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8.2 Pressure of liquids and gases
Why is a soap bubble Pressure of gas
round? Gases always occupy the whole volume of the container in which they are
held. Gas molecules continuously collide with each other, and with the walls
of the container, as the result of chaotic motion. The force exerted by one
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molecule is small, but the effect of all the molecules on the walls or on an
object placed into the gas is considerable. We call this effect of molecular
collisions gas pressure.
The pressure changes if the volume of a gas with constant mass changes.
Gas pressure also changes if the temperature changes while the mass of gas
remains constant.
The atmosphere – the layer of air surrounding the Earth – also exerts
pressure. This atmospheric pressure is measured using a device called
a barometer.
Pascal’s Law
Solid bodies exert pressure in the direction of the action of external
forces. Therefore we can hammer a nail, cut with a knife and dig the
ground. Liquids and gases exert pressure not only in the direction of
force but equally in all directions.
Barometers measure Pascal’s Law was established in 1653 by French mathematician Blaise Pascal.
atmospheric pressure
Pascal’s Law: the pressure exerted on liquid or gas is transmitted equally
in all directions.
The pressure exerted by liquid on the same level is equal in all directions
but increases with depth.
Pressure is measured by a device called a manometer.
Liquids have weight, therefore they exert pressure on all underlying layers
and the bottom of the container in which they are placed. According to
Pascal’s Law, pressure is transmitted equally in all directions. Therefore,
liquids exert pressure on the side walls of the container too. Increasing the
Manometers in use in the oil and depth of a liquid increases the pressure too.
gas industry
The pressure exerted by a still liquid is called hydrostatic pressure. The
pressure of the liquid on the bottom and walls of the container (as well as
on any object immersed in the liquid) is calculated using this equation:
liquid pressure = liquid density × constant physical quantity g
× depth of liquid
p = gh
g = 9.8 N/kg
REMEMBER! Gas pressure is produced by the collision of gas molecules.
If the number or the speed of these collisions increases, the gas
pressure rises.
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