 air
pressure inside the vacuum cleaner is lower than the pressure
Two upright vacuum cleaner models, one with
the conventional bag system (right), and the other with
the new "cyclone" system
As long as the fan is running and the passageway through
the vacuum cleaner remains open, there is a constant stream
of air moving through the intake port and out the exhaust
port. But how does a flowing stream of air collect the dirt
and debris from your carpet? In the next section, we'll find
out how this works.
Picking Up the Dirt
In the last section, we
saw that the suction created by a vacuum cleaner's rotating
fan creates a flowing stream of air moving through the intake
port and out the exhaust port. This stream of air acts just
like a stream of water. The moving air particles rub against
any loose dust or debris as they move, and if the debris is
light enough and the suction is strong enough, the
friction carries the material through the inside of the
vacuum cleaner. This is the same principle that causes leaves
and other debris to float down a stream. Some vacuum designs
also have rotating brushes at the intake port, which
kick dust and dirt loose from the carpet so it can be picked
up by the air stream.
Upright vacuum cleaners usually have rotating
brushes on the bottom to knock dirt loose from your
carpet. The brushes may be rotated by the vacuum's motor
or simply by the rushing
As the dirt-filled air makes its way to the exhaust port,
it passes through the vacuum-cleaner bag. These bags
are made of porous woven material (typically cloth or paper),
which acts as an air filter. The tiny holes in the bag
are large enough to let air particles pass by, but too small
for most dirt particles to fit through. Thus, when the air
current streams into the bag, all the air moves on through the
material, but the dirt and debris collect in the bag.
The vacuum cleaner bag is simply a filter
that lets air pass through but keeps dirt
You can put the vacuum-cleaner bag anywhere along the path
between the intake tube and the exhaust port, as long as the
air current flows through it. In upright vacuum cleaners, the
bag is typically the last stop on the path: Immediately after
it is filtered, the air flows back to the outside. In
canister vacuums, the bag may be positioned before the
fan, so the air is filtered as soon as it enters the vacuum.
Using this basic idea, designers create all sorts of vacuum
cleaners, with a wide range of suction capacities. In the next
section, we'll look at a few of the factors that determine
In the last section, we saw
that vacuum cleaners pick up dirt by driving a stream of air
through an air filter (the bag). The power of the vacuum
cleaner's suction depends on a number of factors. Suction will
be stronger or weaker depending on:
- The power of the fan: To generate strong suction,
the motor has to turn at a good speed.
- The blockage of the air passageway: When a great
deal of debris builds up in the vacuum bag, the air faces
greater resistance on its way out. Each particle of air
moves more slowly because of the increased drag. This is why
a vacuum cleaner works better when you've just replaced the
bag than when you've been vacuuming for a while.
- The size of the opening at the end of the intake
port: Since the speed of the vacuum fan is constant, the
amount of air passing through the vacuum cleaner per unit of
time is also constant. No matter what size you make the
intake port, the same number of air particles will have to
pass into the vacuum cleaner every second. If you make the
port smaller, the individual air particles will have to move
much more quickly in order for them all to get through in
that amount of time. At the point where the air speed
increases, pressure decreases, because of Bernoulli's
principle (see How
Airplanes Work to learn about this physical principle).
The drop in pressure translates to a greater suction force
at the intake port. Because they create a stronger suction
force, narrower vacuum attachments can pick up heavier dirt
particles than wider attachments.
Vacuum cleaner attachm