Warmup 11/20/15
Describe a gas. What's it actually made of? How does it
work?
Objective
Tonight’s Homework
To learn how we model a gas
physically
pp 382: 5, 6, 7
Notes on Kinetic Theory
Molecules in Motion
Imagine we could look at a sample of a gas up
close. On the molecular level.
If solids are just molecules bound together, then
what’s a gas?
Notes on Kinetic Theory
Molecules in Motion
Imagine we could look at a sample of a gas up
close. On the molecular level.
If solids are just molecules bound together, then
what’s a gas?
A gas is just molecules
bouncing around a container
at high speeds. These
molecules bounce off the
walls and off each other. How
fast are we talking here? Try 1,000 mph with 4
billion collisions every molecule every second.
Notes on Kinetic Theory
Pressure
As each molecule hits the edge of the container,
it pushes on it a little. This push isn’t much, but
with quadrillions of molecules it adds up.
Notes on Kinetic Theory
Pressure
As each molecule hits the edge of the container,
it pushes on it a little. This push isn’t much, but
with quadrillions of molecules it adds up.
We measure this pressure in terms of force per
square area. One pascal is a pressure of 1
Newton spread out over 1 square meter.
1 Newton is a small force and 1 square meter
quite big, so 1 pascal is quite small.
Notes on Kinetic Theory
Air exerts quite a bit of pressure. Standard
atmospheric pressure at sea level is 101.325
kilopascals.
This is equivalent to 14.7 pounds per square
inch.
That means every inch of your body is feeling
almost 15 pounds of pressure pushing on it just
from the air!
Notes on Kinetic Theory
Measuring Pressure
So how did we get that number? Measuring
pressure can be quite difficult. Essentially, all
the devices work like this:
Notes on Kinetic Theory
In an open arm manometer, the pressure of the
gas we want is balanced against the pressure of
the air outside.
We have liquid mercury in
the middle. Whichever gas
has more pressure will push
the mercury toward the
other side.
The pressure of the gas we
want is the pressure of the
outside air plus the height difference in the
tube.
Notes on Kinetic Theory
In a closed arm manometer, the end that was
open to the air is now sealed and emptied. If
both sides are empty, the mercury will be level.
When pressure is added to the chamber, it
pushes the mercury up the other end.
The pressure can be
calculated as simply the
height difference in one
end of the mercury from
the other.
Notes on Kinetic Theory
So what’s standard in terms of height
difference?
We said standard air pressure is 101.325 kPa.
Using a closed arm manometer, this same
measurement comes out to 760 mm of
mercury.
As for a conversion factor…
1kPa = 7.501 mm of Hg
Notes on Kinetic Theory
Example:
“An open end manometer shows a difference of
245 mm of mercury favoring the air end. If the
air pressure nearby is standard, what is the
pressure of the gas in the chamber?”
Notes on Kinetic Theory
Example:
“An open end manometer shows a difference of
245 mm of mercury favoring the air end. If the
air pressure nearby is standard, what is the
pressure of the gas in the chamber?”
We know that the pressure we want is given by:
P0 = Pair + h
Pair = 101.325 kPa
H = 245 mm
245 mm •
1 kPa _ = 32.6 kPa
1
7.501mm
P0 = 101.325 kPa + 32.6 kPa
P0 = 133.925 kPa
Notes on Kinetic Theory
Kinetic Energy and Temperature
If we look at the motion of molecules, there’s
something else we can conclude. Motion is
related to temperature. In fact, temperature can
be defined as motion of molecules.
Notes on Kinetic Theory
Kinetic Energy and Temperature
If we look at the motion of molecules, there’s
something else we can conclude. Motion is
related to temperature. In fact, temperature can
be defined as motion of molecules.
If a group of molecules aren’t moving at all, we
say it has a temperature of absolute zero.
Since you can’t have anything move slower than
not moving at all, this temperature represents
the coldest that something can ever get.
Nothing in the universe can ever be as cold as
absolute zero. (Because of quantum stuff.)
Notes on Kinetic Theory
Neither Fahrenheit or Celsius have clean
numbers for absolute zero.
In Fahrenheit, absolute zero is -459.6°
In Celsius, absolute zero is -273.15°
Notes on Kinetic Theory
Neither Fahrenheit or Celsius have clean
numbers for absolute zero.
In Fahrenheit, absolute zero is -459.6°
In Celsius, absolute zero is -273.15°
Given this, in the 1800s, a
scientist named Lord Kelvin
created a scale based on
absolute zero.
This scale is the same as
Celsius but shifted so that
absolute zero is 0 K.
Notes on Kinetic Theory
Neither Fahrenheit or Celsius have clean
numbers for absolute zero.
In Fahrenheit, absolute zero is -459.6°
In Celsius, absolute zero is -273.15°
Given this, in the 1800s, a
scientist named Lord Kelvin
created a scale based on
absolute zero.
This scale is the same as
Celsius but shifted so that
absolute zero is 0 K.
K = °C + 273.15
Exit Question
What does the pressure of a gas physically represent?
a) How tightly packed the gas molecules are
b) How “solid” the gas molecules are
c) The temperature and density of the gas
d) How fast gas molecules hit against each other
e) How hard and how fast molecules are bouncing against
a surface
f) None of the above