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Flexible hose

Pipe

Exhaust
Power cord

Power switch
Motor
Fan

Cleaning
attachment

Caster
Handle

Filter

Section 5.4
Vacuum Cleaners
When we examined garden watering in Section 5.1, we examined tools that permit a fluid
to flow out of a hose. In this section, we’ll look at the reverse, a device that draws a fluid
into a hose. That device is a vacuum cleaner, and the fluid that it draws inward is air.
This moving air gathers dust and debris as it rushes into the vacuum cleaner, which is
why vacuum cleaners are useful.
Questions to Think About: Why does nature “abhor a vacuum”? If you remove some
of the air from a region of space, how does the surrounding air respond? How does wind
push on the objects it passes? Can you think of cases in which moving air actually picks
up at least some of the objects it passes? How does the vacuum cleaner create the partial
vacuum it uses to draw air into the hose? Why does the vacuum cleaner require electric
power?
Experiments to Do: Find a vacuum cleaner with a hose and watch how the airflow
draws in dust. As you shrink the diameter of the cleaning attachment or partially block
the end of the hose, does the air entering the hose move faster or slower? Is a partially
blocked hose more or less effective at trapping dust than an unblocked one? Try to vacuum up tiny objects and then larger ones; which are easier? Block the airflow into the
hose completely and notice what happens to the motor’s pitch. Why should the motor’s
rotation depend on the airflow?
Even without a vacuum cleaner, you can try similar experiments with a drinking
straw or a cardboard tube. You could pretend to be a vacuum cleaner by sucking dust
into your mouth, but you’d do better to blow it around instead. Try blowing on differentsized objects with different-sized openings on the straw or tube. Is a narrow or a wide
opening most effective? Which blow about more easily: small objects or large objects?
© 2001 John Wiley & Sons

1

2

CHAPTER 5. FLUIDS AND MOTION

Air Flowing into the Vacuum Cleaner

Exhaust

Inlet

Stream
lines

Motor Fan

Vacuum
cleaner hose

Total energy

Pressure

Speed

Fig. 5.4.1 - As air flows into the inlet
of a vacuum cleaner hose, its pressure drops and its speed increases.
The fan boosts the air’s total energy, helping it overcome viscous
losses of total energy so that it can
return to the outside air through
the outlet.

Vacuum cleaners use swiftly moving air to sweep up dust. In this section, we’ll
examine how they create th...
© 2001 John Wiley & Sons 1
Section 5.4
Vacuum Cleaners
When we examined garden watering in Section 5.1, we examined tools that permit a fluid
to flow out of a hose. In this section, we’ll look at the reverse, a device that draws a fluid
into a hose. That device is a vacuum cleaner, and the fluid that it draws inward is air.
This moving air gathers dust and debris as it rushes into the vacuum cleaner, which is
why vacuum cleaners are useful.
Questions to Think About: Why does nature “abhor a vacuum”? If you remove some
of the air from a region of space, how does the surrounding air respond? How does wind
push on the objects it passes? Can you think of cases in which moving air actually picks
up at least some of the objects it passes? How does the vacuum cleaner create the partial
vacuum it uses to draw air into the hose? Why does the vacuum cleaner require electric
power?
Experiments to Do: Find a vacuum cleaner with a hose and watch how the airflow
draws in dust. As you shrink the diameter of the cleaning attachment or partially block
the end of the hose, does the air entering the hose move faster or slower? Is a partially
blocked hose more or less effective at trapping dust than an unblocked one? Try to vac-
uum up tiny objects and then larger ones; which are easier? Block the airflow into the
hose completely and notice what happens to the motor’s pitch. Why should the motor’s
rotation depend on the airflow?
Even without a vacuum cleaner, you can try similar experiments with a drinking
straw or a cardboard tube. You could pretend to be a vacuum cleaner by sucking dust
into your mouth, but you’d do better to blow it around instead. Try blowing on different-
sized objects with different-sized openings on the straw or tube. Is a narrow or a wide
opening most effective? Which blow about more easily: small objects or large objects?
Caster
Motor
Power switch
Flexible hose
Cleaning
attachment
Fan
Filter
Handle
Pipe
Exhaust
Power cord
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