Lecture 8 The Gas Laws. Kinetic Theory of Matter. Chapter 4.7 4.16. Outline. Ideal Gas Kinetic Theory of Matter Changes of State Entropy. Boyle’s law relates gaseous volume and pressure under constant temperature. Boyle’s Law. Gas is the simplest state of matter to study.
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Chapter 4.7 4.16
Boyle’s law relates gaseous volume and pressure under constant temperatureBoyle’s Law
Gas is the simplest state of matter to study
p1 initial pressure, p2 final pressure
V1 initial volume, V2 final volume
--- = ----
p1V1 = p2V2
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
the pressure is 0 if the volume was constant
the volume is 0 if the pressure was constant
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)
--- = ---- at constant pressure Charles’ law
Combined Boyle’s and Charles’ laws give the ideal gas law
p1 V1 p2 V2
------- = -------
At constant T (T1 = T2) we have Boyle’s law
At constant p (p1 = p2) we have Charles’ law
----- = const
Basis: all matter is composed of tiny particles called molecules that are in constant motion.
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
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
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
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%).
Thermodynamics is a science of heat transformations
The second law is based on the fact that one cannot line up all the molecules in a volume.
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
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.