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 vacuum.

Vacuums do not actually attract matter; matter is pushed into them by the higher pressure of surrounding air (there being nothing in a vacuum to do any attracting). 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), which is the average atmospheric pressure at sea level. (It is lower at higher elevations.)


Biological uses of suction

Infants, and all baby mammals, are born with a sucking (or suckling) reflex, which they use is nursing liquid foods, such as milk. They do not have to learn this reflex, because it is instinctive. Adults use suction in drinking, particularly when using straws. In breathing, the diaphragm muscle is used to expand the lungs, allowing air to enter due to the outside air pressure.

Suction in physics

A device called a manometer can measure the pressure in a gas (or complete vacuum). The manometer consists of a vessel in which the gas is held, opening into a U-shaped tube containing mercury (or, more rarely, water). The other end of the tube is exposed to the atmosphere. If the vessel contains a gas at atmospheric pressure, the columns within the U-shaped tube will be equal in height. If the contained gas is above atmospheric pressure, the column exposed to the air will be higher, and vice versa if the gas (or vacuum) in the vessel is at sub-atmospheric pressure. The difference in the columns' heights represents the gas's relative pressure, or deviation from the ambient pressure.

A total vacuum exhibits zero pressure, and pressure (for normal matter, as atoms repel at close distances) cannot be negative. Also, a total vacuum is an ideal construct that cannot actually be realized on Earth, though near-total vacuums can be generated. At atmospheric pressure, a manometer whose vessel contains a total vacuum would show a relative pressure of 101.325 kPa. For a mercury manometer, this would create a 760 mm (29.92 in) difference in the mercury levels. For water, the difference would be 10.3 m (33.8 ft). Since straws rely on "suction" (air pressure), it would therefore be impossible to drink water from an elevation exceeding 10.3 m, unless additional pressure were applied at the air/water interface.

Trees rely on water, but many trees exceed 10.3 m in height (some are taller than 110 m [1] ( Therefore, this water must be drawn via some force other than suction. See: transpirational pull.

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.)


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 of the pump. 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 the pump from running dry; i. e. with no fluid entering.

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