physics 102 part ii thermal physics moza m. al-rabban professor of physics [email protected] fluids
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Physics 102 Part II
Thermal Physics
Physics 102 Part II
Thermal Physics
Moza M. Al-Rabban
Professor of Physics
FluidsFluids
OutlineOutline15-1 Density15-2 Pressure15-3 Static Equilibrium in Fluids: Pressure and
Depth15-4 Archimedes' Principle and Buoyancy15-5 Applications of Archimedes' Principle15-6 Fluid Flow and Continuity 15-7 Bernoulli's Equation15-8 Applications of Bernoulli's Equation*15-9 Viscosity and Surface Tension
States Of Matter
• Solids
• Liquids
• gases
Solids:Solids:
Solids tends to hold their shapes.
Although the atoms vibrate around fixed equilibrium positions, they do not have enough energy to break the bound with their neighbors.
FluidsLiquids and Gases
• Liquids and Gases, do not hold their shapes.
• A liquid flows and takes the shape of its container and a gas expands to fill its container.
• The atoms or molecules in Fluid do not have fixed positions, so a fluid does not have a definite shape.
DensityDensity
• Definition of Density,
3/:
/
mkgunitSI
VM
The denser a material, the more mass it has in any given volume.
Example 1:
Find the pressure exerted on the skin of a balloon if you press with a force of 2.1 N using (a) your finger or (b) a needle. Assume the area of your fingertip is , and the area of the needle tip is (c) Find the minimum force necessary to pop the balloon with the needle, given that the balloon pops with a pressure of
24100.1 m27105.2 m
25 /100.3 mN
Solve
(a) Calculate the pressure exerted by the finger:
24
24/101.2
100.1
1.2mN
m
N
A
FP
2627
/104.8105.2
1.2mN
m
N
A
FP
(b) Calculate the pressure exerted by the needle:
(c)
NmmNPAF 075.0105.2/100.3 2725
Atmospheric Pressure and Atmospheric Pressure and Gauge PressureGauge Pressure
• Atmospheric Pressure, Pat
ata
aat
a
at
PPbar
kPP
mNpascalP
mNunitSI
mNP
1101
101
/1)(1
/:
/1001.1
5
2
2
25
Gauge pressure, Pg
• The pressure in a fluid acts equally in all directions, and acts at right angles to any surface.
• In many cases we are interested in difference between a given pressure and atmospheric pressure.
atg PPP
Gauge Pressure Does the flat tire on your automobile have zero air pressure? If it is completely flat, it still has the atmospheric pressure air in it. To be sure, it has zero useful pressure in it, and your tire gauge would read zero Newton per square meter. Most gauges read the excess
of pressure over atmospheric pressure and this excess is called "gauge pressure ."
When a system is at atmospheric pressure like the left image above, the gauge pressure is said to be zero. In this image, the system has been opened so that it is at equilibrium with the atmosphere. In the right image, the system has been closed and the plunger
pushed down until the pressure reads about 15 lb/in2 .
atabsulutegauge PPP
Static Equilibrium in Fluids:Pressure and Depth
ghPP
ghPA
ghAAP
A
FP
ghAAPWFF
ghAVgMgW
hAVVM
APF
at
atatbottom
bottom
attopbottom
attop
,
This expression holds for any liquid with constant density and a pressure Pat at its upper surface.
The relation can be applied to any two points in a fluid.
ghPP at
Dependence of Pressure on depth
ghPP 12
Example3:
A cubical box 20.00 cm on a side is completely immersed in a fluid. At the top of the box the pressure is 105.0 kPa; at the bottom the pressure is 106.8 kPa. What is the
density of the fluid?
Example3:
A cubical box 20.00 cm on a side is completely immersed in a fluid. At the top of the box the pressure is 105.0 kPa; at the bottom the pressure is 106.8 kPa. What is the density of the fluid?
Solve:
3
2
55
12
12
/9202000.0/81.9
10050.110068.1mkg
msm
PaPa
gh
PP
ghPP
A Simple Barometer
An interesting application of the variation of pressure with depth is the barometer, which can be used to measure atmospheric pressure.
The pressure in the tube at depth h below the vacuum is 0 + gh = gh
At the level of fluid in the bowel we know that the pressure is one atmosphere, therefore, Pat = gh
A fluid that is often used in such a barometer is mercury (Hg), with a density of
34 /103595.1 mkg
The corresponding height for a column of mercury is
mmHgPatmosphere
mmsmmkg
Pa
g
Ph
at
at
7601
760/81.9/103595.1
10013.1234
5
Fluid Seeks Its Own LevelFluid Seeks Its Own Level
(a)When the levels are equal, the pressure is the same at the base of each arm of the U tube. As a result, the fluid in the horizontal section of the U is in equilibrium.
(b) With unequal heights, the pressure are different. In this case, the pressure is greater at the base of the right arm, hence fluid will flow toward the left and the level will equalize.
Example 4: Oil and Water Don’t Mix
A U-shaped tube is filled mostly with water, but a small amount of vegetable oil has been added to one side, as shown in the sketch. The density of the water is
, and the density of the oil is
If the depth of the oil is 5.00 cm, what is the difference in level h between the top of the oil on one side of the U and the top of the water on the other side?
33 /1000.1 mkg 32 /1020.9 mkg
Solution
1 .Find the pressure at point A, where the depth of the water is h1
1ghPP wateratA
2 .Find the pressure at point B, where the depth of the water is h2 = 5.00 cm
2ghPP oilatB
3 .Set PA equal PB :21 ghPghP oilatwaterat
Example 4: Oil and Water Don’t Mix
A U-shaped tube is filled mostly with water, but a small amount of vegetable oil has been added to one side, as shown in the sketch. The density of the water is
, and the density of the oil is
If the depth of the oil is 5.00 cm, what is the difference in level h between the top of the oil on one side of the U and the top of the water on the other side?
4. Solve for the depth of the water,h1
cmcmcmhhh
cmmkg
mkgcm
hhwater
oil
40.060.400.5
60.4/1000.1
/1020.900.5
12
33
32
21
Pascal’s PrincipalPascal’s Principal
11 / AFP
11
212112
22
/ FA
AFAAFF
APF
An external pressure applied to an enclosed fluid is transmitted unchanged to every point within the fluid.
An external pressure applied to an enclosed fluid is transmitted unchanged to every point within thefluid.
A hydraulic lift
2
112
2211
A
Add
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