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Suction

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Suction is the creation of 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. Physicists consider the notion of "suction" to be specious, since vacuums do not innately attract matter.

Higher pressure of surrounding air can push matter into a vacuum but a vacuum cannot attract matter. At zero air pressure, such as in space, suction would have no effect. However, most humans live at air pressure near 101.325 kPa (14.7 lbf/in² or 760 mmHg), which is the average atmospheric pressure at sea level. (It is lower at higher elevations.)

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.

Pumps

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.

Trivia

Some physicists consider the notion of "suction" to be apocryphal, since vacuums do not innately attract matter. For this reason, a common joke among physicists is that, "There is no such thing as gravity: the earth sucks." (In fact, atmospheric pressure is set by an equilibrium between the Earth's gravity and the outward pressure generated by the concentration of air. If there were no gravity, the atmosphere's pressure would result in its outward dispersal into space.)

See also

Look up suction in Wiktionary, the free dictionary.
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