CHAPTER 7
THE OUTER PARTS OF THE ATOM
:
7—1.
Introduction.—Two spheres are said to collide with each
other when they just touch, i.e., when the distance between their
centers is equal to the sum of their radii. This denition of a
“
collision
”
is not satisfactory when small bodies such as electrons
or
atoms
are
considered since these particles carry electrical
charges capable of repelling each other even at comparatively large
distances.
For such cases, we shall consider an elastic collision as
one in which the charged bodies are deected from their normal
paths without loss of energy; both the total energy and the
«
momentum shall be conserved.
On the other hand, we shall say
that an inelasz‘ic collision has taken place between charged particles
when one of them gains energy at the expense of the other. We
shall see that the study of collisions between charged particles hasyielded a great deal of information as to the nature of the bodies);
themselves.
-»
An electron gun is used to direct a beam of electrons through a
chamber lled with gas at a low pressure.
Occasionally the
trous collide with the gas atoms and are
“
absorbed,” either by£
loss of energy, scattering or by combination with the atoms. Let
’
.X be the average' distance or mean—fræ-potz which an electron
travel between collisions and let  be the qect‘ioe-cross—sm‘ion ofa
gas
for the electrons.
It will be of interest to compare
the
‘
meaSured values of these quantities with those computed from“ the
,
.
kinetic theory; a theory which treats
as solid sphereS
,_
_
The thermal velocities o£‘the atoms of a gas at ordinary tem—
small in comparison with the usual Vcloc1t1€S
root—mean—square values are equ1valentto
…
;… _. grad1usofan Çlëèfr0ñ“(äbout 10—13
small com
With't‘hat ;Ôf
an atoir1_(about 1—0—8 cm.). .Hence, hekmet1€
THE OUTER PARTS OF THE ATOM
:
7—1.
Introduction.—Two spheres are said to collide with each
other when they just touch, i.e., when the distance between their
centers is equal to the sum of their radii. This denition of a
“
collision
”
is not satisfactory when small bodies such as electrons
or
atoms
are
considered since these particles carry electrical
charges capable of repelling each other even at comparatively large
distances.
For such cases, we shall consider an elastic collision as
one in which the charged bodies are deected from their normal
paths without loss of energy; both the total energy and the
«
momentum shall be conserved.
On the other hand, we shall say
that an inelasz‘ic collision has taken place between charged particles
when one of them gains energy at the expense of the other. We
shall see that the study of collisions between charged particles hasyielded a great deal of information as to the nature of the bodies);
themselves.
-»
An electron gun is used to direct a beam of electrons through a
chamber lled with gas at a low pressure.
Occasionally the
trous collide with the gas atoms and are
“
absorbed,” either by£
loss of energy, scattering or by combination with the atoms. Let
’
.X be the average' distance or mean—fræ-potz which an electron
travel between collisions and let  be the qect‘ioe-cross—sm‘ion ofa
gas
for the electrons.
It will be of interest to compare
the
‘
meaSured values of these quantities with those computed from“ the
,
.
kinetic theory; a theory which treats
as solid sphereS
,_
_
The thermal velocities o£‘the atoms of a gas at ordinary tem—
small in comparison with the usual Vcloc1t1€S
root—mean—square values are equ1valentto
…
;… _. grad1usofan Çlëèfr0ñ“(äbout 10—13
small com
With't‘hat ;Ôf
an atoir1_(about 1—0—8 cm.). .Hence, hekmet1€