Gas Laws Introduction of pressure Pressure is defined as : Next Slide 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 Pressure 1

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

Gas Laws Relationships between P, V and T for a fixed amount of gas Next Slide 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 Gas Laws 1

Gas Laws 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. Next Slide 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. Kinetic Theory 1

Gas Laws Kinetic Theory Next Slide 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. Kinetic Theory 2

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

END of Gas Laws

Gas Laws Click Back to gas molecule is moving around wall of the container Force is experienced by the wall of container (change of momentum) Back to Pressure 1

Gas Laws Next Slide The atmospheric pressure is large enough to hold the cover. atmospheric pressure water cover Pressure 1

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

Gas Laws Click Back to Bourdon gauge: Back to Pressure 2

Gas Laws Next Slide 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. Gas Laws 1

Gas Laws Next Slide 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 Gas Laws 1

Gas Laws Click Back to The relation between P and V: Back to P 1/V0 P V0 Gas Laws 1

Gas Laws Next Slide 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. Gas Laws 1

Gas Laws Next Slide 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 Gas Laws 1

Gas Laws Click Back to The data obtained is plotted in the following graphs. (Different temperature scales lead to different graphs) Back to P T (in  C) 0  273  C P T (in K) 0 Gas Laws 1

Gas Laws Next Slide 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. Gas Laws 1

Gas Laws Next Slide 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 Gas Laws 1

Gas Laws Click Back to The data obtained is plotted in the following graphs. (Different temperature scales lead to different graphs) Back to V T (in  C) 0  273  C V T (in K) 0 Gas Laws 1

Gas Laws Next Slide All the gas laws can be summarized by general equation : Example : A weather balloon contains 5 m 3 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? Gas Laws 1

Gas Laws Click Back to Back to Gas Laws 1

Gas Laws Click Back to Back to 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. Kinetic Theory 3

Gas Laws Click Back to Back to 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. Kinetic Theory 3

Gas Laws Click Back to Back to 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. Kinetic Theory 3

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

Gas Laws Click Back to Back to 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. Kinetic Theory 3

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

Gas Laws Next Slide 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 Kinetic Theory 3

Gas Laws Next Slide 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). Kinetic Theory 3

Gas Laws Click Back to 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. Back to Kinetic Theory 3