Gases

Chapter 20

Gases

• Gases and liquids are both fluids – they are substances that flow.

• Because of this, the behavior of gases is very similar to the behavior of liquids in many regards.

• The biggest differences are:1. Gases are compressible2. Gases expand to fill their container

completely.

Earth’s atmosphere

• The density and pressure of the atmosphere both increase with depth.

Atmospheric Pressure

• The weight of miles and miles of air is pushing down on you from above at all times.

• About 14.7 psi at sea level.

• In metric units, this is about 100,000 Newtons per square meter.

• We call this “1 atm”.

Barometer

• We measure air pressure with a device called a barometer.

• Barometers detect changes in atmospheric pressure due to changing weather.

• Barometers also read differently at different altitudes.

Barometer

• A barometer basically shows a tube full of mercury fighting against air pressure.

• The higher the air pressure, the higher into the tube the mercury is pushed.

• As air pressure lowers (e.g. in a storm), the mercury is able to push its way down (due to gravity).

Barometer

• Why 76 cm?• Remember from chapter 19, P = density x g x depth.• The density of mercury is 13.6 times water – that is,

13,600 kg/m3. • So 76 cm of mercury causes a pressure of

P = (13,600) x (10) x (0.76 meters)

P = 103,360 Pascals

P = 100,000 Pascals (rounded off)

A few slides back, we said that the pressure of the atmosphere is about 100,000 N/m2, or 100,000 Pascals.

Barometer

• The 100,000 Pa of air pressure pushes up.

• The 100,000 Pa of mercury pushes down.

• They equal out at 76 centimeters of mercury and 1 atm.

The Physics of Drinking Straws

Drinking Straws

• Drinking straws work just like barometers.

• Draw out the air from the column, let atmospheric pressure push the fluid up.

• The straw will not work if the glass is sealed from the atmosphere – try it!

Drinking Straws

• How tall could a drinking straw be?

P = density x g x depth.

100,000 Pa = (1000 kg/m3) x (10) x (depth)

10 meters = depth

Drinking Straws

• If a drinking straw were longer than 10 meters, it would not work, because the atmospheric pressure would not be strong enough to push the liquid up.

So how does a table-top barometer work?

So how does a table-top barometer work?

• Remember, a barometer is supposed to have no air inside. A table-top barometer has air inside, adding to the pressure, so it can’t measure pressure. Accurately.

• All it does is tell you whether pressure is going up, or going down, which can sometimes help predict the weather.

• Or you could just go to Weather.com!

Chapter 20

• 20.4, the Aneroid Barometer– Aneroid means ‘without liquid’– Has a dial indicator– Concept behind the altimeter

• 20.5 Boyle’s Law– P1V1 = P2V2– P1V1/T1 = P2V2/T2– Example: Tires on your car

Remember! Buoyancy

• When an object is placed in water, it weighs less than it did in air. This is due to an upward force exerted on the object by the water.

• The upward force is called the buoyant force.

• If the buoyant force on an object is greater than the weight, the object will float in water.

Remember! Buoyancy

• Air provides buoyancy, too!• An object weighs less in air than it would if

Earth had no atmosphere. This is due to an upward force exerted on the object by the air!

• The upward force is still called the buoyant force, even though we are talking about air.

• If the buoyant force on an object is greater than the weight, the object will float upwards!

Remember! Buoyancy

• If the buoyant force on an object is greater than the weight, the object will float upwards!

• For objects floating in the air instead of in the water, buoyant force is still due to thepressure differencebetween top and bottom of the object.

Buoyancy

• Is there a buoyant force acting on you right now?• Yes, the air is buoying you upwards at all times!

• When you see a helium balloon rising through the air, it is because the buoyant force on that balloon is greater than the weight of the helium balloon.

• The density of the helium balloon is less than the density of the air.

Bernoulli’s Principle

• Daniel Bernoulli published works in 1738 regarding the physics of fluids.

• The difference between this section and our previous studies is that Bernoulli was concerned with moving fluids, not stationary ones.

Bernoulli’s Principle

• Bernoulli’s Principle states that when the speed of a fluid increases, the pressure drops.

• This also means that a fluid’s pressure is greatest when it is stationary.

• This can be seen very easily by feeling the pressure in a hose when water is flowing through it, compared to when the hose is kinked or blocked.

Bernoulli’s Principle

• Bernoulli’s Principle works because of conservation of energy.

• Bernoulli’s Principle only works with steady flow – not turbulent flow.

• If the streamlines are smooth, this principle works.

Bernoulli’s Principle

• If the fluid has eddies or swirls, Bernoulli doesn’t work.

Bernoulli’s Principle

• This is how the physics of an airfoil or airplane wing is often explained. The air must go over the top faster than under the wing, creating a low pressure area above – and an upward force on the wing called lift.

Bernoulli’s Principle

• Moving air over the top of the roof is at a lower pressure than the stationary air under the roof.

• When the wind blows the roof off of a building, this is how it occurs – it is actually blown off by higher pressure air inside the building.

Bernoulli’s Principle

• This is also why a tornado blows the windows out of a building, not in.

Bernoulli’s Principle

Bernoulli’s Principle

Homework

• Read ch 20 and do the following review questions:

# 1,2,3,7,8,9,10, and 13-18 on pages 296 and 297.

• Due Thursday, 3/23