Summary notes for topic 5.1 Thermal Physics (chapters 14 and 15 in the textbook) including explanations, definitions, diagrams, derivations, highlighted key points and equation break downs
The number of atoms / Molecules in a of gas
moles ( Md )
given volume is
expressed using
The amount of a substance indicates the number of elementary
entities within a given sample of substance
atoms in
One mole number of 0.012kg of carbon 12
: -
Avogadro constant Na , ,
6-02×1023
Therefore ,
the total number
of atoms / molecules in a substance is
given by :
N n × Na where is the number of moles of the
=
n
substance
The Kinetic
Theory Of Gases
Amodel used to describe the behaviour of atoms / molecules in
an ideal
gas
Assumptions made in the kinetic model for an ideal gas
:
The gas / molecules moving
contains a
very large number of atoms with random
•
directions speeds
+
•
Atoms / molecules of the gas occupy a negligible volume compared to the
volume of the
gas
•
Collisions of atoms / molecules with each other and with container walls
are
perfectly elastic
•
Time of collision is negligible compared to time between collisions
•
Electrostatic forces between atoms are
negligible except during collisions
, How do the atoms /Molecules in an ideal
gas cause
pressure ?
Atoms /molecules
always moving
•
• When atoms / molecules collide with the walls of a
container , the container exerts a force on them .
Atoms / molecules off the
•
change in momentum as
they bounce
wall
•
Atom also exerts an equal and opposite force on the Wall
( Newton 's third low)
•
p =
f- where A is the cross-sectional area of the wall As . a
A
large number of atoms / molecules collide randomly with and exert a
force on the walls pressure is caused
,
/
WM -
•
When an atom collides with a container wall elastically .
its speed does not
change
mu
mu
-
.
velocity charges from
'
to Ums !
-
Ums
-
Its
-
•
The total
atom •
Therefore its momentum
changes from mu to -
met .
change in
momentum is -2mn
-2mn and At is the time between
Newton 's second law Falun If where Dp
- -
=
•
=
before after collisions
Gas Laws
}
for constant mass and temperature :
p
¥ pV= constant
a
:
p¥ constant pill pzvz
= =
o
T, Tz
for • meant now me and mass ,
a T constant
p
p
=
for one mole of gas the ,
constant in the equations above is the
molar gas constant R which is equal to 8.31 JK 'm 01 "
, ,
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