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1
lesson 2
I. Concepts and Definitions
F. Properties of a system (we use them to calculate changes in energy)
1. A property is a characteristic of a system that can be given a
numerical value without considering the history of the system.
Examples include T, P,
ρ
, velocity, E, U, volume (we can only
measure T, P, velocity, mass, volume)
2. Examples and definitions
a. pressure, P
b. density,
ρ
2
5
()
1
1
1
101325
1.01325
14.696
11
0
force exerted by fluid
P
unit area
N
P
pascal
Pa
m
atm
Pa
bar
psia
bar
Pa
=
==
=
=
=
3
m
kg
,
V
m
volume
mass
=
=
ρ
(14)
lesson 2
I. Concepts and Definitions
c.
atmospheric pressure (absolute) = P
atm
d.
gage pressure
= P
gage
= P
abs
P
atm
(115)
e.
absolute pressure
= P
abs
= P
atm
+ P
gage
f.
vacuum pressure = P
vac
= P
atm
abs
(116)
g.
specific volume = v = 1/
ρ
= Vol/mass, m
3
/kg
h.
intensive
properties are independent of the
size of the system:
T, e, P,
ρ
, specific volume
(v), u.
The units on e are kJ/kg
.
i.
extensive
properties depend on the size of the
system:
actual volume (V), E, m, U.
The units
on E are kJ
.
Example
.
The pressure gage attached to the air storage tank
of a compressor reads 100 psi.
If the atmospheric
pressure of the surroundings is 14.7 psi, what is the
absolute pressure in the tank? (Answer:
114.7 psia)
p
Stiff
metal tube
Bourdon tube
gage gives P
gage.
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lesson 2
I.
Concepts and Definitions
Example
. You are designing a snorkel that will allow you to remain
submerged in water at a depth of 1 m.
Discuss any problems you might
experience in using this device.
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 Summer '08
 thermo

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