Transcript Unit 4 Sections A.4-A.6

Unit 4
Sections A.4-A.6
In which you will learn about:
•Definition of pressure
•Units of pressure
•Applications of pressure
•Atmospheric pressure
1
A.4 Pressure
• In everyday language, the word pressure can
have many meanings
– Feeling too busy
– Feeling forced to behave a certain way
• The greater the pressure, the more “boxed in”
you feel
– To scientists, pressure also refers to force and
space, but not in the same way!
2
• Pressure is the force applied divided by the
surface area upon which the force is applied
• Pressure = force/area
– Pressure is directly proportional to the force
applied
– Pressure is inversely proportional to the area upon
which the force is applied
P
P
F
A
3
• Think back to a common experience: someone
steps on your foot
– The pressure (or maybe pain) you feel depends on
two variables: force and area
– You can feel the difference between a large person
and a smaller person (force)
– If same weight (force) is applied, then shoe size
will give different effects (area)
4
• In the same way someone exerts pressure if
they step on your foot, gas molecules exert
pressure on the walls of their container
• Gas pressure = # of collisions between gas
particles and container wall
5
Units of Pressure
• Pressure has several different units
– millimeters of mercury (mm Hg)
– torr
– atmospheres (atm)
– pascals (Pa)
– pounds per square inch (psi)
6
SI Base Units & Derived Units
• Remember that the SI system creates
standards for measurements and there are
seven base units for quantities of length,
mass, temperature, luminosity, amount of
substance, electric current, and time.
• Any units that are a combination of SI base
units are considered derived units
7
• Revisiting all of the pressure units:
– mm Hg = SI base units of length (refers to
barometers)
– torr are based on mm Hg (Torricelli was the
scientist that invented barometers)
– Atmospheres are based on Pascals
– pascals are N/m2 where N= newton which is a unit
of force used by physicists
• 1 N = 1 Pa = 1 kg∙m/s2 (note that these are all SI base
units)
– psi is the English (non-metric) way to measure
things that we see in the U.S. (think of tires)
8
Which units should we use?
• Well, unfortunately, we use all of them!
– SI unit is the pascal or kilopascal
– Atmospheres are the easiest numbers to work with
– mm Hg come from barometer readings (and so do
torr)
– psi is used in the U.S.
• To convert from one unit to the other, use the
following equivalents for standard pressure at sea
level:
• 101,325 Pa = 101.325 kPa = 1.00 atm = 760 mm
Hg = 760 torr = 14.7 psi
9
A.5 Applications of Pressure
• Please answer the following questions as part
of your homework:
1) Two bricks of the same mass are lying on the
ground. One is standing upright, and the
other is lying flat.
a) Is each brick exerting the same total force upon
the ground? Explain.
b) Is each brick exerting the same pressure upon
the ground? Explain.
10
2) Calculate the pressure, expressed in pascals,
that the upright brick exerts upon the ground.
It is exerting a force of 18 N, and the
dimensions of the brick surface touching the
ground are 9.3 cm x 5.5 cm.
3) You need to chop some wood to build a fire.
You quite obviously reach for an axe rather
than a hammer to complete this task. Explain
your choice in terms of the concept of
pressure.
11
A.6 Atmospheric Pressure
• The unit mm Hg (or sometimes inches of Hg, used
by weather forecasters) suggest that pressure is
measured from the height of a column of
mercury
• How is this possible?
– Recall that in Activity 7, you covered a test tube filled
with water and inverted it into a container of water.
– You then uncovered the test tube and the air pressure
forces more water into the test tube so the water level
doesn’t change
– Now imagine the tube is taller – eventually it gets to a
point where the water level doesn’t reach the top
12
• The puzzling effect of the water level not
changing until the tube is large enough was first
observed by scientists in the mid-1600s.
• Scientists discovered that 1 atm of pressure can
only support a column of water 10.3 m (33.9 ft)
high, even if the column was taller.
• A barometer (a device that measures
atmospheric pressure) based on a tube filled with
water would be much too tall to be useful.
– Scientists replaced water with mercury, a liquid 13.6x
more dense than water.
– The resulting column is 13.6 times shorter.
– Thus, at 1 atm of pressure, the mercury column has a
height of 760 mm; 1 atm = 760 mm Hg
13
HOMEWORK QUESTIONS
4) U.S. weather reports generally express air pressure in
units of inches of mercury. During a severe storm, the
barometric pressure can drop to as low as 27.2 inches
of mercury. Convert this air-pressure value to
A. mm Hg (HINT: 1 inch = 25.4 mm)
B. atmospheres (atm)
C. kilopascals (kPa)
5) Heavy vehicles that must move easily over loose sand
are often equipped with special tires.
A. Would you expect these tires to be wide or narrow?
B. Explain your answer using the concept of pressure.
14