Classical free Electron Theory of Metals Video Lecture - Civil Engineering (CE)

in this lesson we are going to talk
about the conductivity of materials okay
first we are going to learn about the
free electron theory and then we are
going to talk about the band theory of
solids okay so you must already be
knowing about your free electron theory
however this theory is not sufficient
enough to answer why the conductivity of
a material is affected by temperature or
by radiation or by other external
factors okay like addition of impurities
okay so your free electron Theory does
not help you understand the phenomena of
conductivity in semiconductor for that
what you need is your band Theory so
first we are going to quickly review the
free electron theory and then we are
going to move on to the band theory of
solids that answers all of these
questions okay my free electron theory
tells me that the phenomena of
conductivity is due to presence of
valence electrons okay so we already
know what our valence electrons the
valence electrons are the electrons that
are present in the outermost partially
filled shell okay so whenever solid is
formed especially a solid like a metal
what happens is these valence electrons
they detach themselves from the @mk so
if this is your nucleus and this is your
valence electron then these valence
electrons they become free okay now
these valence electrons which have now
become free they travel within the solid
and this constitutes your current okay
so these free electrons they travel only
within the solids because the potential
outside this solid for a stationary
electron is higher as compared to the
potential energy
she offer electron within this solid
okay so it would not go out of the metal
rather it stays inside the metal and it
moves within the metal okay so this
movement is going to be responsible for
conduction of current in a metallic
solid okay so what happens is whenever
you have a electric field then these
electrons tend to move in a direction
opposite to the electric field okay so
if you have electric field like this
then your electron is going to move like
this and when this happens this
constitutes your current okay so your
free electron Theory tells you that the
valence electrons which become free
during the formation of a solid are
responsible for conduction of current
whenever an external applied electric
field exists okay
just understand one thing under normal
conditions your electrons are actually
moving in a very high speeds because of
the thermal energy which is inherently
present okay so at every temperature you
have a certain amount of thermal energy
that's inherently present rate so
because of this inherent thermal energy
your electrons are actually moving in
all random directions at a very high
speed okay since electrons are moving in
all random direction there is no
effective current flowing okay the speed
of electrons under this condition is
very high it is of the order of 10 raise
to power 6 meter per second okay so this
motion due to the inherent thermal
energy is known as your thermal motion
of the electrons okay so your thermal
motion is a random motion in which
electrons are moving in all possible
directions therefore there is
no net current flowing okay now let's
see what happens when you have an
applied external electronic field okay
so let's just say that I have an applied
external electronic field which is like
this okay now your electron tends to get
accelerated in this direction okay so
your electron is going to feel an
acceleration in this direction is going
to move in this direction however your
electron is moving in a solid okay so
within this solid you have your positive