Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not innately attract matter. Dust being "sucked" into a vacuum cleaner is actually being pushed in by the higher pressure air on the outside of the cleaner.
The higher pressure of the surrounding fluid can push matter into a vacuum but a vacuum cannot attract matter.
Suction in biology
Infants, and all baby mammals, are born with a sucking (or suckling) reflex, which they use in nursing liquid foods, such as milk. They do not have to learn this reflex, because it is instinctive. Some adult animals use suction in drinking, as do humans when using drinking straws. In breathing, the diaphragm muscle is used to expand the lungs, allowing air to enter due to the outside air pressure.
Large plants can actually create a negative pressure by transpirational pull.
Pumps used for pumping or moving fluids typically have an inlet where the fluid enters the pump and an outlet where the fluid comes out. The inlet location is said to be at the suction side of the pump. The outlet location is said to be at the discharge side of the pump. Operation of the pump creates suction (a lower pressure) at the suction side so that fluid can enter the pump through the inlet. Pump operation also causes higher pressure at the discharge side by forcing the fluid out at the outlet. There may be pressure sensing devices at the pump's suction and/or discharge sides which control the operation of the pump. For example, if the suction pressure of a centrifugal pump is too low, a device may trigger the pump to shut off to keep it from running dry; i. e. with no fluid entering.
Under normal conditions of atmospheric pressure suction can draw pure water up to a maximum height of approximately 10.3 m (33.9 feet or suction head). There is a theoretical limit of height that a perfect pump can raise water up a pipe and that is when the pump draws a perfect vacuum with no air in the suction pipe and atmospheric pressure of around 14 psi. In reality, no pumps are perfect. Leakage around the moving parts of the pump limit normal water pumps from starting if they lose prime.
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- sucks - a (sometimes vulgar) term of abuse (see definition 3 of suck at Wiktionary)
- suction cup
- suction cupping
- vacuum cleaner