Lecture%208%20The%20Gas%20Laws.%20Kinetic%20Theory%20of%20Matter.

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Lecture 8The Gas Laws. Kinetic Theory of Matter.
Chapter 4.7 ? 4.16
Outline

• Ideal Gas
• Kinetic Theory of Matter
• Changes of State
• Entropy

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Boyles Law
Gas is the simplest state of matter to study

• Boyles law relates gaseous volume and pressure
  under constant temperature

p1 ? initial pressure, p2 ? final pressure V1 ?
initial volume, V2 ? final volume
p1 V2 --- ---- p2 V1
or
p1V1 p2V2
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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
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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
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Ideal Gas
Combined Boyles and Charles laws give the ideal
gas law
p1 V1 p2 V2 ------- ------- T1
T2
At constant T (T1 T2) we have Boyles law At
constant p (p1 p2) we have Charles law
p V ----- const T
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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
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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
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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
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Energy Transformations
Heat cannot be converted into other forms of
energy efficiently
The reason is random molecular motion

• Heat can be turned into mechanical energy by a
  heat engine.

The maximum efficiency of a heat engine is a
ratio of the (work output)/(energy input) or 1 ?
Tcold/Thot (typical actual efficiency lt 40).
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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.
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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
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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.
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Summary

• The concept of ideal gas is a good model to study
  properties of substances

• 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.