1.7.Pressure

David Raju Vundi

PressurePressure is the amount of force applied to a

known area.We can calculate it using the equation

Pressure = Force (N) / Area (m2)

We can reduce pressure by reducing the size of the

force or increasing the size of the area.

3

Pressure and DensityDensity = Mass/Volume

A property of the material.Pressure = Force/Area

Depends on the height of the fluid.Same in all directions.Units are:

Force/Area = N/m2.Pascals 1 Pa = 1 N/m2.Atmosphere 1 atm = 1.013 X 105 N/m2.

Pressure is due to the net force of the molecules in a fluid colliding with the walls.

A very large number of collisions happen each second.

Each collision exerts a tiny net force on the wall.

Aneroid Barometer

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Barometer– measures atmospheric pressure

vacuum

PatmPHg

barometerbarometer: device to measure air pressure

air pressure

vacuum

mercury (Hg)

Barometer

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 401

Empty space(a vacuum)

Hg

Weight of themercury inthe column

Weight of theatmosphere(atmosphericpressure)

BarometerMercury filled

760 mm = 1 atm

Water filled10400 mm = 1 atm

The barometer measuresair pressure

Water column(34.0 ft. high

or 10.4 m)

Atmosphericpressure

Mercury column(30.0 in. high

or 76 cm)

BarometersMount Everest

Sea level On top of Mount EverestSea level

SAND

100g

I think the bigger

mass will sink

further

I think a bigger area will stop the

mass sinking

Pressure Experiment

Torricelli Barometer

The mercury in the tube pushes down with its weight.

The bottom of the tube is open to the atmosphere.

The air pushes on the open surface of the mercury.

On an average day, the pressure of the air equals the pressure exerted by a column of mercury 760 mm high.

Above 760 mm, there is a vacuum in the tube.

Weight of mercury

Barometer DetailWhy doesn’t the diameter of the

column of Hg make a difference?Recall that Pressure = force/area.The “force” is the weight of the

mercury, but the pressure that results is that weight divided by the area of the column. So … a bigger column weighs more but also has a proportionally bigger area, and the two factors cancel one another out.

The pressure caused by the column of mercury pressing down is independent of the diameter of the column.

Manometer

A manometer is comprised of a bulb containing a gas and a U-shaped tube.

The U-shaped tube is partially filled with mercury. The weight of the mercury puts pressure on the gas.

If the U-tube is OPEN there is also air pressure acting on the gas.

The gas molecules put pressure on the mercury.

PHg

Closed ManometersThere is a balance

between the weight of the mercury on the left (PHg) and the pressure of the gas on the right (Pgas).

The difference between the heights of the mercury on each side of the tube is a measure of the pressure of the gas.

Pgas = h

vacuum

PHg

Open ManometersWhen gas pressure is

greater than atmospheric pressure, the mercury is pushed toward the open end.

The balance is between the gas on the right, and the air plus mercury on the left.

Pair + PHg = Pgas

The weight of the mercury is measured as the height difference:

PHg = h

So Pgas = Pair + h

Pair

PHg

Open ManometersWhen gas pressure is less

than atmospheric pressure, the mercury is pushed toward the gas reservoir.

The balance is between the air on the left and the gas plus mercury on the right:

Pair = Pgas + PHg

The weight of the mercury is measured as the height difference:

PHg = hSo Pair = Pgas + h

Or Pgas = Pair- h

PHgPair

Sample Problems

PAIR = 765 mm

h = 27 mmh = 13 mm

PAIR = 753 mm

h = 20 mm

Pair = 790 mm

Find the pressure of the gas in each manometer.Pay attention to whether the manometer is open or closed!

1. 2. 3.

Sample Problem Answers1. Pgas+ h = Pair

Pgas= 790 mm - 20 mm = 770 mm Hg

2. Pgas = vacuum + 13 mm = 13 mm Hg

3. Pgas = Pair + h

Pgas = 765 + 27 = 792 mm Hg