Suction
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 apocryphal (not revealed), 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.)
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. Adults use suction in drinking, particularly 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.
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.
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.
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 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.
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.)
*
implosion*
sucks - a (sometimes vulgar) term of abuse
*
suction cup*
vacuum*
vacuum cleaner
