln¬

V 2

V 1

mRT 0 ln¬

V 2

V 1

Wb

P 2 V 2 
P 1 V 1

1 
n

¬ 1 n 12

WbP 01 V 2 
V 12

Wb

2

1

¬P^ dV

200 | Thermodynamics

Net energy transfer Change in internal, kinetic,

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

It can also be expressed in the rate formas

Rate of net energy transfer Rate of change in internal,

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

Taking heat transfer tothe system and work done bythe

system to be positive quantities, the energy balance for a

closed system can also be expressed as

where

For a constant-pressure process, WbU H. Thus,

Q
Wother¢H¢KE¢PE¬¬ 1 kJ 2

¢PEmg 1 z 2 
z 12

¢KE^12 m 1 V^22 
V^212

¢Um 1 u 2 
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WWotherWb

Q
W¢U¢KE¢PE¬¬ 1 kJ 2

E

.

in
E

.

out¬^ ¬^ dEsystem>dt¬¬^1 kW^2

Ein
Eout¬

¬

¢Esystem¬¬ 1 kJ 2

SUMMARY

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