f2 Sc c6 Air Pressure

Air Pressure

6.1 Understanding Air Pressure • The kinetic theory of gases states that gas consists of tiny particles. • These particles are constantly in random motion and collide against the walls of the container. • These collisions produce the gas pressure.

Atmospheric Pressure

Atmospheric pressure • The thick layer of air surrounding the Earth is called the atmosphere. • The air particles in the atmosphere collide with one another and collide against the surfaces of all things. • These collisions result in air pressure or atmospheric pressure.

GAS PRESSURE AND ATMOSPHERIC PRESSURE

Atmospheric pressure • Atmospheric pressure decreases with the altitude or the height above sea level.

Show existence of atmospheric pressure • The cardboard does not fall and the water remains in the glass even though it’s not supported by anything. • Explanation: The force caused by the atmospheric pressure acts on the surface of the cardboard is greater than the weight of the water in the glass.

Show existence of atmospheric pressure • When a can filled with hot water is closed and is cooled down rapidly by pouring cold water on it, it will crash instantly. • Explanation: When the air inside the can is cooled, its pressure decreases. The high atmospheric pressure outside exerts a great force on the can and causes it crashes.

Instrument Used to Measure Atmospheric Pressure

-Fortin Barometer -Aneroid Barometer -Simple Barometer

Fortin Barometer

Aneroid Barometer

Simple Barometer

• Atmospheric pressure is 76 cm mercury at sea level. The unit of air pressure is atmosphere and the air pressure near sea level is one atmosphere.

Low pressure or high pressure • If in some area the pressure is higher than it is in the surrounding area, we say that it is an area of high pressure.

Air pressure

GAS PRESSURE AND ATMOSPHERIC PRESSURE • Gas pressure is the force per unit area exerted by the gas molecules as they collide with the walls of their container. • Atmospheric pressure is the pressure caused by the weight of the air above us.

• The factors which affect the air pressure are: (a) Volume of air (b) Temperature of air

Volume of the container • When the volume of the container is reduced, reduced the air particles become closer. • The air particles collide more frequently among themselves and against the walls of the container. Hence, the air pressure increases or vice versa.

Temperature • When the temperature is increased, the air particles gain more energy and move faster. • The rate of collisions between the particles and against the walls increases. • This increases the air pressure or vice versa.

Application of air pressure • • • • •

Drinking straw Rubber sucker Syringe Vacuum cleaner Lift pump

Drinking Straw • When the water is sucked, the air inside the straw is sucked out. • The air pressure inside the straw is reduced. • The higher air pressure outside the straw forces the water to enter the straw.

Syringe • When the piston is pulled, the air pressure in the syringe decreases. • The higher air pressure outside forces the liquid into the syringe. • When the piston is pushed, the air pressure in the syringe increases . • The higher pressure in the syringe forces the liquid to flow out.

Rubber sucker • The higher air pressure at Y presses the pump and it sticks firmly on the surface of wall. • The air pressure that presses on it is enough to support light objects.

Y

Plunger / Suction pump • When the plunger is pulled upwards, the higher pressure in Z forces the blockage out.

Z

Siphon • The air pressure at P is higher. • It pushes the water up the siphon to beaker Q.

6.3 Gas under High Pressure

Gas cylinder • When gases are compressed under high pressure, they turn into liquids. • This property is used in the storage of gas in gas cylinders. • A larger volume of gas can be stored in the form of liquid.

Aerosol cans • Similarly, perfumes, insecticides and paints can be stored in the form of liquids in aerosol cans.

Safety precautions when using gas under high pressure. (a) Gas cylinders should be kept away from heat sources. (b) Gas cylinders should be placed vertically.

Safety precautions when using gas under high pressure. ( c) Gas cylinders should be kept in rooms with good ventilation. (d) Always check the gas cylinders for leakage. (e) Light a match first before turning on the gas.