Transcript Chapter 20 Gases

Chapter 20
Gases
• Gases are similar to liquids in that both flow and are both
considered fluids.
• The primary difference between liquids and gases is the
distance between molecules. In liquids, molecules are close
together, in gases they are spread apart.
• If two molecules of a gas colide, if one gains speed in the
collision, the other loses speed, but their total kinetic energy
is unchanged
• Gases expand to fill all the space available to it and thus
takes the shape of its container
• The earth’s atmosphere can be though of as an ocean of
air, with the greatest air pressure near the surface and
decreasing air pressure as altitude increases
• Air pressure at sea level:
101 kPa
760 mm Hg or Torr
14.691 psi
1013.25 millibars
• 50% of the earth’s
atmosphere is under 18,000
feet altitude
• 75% of the earth’s
atmosphere is under 56,000
feet altitude
The SI unit for pressure is the pascal.
1 pascal (Pa) = 1 N/m2
Air pressure at sea level is 101,325 Pa or
101.3 kPa
Pascal
(Pa)
Bar
(bar)
Technical
atmosphere Atmosphere Torr
(at)
(atm)
(Torr)
Pound-force
per
square inch
(psi)
1 Pa
≡ 1 N/m2
10−5
1.0197×10−5
9.8692×10−6
7.5006×10−3
145.04×10−6
1 bar
100,000
≡
106 dyn/cm2
1.0197
0.98692
750.06
14.5037744
1 at
98,066.5
0.980665
≡ 1 kgf/cm2
0.96784
735.56
14.223
1 atm
101,325
1.01325
1.0332
≡ 1 atm
760
14.696
1 torr
133.322
1.3332×10−3
1.3595×10−3
1.3158×10−3
≡ 1 Torr;
≈ 1 mmHg
19.337×10−3
1 psi
6.894×103
68.948×10−3
70.307×10−3
68.046×10−3
51.715
≡ 1 lbf/in2
• The barometer is used for measuring air pressure.
• A simple mercury barometer can be a glass tube longer
than 76 centimeters closed at one end, tipped upside
down in a dish of mercury
Boyle’s Law
P1V1 = P2V2
If the temperature of a gas is constant, when the pressure of a gas
increases, the volume that it contains decreases
Practice Problems:
1. If you squeeze a balloon to one-third its volume, by how
much will the pressure inside increase?
Three times
1. A piston in an airtight pump is withdrawn so that the volume
of the air chamber is increased five times. What is the
change in pressure?
Decreases to 1/5th
1. A scuba diver 10.3 meters deep breathes compressed air.
If she holds her breath while returning to the surface, by
how much does the volume of her lungs tend to increase?
Twice the volume
Charle’s Law
V1
V2
T1 = T2
At constant pressure, the volume of a given mass of an ideal gas
increases or decreases by the same factor as its temperature on
the absolute temperature scale
Buoyancy
An object surrounded by air is buoyed up by a force equal
to the weight of the air displaced
For example, a cubic meter of air at ordinary atmospheric
pressure and room temperature has a mass of about 1.2
kg, so its weight is about 12 N. If the mass of the 1 cubic
meter object is greater than 1.2 kg, it will fall to the
ground. If it is less than 1.2 kg, it will rise in the air.
Gay-Lussac’s Law
P1
P2
T1 = T2
The pressure of a gas of fixed mass and fixed volume is directly
proportional to the gas's absolute temperature.
Combined Gas Law
From our three equations:
P1V1 = P2V2
V1 = V2
T1
T2
P1 = P2
T1
T2
We get the following relationship:
PV = k
T
(where k is a constant)
This relationship can also be expressed in the Ideal Gas Law which states:
PV = nRT
P = Pressure (atmospheres)
V = Volume
(Liters)
n = moles of gas
R = gas constant (.082 L * atm K-1 mol-1)
T = Temperature (Kelvin degrees)
Bernoulli’s Principle:
When the speed of a fluid increases, the pressure drops.
Low speed
High Pressure
High Speed
Low Pressure
Air Flow through a pipe
Bernoulli’s Principle Provides the lift for an airplane’s wing: