The illustration above shows a shower, a sink, their wastewater pipe, and a “vent” pipe.
Water can flow down the shower drain, through a P-shaped “water trap,” and from there
to a wastewater pipe that connects to a sewer line. The water trap retains a seal of
water that prevents noxious sewer gas from flowing up the waste pipe into the house.
The sink in the illustration is likewise connected through a protective P-trap to the waste
pipe.
In the illustration, the sink drain is “vented” by an air pipe that extends through the roof
of the house. Because it is called a vent pipe, you might think it allows air to leave, but it
actually allows air in. Several such vents can be seen on the roofs of most houses. The
vents work in conjunction with the water traps that protect the home. What purpose
does a vent serve?
Waste pipes are not pressurized like water supply pipes, and they are always at least
slightly inclined so that they ordinarily stand empty. Imagine that someone turns on the
shower, causing a stream of water to flow through the waste pipe. As the flow
increases, the pressure decreases, in accordance with the Bernoulli effect. If the
pressure in the waste pipe were to decrease enough, it could “suck” the water out of the
sink’s P-trap, thwarting its protective function.
This is where the vent comes in: When the pressure decreases at the bottom of the sink’s waste connection due to the shower flow, air flows in
from the exterior (due to atmospheric pressure), restoring the pressure in the connection. The water remains in place in the trap.
Traps are vented
To counter Bernoulli effect in
wastewater pipes13.16 - The Earth’s atmosphere
The atmosphere is the layer of gas, including the oxygen we need in order to survive,
surrounding the Earth and bound to it by gravity. The atmosphere receives a great deal
of press these days due to environmental concerns such as global warming and ozone
holes. Understanding the nature and dynamics of the atmosphere is proving
increasingly important to human life.
In this section we give a brief overview of three topics relating to the Earth’s
atmosphere. The first is the magnitude of atmospheric pressure, and how its existence
was first demonstrated. Next, we discuss briefly why there is no hydrogen in the Earth’s
atmosphere (unlike the atmosphere of Jupiter). Finally, we provide a general sense of
the range of pressures, densities and temperatures of the Earth’s atmosphere at
different altitudes.
The very existence of atmospheric pressure was once a topic of debate. After all, you
do not “feel” air pressure (since the fluid inside your body exerts an equal and opposite
pressure) any more than a fish feels water pressure. (At least no fish has ever told us
that it feels this pressure.) In a famous demonstration that showed the existence of
atmospheric pressure, the German scientist Otto von Guericke created a near vacuum
between two copper hemispheres, as shown in Concept 1. Von Guericke, also the
mayor of the town of Magdeburg, where he conducted the demonstration in 1654,
challenged teams of horses to pull the hemispheres apart. The horses failed: The force
of the air pressure was too great for them to overcome.
This clever demonstration of air pressure may seem incredible until you consider that
the air pressure on the Earth’s surface is about 101 kilopascals
(14.7 pounds per square inch). To get an approximate value for the pressure holding
the hemispheres together, we can simplify matters and assume they acted like halves
of a cube, one meter on a side. We will further assume that a perfect vacuum was
created inside them, so the pressure inside was zero. Each team of horses would then
have had to overcome 101,000 N of force, more than 10 tons. Whoa, Nellie!
Von Guericke demonstrated the surprisingly large magnitude of atmospheric pressure.
He might have been startled to know how fast the particles that make up the
atmosphere move. Sunlight energizes the molecules in the atmosphere, keeping it in a
gaseous state, and causes them to fly energetically about, reaching speeds of up to
1600 km/h. The gravitational force of the Earth keeps most of these molecules from
flying off into space, but some molecules í especially the lightest ones í do reach
escape velocity and leave the planet’s atmosphere. This is why there is little or no hydrogen, or any other light gas, in the Earth’s atmosphere.
On the average, a lighter molecule moves faster than a heavier molecule of the same energy, meaning the lightest molecules most easily
reach the 11.2 km/s required to escape the Earth’s gravitational pull. The escape velocity is greater on Jupiter, allowing that planet to keep
hydrogen in its atmosphere.
The Earth’s atmosphere is a gas, which makes it a fluid. However, since it is a gas, its density changes with its height above the planet’s
surface. The weight of the atmosphere above “compacts” the atoms and molecules below, increasing their concentration (density). We live in
an ocean of air, just as fish dwell in an ocean of water. An important difference between the two is that water is relatively incompressible and
the ocean has essentially the same density, although different temperatures and pressures, at any depth.
The diagram in Concept 2 shows the Earth’s atmosphere schematically. The pressure and density of the atmosphere lessen, as does its
temperature, with height. The density of air at sea level and 15°C equals 1.23 kg/m^3 ; at the higher and chillier altitude of 30,000 meters, where
Air pressure demonstration
Near vacuum inside sphere
Horses pull against air pressure
Air pressure can cause great force!At greater altitudes
Pressure, density, temperature
decrease