General Physics 1

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General Physics 1

• Hongqun Zhang
• The Department of Physics, Beijing Normal
  University
• June 2005

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Chapter 13 Heat

13.1 Introduction 13.2 Ideal gas-A
Macroscopic Description 13.3 Ideal gas-A
Microscopic Description 13.4 Kinetic
Calculation of Pressure 13.5 Kinetic
Interpretation of Temperature
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13.1 Introduction

Heat is a branch of physics that is concerned
with the analysis of thermal phenomena and
thermal properties of matter.
Macroscopic description
Microscopic description
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Macroscopic parameters pressure (p), volume
(V), temperature (T), internal energy (U),
entropy (S) etc.
Microscopic parameters speeds, energies,
masses, angular momenta, etc.
Macroscopic description Thermodynamics
Microscopic description Statistical Physics,

Kinetic Theory
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Thermodynamics
All laws of thermodynamics are based on
experiments.
Thermodynamics System any limited
macroscopic system
that consists of a huge number of molecules or
atoms.
Statistical Physics, Kinetic Theory
The Laws of Mechanics Statistical Method
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13.2 Ideal gas-A Macroscopic Description
Given a gas in a state of thermal equilibrium
P, V, T
For low enough values of the density, experiment
shows that
At constant T, pVconstant (Boyles Law)
At constant p, V/Tconstant (Law of Charles
and Gay-Lussac)
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Combined pV/Tconstant
Equation of State of Ideal Gas
Where ? is the number of moles of gas, M is the
mass of gas, ? is the mass of one mole of gas, R
is the universal gas constant, 8.314 J/mol K.
This relation is also the macroscopic definition
of ideal gas.
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13.3 Ideal gas-A Microscopic Description
We assume
1. The gas consists of a large number of
identical particles (atoms or molecules).
1 ?m3 water contains 3.3?1010 molecules
2. The molecules are in random motion and obey
Newtons laws of motion.
3. The actual molecules take up a negligible
small part of the volume occupied by the gas.
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Vol. of gas can be changed over large range vol.
of liquid thousands timeslt vol. of gas.
4. No appreciable forces act on the molecules
except during a collision.
Molecular forces act over molecules size
distances and molecule spacing gtgt size of molecule
5. Collision are elastic and of negligible
duration.
6. The molecules moves in all directions with the
same probability.
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13.4 Kinetic Calculation of Pressure
(1) For one molecule
Impulse

• Dividing all molecules into groups according
• to their velocities as follows

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Velocities
Number of molecules
Number per unit volume
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(4) For all molecules
Define the statistical average value of v2x for
all molecules as follows
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According to statistical assumption,
we have
then
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Pressure formula of ideal gas

• Discussion
• p is determined by n and , and is
• proportional to them.
• (2) It is a statistical law, not a mechanics
  law.

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13.5 Kinetic Interpretation of Temperature
From
We obtain
Avogadro law
in which
is Boltzmanns constant
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By using of
We finally get
Temperature formula of ideal gas