Gas Pressure

Let’s start with straws….

• Both the red and blue arrows represent the same thing—the pressure we feel from the atmosphere

• What do you notice about the level (height) of the liquid IN the straw?

Equal to the height of the liquid OUTSIDE the straw

• Why doesn’t the liquid IN the straw move up higher or down lower?

Pressure INSIDE the straw is equal to the pressure OUTSIDE the straw (same atmosphere is pressure down)

Then you decide to take a drink..What will happen to our arrows we have used to depict pressure?

• When you drink from the straw, you REDUCE the air pressure in the straw

Suddenly, the forces are unbalanced

Now that we have unbalanced forces acting outside and inside the straw…

• As a result of the unbalanced forces, nature searches for equilibrium (to make the forces equal again)

Consequently, the liquid OUTSIDE of the straw is pushed down while the liquid INSIDE the straw is pushed up

When you drink from a straw, you are actually allowing liquid to be pushed UP the straw by the atmosphere

So how do we define pressure?• Not just atmospheric pressure, but ANY pressure

• Let’s start by asking a simple question: Would you rather have a woman step on you with her heels or her regular shoes? Why? What variables affect your decision?

Far less area in the heels

So the 2 variables we decided affect pressure are: Force & Area

• Scientists give a formal definition of pressure as: force per unit of area

𝑃=𝐹𝐴

Force• Usually measured in either

lbs or newtons

Area• Usually measured in in2

or m2

Because these are the units we tend to use for force & area, we have developed different names for the combination of the units

• Pascal (N/m2)

• PSI (Pounds per Square Inch): BTW, our atmospheric pressure @ sea level is 14.7 psi

How much force can our atmosphere really contribute?

http://www.youtube.com/watch?v=2WJVHtF8GwI

Torricelli, Manometers, & Pressure-Measuring Devices

• Evangelista Torricelli (1608-1647) is generally given most of the credit for the invention of the barometer

Device used to measure air pressure

Height of mercury= 760 mm = 29.92 in.

• It is worth noting that Torricelli originally used water for his barometer, unfortunately, atmospheric pressure shot the water up 34 feet!

• This is why Torricelli used mercury (Hg)—it’s much more dense (13.6 g/mL)

1 standard pressure (SP) =760 mm Hg = 101.3 kPa

Manometers• Assist in measuring the pressure of a gas by comparing it to

the pressure of its surrounding environment

• The difference in height of the mercury is the key to solving the pressure of the gas

How can we calculate pressure of gas using manometer?

Pgas = Ph1 Pgas = Ph2 + Patm Pgas= Patm – Ph3

Brief Overview of Conversions Between Different Units of Pressure

1 atm = 760 mm Hg = 101.3 kPa = 14.7 psi

1.) Convert 320 mm Hg to atm

2.) Convert 30.0 psi to mm Hg