Kinetic Theory Of Kinetic Theory Of GasesGases

• Gases or atmosphere consist of molecules which far apart and in random motion at high speed.

GAS PRESSUREGAS PRESSURE

Atmospheric pressure1. The earth is surrounded by a thick layer of

atmosphere.2. Atmospheric pressure is the pressure exerted pressure exerted

by the atmospheric on the surface as well as all by the atmospheric on the surface as well as all objects on Earth.objects on Earth.

Activity

Like all gases, air also have weight hence exerts pressure.

The force exerted by the atmosphere on unit area is called atmospheric pressure.

Air exerts huge pressure in every direction.

Characteristics of atmospheric or gas Characteristics of atmospheric or gas pressure.pressure.

1. Unit : 1 atmosphere= 1 x 105 Pa.= 1 x 105 Nm-2

= 1000 milibar= 760 mm Hg= 10 m water

1 milibar = 100 Pa

2. Atmospheric pressure act in all direction.3. It not affected by surface area of the object.

Continued..

4. The atmospheric pressure depend on altitude. Hence as the altitude increases, the atmospheric decrease.

5. The atmospheric pressure can support a column of mercury at 760mm in high or column of water at 10m high.

6. The gas pressure inside a container depends on;

– Density of molecules of gases– Temperature – Volume of the container

MEASURING INSTRUMENTSMEASURING INSTRUMENTS

1. The height of the mercury column, h = 76 cm, represents the atmospheric pressure.

2. The atmospheric pressure, P P = h ρ g

= 0.90 m x 1.36 x 104 kg m-3 x 10 N kg-1 = 101 300 Pa

Mercury BarometerMercury Barometer

Aneroid barometer

1. Consist of a flexible metal can be which most of the air inside the can has been taken out of it.

2. When the surrounding pressure is higher, the top of the can is squeezed down slightly causing the pointer to move along the curved scale measuring the corresponding pressure.

3. The spring prevents the flexible metal can from collapsing completely by the atmospheric pressure.

ManometerManometer

1. Consist of a U-Tube that is filled with a liquid like water, oil or mercury

2. When both ends of the tube is exposed to atmospheric pressure, both level are the same.

3. When one end is connected to a gas supply, the difference in level, h gives the pressure of the gas, Pwhere

P = Atmospheric pressure + h ρ g

Bourdon GaugeBourdon Gauge

1. The movement of a gas into a hollow copper tube causes it to straighten slightly.

2. A system of lever and gear will enable a pointer to move along the round scale indicating the pressure of a gas measured.

APPLICATION OF GAS / APPLICATION OF GAS / ATMOSPHERIC PRESSUREATMOSPHERIC PRESSURE

• Straw

• Siphon

• Rubber sucker

• Lifting pump

• Vacuum cleaner

Drinking Straw• When we suck

through a straw, the air pressure in the straw is lowered. Then the pressure of the atmosphere acting on the surface of the drink in the glass pushes the water up the straw

Rubber Sucker• When the sucker is

pressed into place, most of the air behind it is squeezed out. The sucker is held in position by the pressure of atmosphere on the outside surface of the rubber. If the seal between the sucker and the surface is airtight, the sucker will stick permanently.

Syringe

• Pulling up the piston reduces the atmospheric pressure inside the cylinder.

• The atmosphere on the liquid surface then pushes the liquid up into the syringe.

• If we then hold the plunger in place and lift the syringe out of liquid, none will fall out. This is again due to atmospheric pressure.

Vacuum Cleaner

• Produces only a partial vacuum.

• The fan inside the cylinder blows air out of the vents. – less air inside, air pressure drops.

• The atmospheric pressure outside then pushes air up the cleaner hose, carrying dust and dirt with it.

Example & Example & exercise;exercise;Example 1 :

The figure below shows a simple barometer used in the laboratory to measure atmospheric pressure. The length of the glass tube is 100 cm. The height of a vacuum column is 10 cm and the height of the mercury column is 76 cm.

a) What is a value of the atmospheric pressure in cm Hg ? 76 cm Hgb) What is the pressure at point X in cm Hg ? 90 cm Hgc) If the density of a mercury is 1.36 x 104 kg m-3 and the g = 10 m s-2.

Calculate the pressure at X in Pascal.

Answer;

Atmospheric pressure = 76 cm Hg.Pressure at point X = ( 76 + 14 ) cm Hg

= 90 cm Hg.Pressure at point X in Pascal

P = h ρ g= 0.90 m x 1.36 x 104 kg m-3 x 10 m s-2.

= 1.224 x 106 Pa.

Example 2 :

A mercury manometer with one end attached to a gas supply measures a difference in the level of mercury of 32 cm as shown in figure below.

Calculate the pressure of a gas supply in a) cm Hgb) pascal( Atmospheric pressure = 76 cm Hg, g = 10 m s -2 and density of mercury = 1.36 x 104 kg m-3 )

Solution :Solution : a) P gas = Atmospheric pressure + Pressure the column of mercury. = ( 76 + 32 ) cm Hg = 108 cm Hg. b) P gas = 1.08 m x 1.36 x 104kg m-3 x 10 m s-2. = 1.47 x 105 Pa.

ExerciseExercise;;

The diagram shows a manometer which is connected to a gas supply. The atmospheric pressure is 76 cm Hg

What is the pressure at point A?(a)Find the pressure of the gas supply,(b)The tap is opened and the mercury is allowed to flow out until the level B falls to the division 60 cm of the ruler. State the new position of level A.

1)

Figure shows an apparatus used in the laboratory to measure atmospheric pressure. The length of the glass tube is 100 cm. The height of the vacuum column is 8 cm and the height of the mercury column is 76 cm.(a) Name the apparatus as shown in the figure(b)(i) What is the value of the atmospheric pressure in cm Hg?

(ii) What is the pressure at point X in cm Hg?(iii) If the density of the mercury is 1.36 × 104 kg m-3 and the acceleration due to gravity is 10 ms-2.Calculate the atmospheric pressure in Pascal.

2.)

(c) What will happen to the height of mercury column from the surface of the mercury in the container if (i) the tube is raised through a height of 12 cm? (ii) the tube is lowered through a depth of 6 cm? (iii) the glass tube is inclined about 2o from the vertical line? (iv) give reason for you answer in (c) (iii) (d) The apparatus above can be used to measure altitude

(i) Explain why this can be done(ii) Explain why the apparatus above is not suitable to

measure altitude.