1. Introduction of pressure
Gas Laws
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Introduction of pressure
Pressure is defined as :
Pressure = force perpendicular to an area  area
Gas pressure comes from the collisions between the gas molecules and the wall of container
Diagram
Molecules in air also exert pressure on all objects (even on our body) : atmospheric pressure
Diagram

2. Pressure Pressure 2 Gas Laws
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Pressure
Pressure can be measured by an instrument called Bourdon gauge
Photo

3. Gas Laws Gas Laws 1 Gas Laws
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Gas Laws
Relationships between P, V and T for a fixed amount of gas
Boyle’s law : P and V’s relation at a fixed T
Diagram
Pressure’s law : P and T ’s relation at a fixed V
Diagram
Charle’s law : V and T’s relation at a fixed P
Diagram
General gas equation and examples
Example

4. Real gas and Ideal gas Kinetic Theory 1 Gas Laws
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Real gas and Ideal gas
Real gas : a gas which does not obey the general gas equation for all values of P, V and T.
Ideal gas : a gas which obeys the general gas equation for all values of P, V and T.
A real gas behaves almost exactly the same as an ideal gas at low pressures and temperatures well above its boiling point.

5. Kinetic Theory Kinetic Theory 2 Gas Laws
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Kinetic Theory
1. all matter is made up of very tiny constantly moving particles, namely atoms, ions and molecules.
2. the particles are in constant random motion.
3. the particles move more quickly when the matter is heated.
4. the particles exert practically no force on each other when they are widely separated.
5. the particles attract each other when they are slightly apart, but if they are forced too close together, they will repel each other strongly.

6. States of matter Kinetic Theory 3 Gas Laws Structure of a solid
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States of matter
Structure of a solid
Diagram
Structure of a liquid
Diagram
Structure of a gas
Diagram
Brownian motion in gas
Diagram
Three-Dimension Kinetic Theory Model: Explanation of Gas Laws
Diagram

7. END of Gas Laws

8. Force is experienced by the wall of container (change of momentum)
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Pressure 1
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gas molecule is moving around
wall of the container
Force is experienced by the wall of container (change of momentum)

9. The atmospheric pressure is large enough to hold the cover.
Gas Laws
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The atmospheric pressure is large enough to hold the cover.
atmospheric pressure
water
cover

10. Pressure 1 Back to Gas Laws
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Atmospheric pressure presses the water through the sucking tube into your month.
atmospheric pressure
water

11. Pressure 2
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Bourdon gauge:

12. Gas Laws 1
Gas Laws
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An air column is trapped in the glass tube as shown. The pressure and the volume of the gas in the glass tube can be read from the Bourdon gauge and the ruler respectively.

13. Gas Laws 1
Gas Laws
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The volume of the rubber tube will cause an error. The volume is unknown and should be made as small as possible.
The volume of the gas should be changed slowly to avoid error in measuring the pressure and the volume.
Boyle’s law :
the pressure of a fixed mass of gas at constant temperature is inversely proportional to its volume

14. The relation between P and V:
Gas Laws 1
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The relation between P and V: P V P
1/V

15. Gas Laws 1
Gas Laws
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A gas is placed in a fixed volume container. The container is placed in a water bath. The pressure and the temperature of the gas can be read from the Bourdon gauge and the thermometer respectively.

16. Gas Laws 1
Gas Laws
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The length of the rubber tube should be made as short as possible since we cannot control the temperature of the gas in the tube.
The flask should be completely immersed in water to ensure a homogeneous temperature in the gas.
Pressure law :
the pressure of a fixed mass of gas at constant volume is directly proportional to its Kelvin temperature

17. Gas Laws 1 Back to Gas Laws
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The data obtained is plotted in the following graphs. (Different temperature scales lead to different graphs) P
T (inC)
273C P
T (in K)

18. Gas Laws 1
Gas Laws
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A gas is trapped in a capillary tube by a mercury thread. The tube is placed in a water bath. The volume and the temperature of the gas can be read from the ruler and the thermometer respectively.

19. Gas Laws 1
Gas Laws
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Since the mercury thread can freely move up and down and is in contact with atmosphere, the pressure of the gas is equal to the atmospheric pressure.
The friction between the mercury thread and the container should be small.
Charle’s law :
the volume of a fixed mass of gas at constant pressure is directly proportional to its Kelvin temperature

20. Gas Laws 1 Back to Gas Laws
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The data obtained is plotted in the following graphs. (Different temperature scales lead to different graphs) V
T (inC)
273C V
T (in K)

21. All the gas laws can be summarized by general equation :
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All the gas laws can be summarized by general equation :
Example :
A weather balloon contains 5 m3 of helium at the normal atmospheric pressure of 100 kPa and a temperature of 27 C. What will be its volume when it rises to an altitude where the pressure is 80 kPa and the temperature is 7 C?

22. Gas Laws 1
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23. Kinetic Theory 3 Back to Gas Laws
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Solid: Atoms or molecules are close together and arranged in a regular pattern. They are held together by a strong attractive force. Hence, vibrations, though possible, is not vigorous enough to set the particles free.

24. Kinetic Theory 3 Back to Gas Laws
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Liquid: Weak attractive force can no longer hold the particles in their regular positions and the liquid does not have any definite shape. However, the particles are still very close together.

25. Kinetic Theory 3 Back to Gas Laws
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Gas: The attractive force between the particles is negligible and therefore the particles can move around randomly at high and uniform speeds. In addition, all collisions are elastic.

26. Kinetic Theory 3 Gas Laws
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The following experimental set-up is used to observe the Brownian motion of smoke particles.
smoke particle
lamp
glass rod
microscope
glass
cell

27. Kinetic Theory 3 Back to Gas Laws
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When we observe the smoke particles in a smoke cell under microscope, the smoke particles move jerkily about. The motion is called Brownian motion. The collisions between the air molecules and the smoke particles cause the random motions of the smoke particles.

28. Kinetic Theory 3 Gas Laws
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The following experimental set-up is used as a Three-Dimensional Kinetic Theory Model :
disc

29. Pressure of the ‘gas’ : weight of the disc
Kinetic Theory 3
Gas Laws
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The motor under the cylinder set the bearings in motion. Due to the collisions between the disc and the bearings, the disc rises to a certain level.
Pressure of the ‘gas’ : weight of the disc
Volume of the ‘gas’ : volume occupied by the bearings
Temperature of the ‘gas’ : voltage of the power supply which drives the motor

30. Boyle’s law’s demonstration :
Kinetic Theory 3
Gas Laws
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Boyle’s law’s demonstration :
We keep the voltage of the motor constant (T constant), and add discs one after another (P increases). We can observe a decrease in volume.
Pressure law’s demonstration :
We add discs one after another (P increases). In order to maintain a constant volume (V constant), we must increase the voltage gradually (T increases).

31. Charles’ law’s demonstration:
Kinetic Theory 3
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Charles’ law’s demonstration:
We keep the no. of discs constant (P constant) and increase the voltage (T increases). We can observe an increase in volume.