Lecture 8 The Gas Laws. Kinetic Theory of Matter.

1. Lecture 8The Gas Laws. Kinetic Theory of Matter. Chapter 4.7  4.16 Outline • Ideal Gas • Kinetic Theory of Matter • Changes of State • Entropy

2. Boyle’s law relates gaseous volume and pressure under constant temperature Boyle’s Law Gas is the simplest state of matter to study p1 initial pressure, p2 final pressure V1 initial volume, V2  final volume p1 V2 --- = ---- p2 V1 or p1V1 = p2V2

3. Charles’ Law Changes in volume are related to the gas temperature under a constant pressure Cooling a volume of gas steadily from 0oC at a constant pressure decreases its volume by ~1/273 for every degree The same rule applies to pressure if volume is constant Conclusion: at 273oC the pressure is 0 if the volume was constant or the volume is 0 if the pressure was constant

4. Absolute Zero Temperature It is impossible to achieve such a low temperature (273oC) Gases turn into liquids before this temperature is reached The temperature 273oC is called absolute zero Absolute temperature is temperature measured above absolute zero in degrees celsius (the Kelvin scale) T1 V1 --- = ---- at constant pressure  Charles’ law T2 V2

5. Ideal Gas Combined Boyle’s and Charles’ laws give the ideal gas law p1 V1 p2 V2 ------- = ------- T1 T2 At constant T (T1 = T2) we have Boyle’s law At constant p (p1 = p2) we have Charles’ law p V ----- = const T

6. Kinetic Theory of Gases Basis: all matter is composed of tiny particles called molecules that are in constant motion. Gas molecules: are small compared with the average distances between them collide without loss of kinetic energy exert almost no forces on one another outside of collisions Thus, a gas is mostly an empty space The absolute temperature of a gas is proportional to the average kinetic energy of its molecules

7. Why Molecules Keep Moving? Motion is affected by friction When friction is applied, it converts kinetic energy into heat Heat is molecular energy! Thus, there is no change in molecular energy by friction  molecular motion is unstoppable

8. States of Matter Solid Liquid Gas Matter can exist in these 3 states. Changes of state may occur under specific conditions. Liquid into Gas: evaporation and boiling Solid into Liquid: melting Solid into Gas: sublimation

9. Heat can be turned into mechanical energy by a heat engine. Energy Transformations Heat cannot be converted into other forms of energy efficiently The reason is random molecular motion The maximum efficiency of a heat engine is a ratio of the (work output)/(energy input) or 1  Tcold/Thot (typical actual efficiency < 40%).

10. Thermodynamics Thermodynamics is a science of heat transformations • 2 fundamental laws of thermodynamics: • Energy cannot be created or destroyed. • It is impossible to convert all the heat of a source into mechanical energy. The second law is based on the fact that one cannot line up all the molecules in a volume.

11. Fate of the Universe Other energy forms can be turned into heat, but heat cannot be efficiently converted back. Thus, heat energy in the Universe increases with time. Stars is the warm reservoir, everything else is the cool reservoir. With time the temperature difference between the two decreases, and finally all the particles will have the same average energy  “heat death” of the Universe

12. Entropy Entropy is defined as a measure of the disorder of the molecules in a material body. A liquid has more disorder than a solid. A gas has more disorder than a liquid. The entropy of an isolated system cannot decrease.

13. The concept of ideal gas is a good model to study properties of substances Summary • The absolute zero temperature corresponds to the absence of molecular motion and cannot be reached. • Heat cannot be effectively converted into other forms of energy. • Entropy or disorder in isolated systems can only be increased.