CHAPTER 3.3 GAS PRESSURE

GAS PRESSURE

Force per unit area exerted by the gas

particles as they collide with walls of the

container.

Kinetic Theory of Gases

• Discrete. • Moves freely in random motion.

Kinetic Theory of Gases

• Kinetic energy increases with the temperature.

Kinetic Theory of Gases

• Collisions with each other and the walls of the container are elastic.

• Forces of attraction

NEGLIGIBLE.

ATMOSPHERIC

PRESSURE

It is a thick layer of air that surrounds the earth.

It occurs when gas molecules collides to each other in the atmosphere and an object.

It acts in all direction.

The value of atmospheric pressure is about 1.0 x 105 N m-2 or 1.0 x 105 Pa.

INSTRUMENTS FOR MEASURING GAS PRESSURE

U-tube manometer

Bourdon gauge

U-TUBE MANOMETERThe gas whose pressure is to be measured is connected directly to one side of the U-tube.The pressure of the gas is equal to the sum of atmospheric pressure and the pressure due to the difference in the height of the liquid in the manometer.

BOURDON GAUGECommonly used in school laboratories, air

conditioner service shops, hospitals and factories.

When a Bourdon gauge is attached to a gas cylinder, gas flows from the cylinder into the flattened curved tube in the gauge.

The pressure of the gas causes the curved tube to straighten out.

Through a lever and a gear system, the pointer rotates to measure the pressure of the gas.

INSTRUMENTS FOR MEASURING ATMOSPHERIC PRESSURE

oMercury barometer

oFortin barometer

oAneroid barometer

MERCURY BAROMETERMade by inverting a long glass tube filled with

mercury.

Due to the inversion, a vacuum is created at the base of the tube and a column of mercury is supported by the atmospheric pressure.

The units for the measurement of atmospheric pressure can be in terms of centimeters of mercury (cm Hg) or Pascal (Pa). Another unit of pressure is torr, where 1 torr is equal to 1 mm Hg.

FORTIN BAROMETER• Essentially modified from a simple mercury

barometer.

• The Fortin barometer has a metal scale with a vernier attached and a mirror behind the top of the mercury column to avoid parallax error when taking readings.

• With these features, the Fortin barometer measures the atmospheric pressure more accurately than the simple mercury barometer.

ANEROID BAROMETER More convenient form of barometer because it

does not use liquid and can be as small as a wristwatch.

Inside an aneroid barometer there is a partially evacuate metal case made of thin and flexible alloy.

A change of atmospheric pressure will cause the centre of the metal case to move in or out. This movement is magnified by a system of levers.

A chain attached to the last lever rotates the pointer to show the atmospheric pressure measured.

THE END !

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