Ohms Law & Resistivity (Part - 5) - Electricity Video Lecture - Class 10

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so what does ohm slot all who was a
scientist who performed an experiment
and from his experimental observation he
found that as the potential difference
between the ends of a conductor
increases the current flowing through
the conductor also increases so he said
that the current flowing through a
conductor is directly proportional to
the potential difference applied between
the ends of the conductor provided
temperature remains constant so what he
actually did he set up a he had set up a
circuit and he kept on increasing the
potential difference between the ends of
the conductor and he kept measuring
currents for different values then he
plotted a graph where he took voltage
along the x-axis and current along the
y-axis and he found that the line which
was obtained was something like this
which showed that current is directly
proportional to the potential difference
so this means if you want to convert
this proportionality into an equality
you should in bring in picture of
constant so this current was equal to 1
by I into V this constant was introduced
so from this it was written as V is
equal to I at so V is equal to I I
became the mathematical expression of
Ohm's law where R is the resistance of
the conductor so resistance was a new
property which was introduced from here
so what is resistance let us look into
that knob
resistance as the name suggests it
resists the flow of current it opposes
the flow of current so
if you have any if you eat for example
we say that metals are good conductors
of electricity that itself means that
the resistance of metals are very less
so if the resistance is less the amount
of current flowing will be more if the
resistance is more than the current flow
will be less for example in case of
insulators there assistances mode
therefore the amount of current flowing
fluency lattices less right let us look
at this picture here if you see these
are the free electrons which are
carrying current now since there are so
many na atoms present inside a so this
electrons are facing obstructions
several number of times so therefore in
this case this is Lauren's resistance so
look at this picture which will give you
an idea of high resistance and low
resistance so if you look at this figure
here you see that the movement of this
free electron is little smooth because
it doesn't it is not colliding or it
does not hit getting hit by any other
atoms so here relatively the resistance
is less now if the resistance is less
the amount of current flowing will be
more whereas in this case the the free
electron is getting hit or getting
collided by so many atoms as a result
the resistance in this cases more so
whenever you have whole resistance the
current flow will be less and vice versa
right so now let us see what are the
factors on which resistance of a
material dependent the first one is
length of a conductor so greater the
length of a conductor more will be the
collision because as as you saw in our
last slide if the length of the
conductor is more if you see if this is
my conductor so if the length of the
conductor is more that means the free
electron will experience more and more
cohesion so more cohesion means more
resistance right so that means
resistance will increase with increasing
length so we can say that resistance is
directly
proportional to the length of the
conductor next one is cross-sectional
area of the conductor so similarly for
cross-sectional area also now if you
increase the cross-sectional area of the
conductor what will happen again let us
look at the slide let us suppose now if
I increase the cross-sectional area that
means this area if I increase I mean if
this width is increased what will happen
the number of coalition-building is
because the number of atoms will remain
the same if you increase the cross
sectional area that will not increase
the number of atoms the number of atoms
will remain the same but we are adding
up some extra space therefore the
collision be reduced because the free
electrons will get some extra space to
move about freely so the collision will
reduce so we can say that resistance is
inversely proportional to area if you
increase the area the resistance will
decrease it is something like this let
us suppose if this is the length of a
conductor in this length you have these
many atoms so that means the free
electron will collide with these many
atoms now if you increase the length of
the conductor so when you increase the
length of the conductor the number of
atoms will also increase right therefore
the number of collisions will increase
therefore resistance will increase now
let us suppose instead of increasing the
length if you increase the
cross-sectional area now increasing the
cross sectional area will not increase
the number of atoms so the number of
atoms are still the same but now this
free electron will experience lesser
condition so therefore the resistance
will decrease with increase in area the
third one is the nature of material of
the conductor so value of resistance is
also dependent on the nature of material
of a conductor next is temperature of
the conductor so how does it change with
temperature when the temperature
increases what happens for any object if
you increase the temperature that the
particles of that object will start
moving randomly so they will be set in
the motion so whenever there is random
motion there will be more collisions so
wherever you have more collision the
frequency of collision will also
increase
now if the frequency of collision
increases what will happen to your
resistance the resistance will also
increase with the increase in
temperature right so these are some of
the things on which the resistance of
the resistance depends so from this we
can see that the expression for
resistance can be written as Rho is
directly proportional to n by a because
it is directly proportional to length on
the inversely proportional to a again to
remove this proportionality we will
introduce a constant called Rho and what
is this row this row is known as
specific resistance what is Rho it is
known as specific resistance for
sometimes it is also known as
resistivity this specific resistance is
specific of a material that means the
value of Rho depends on the material of
the conductor right so the value of the
resistivity is specific to every
conductor so if we are talking about a
metal the value of resistivity will be
very low if we are talking about an
insulator the value of resistivity
builting very high so resistivity is a
constant which is often defined as
resistance of unit length and unit area
so that is how we define specific
resistance so it is something it is
constant for a specific material okay so
this is how we I mean we express
resistance of a conductor a resistance
is given as Rho into L by a now you will
you might ask that how does the
dependence of resistance on temperature
taken into account in this case well the
dependence of resistance on temperature
is not exactly taken into account in
this case however that has been observed
experimentally
that in case of metals or insulators or
semi
ters the resistance changes with change
in temperature so you will study about
the dependence of resistance on
temperature now in your higher classes
because that involves some other
concepts as well so we will talk about
that in in class 12 so for now you
should know that this is how we express
resistance and these are the factors on
which resistance of a substance depends
now we studied Ohm's law but do you
think that Ohm's law is a foolproof law
or there are certain limitations of
Ohm's law as well yes there are certain
limitations of Ohm's law Ohm's law is
not universally true for every object
when we look at the behavior of diodes
and not sure if you are aware of water
diodes they are also a kind of a
electronic element which allows flow of
current in one direction so diodes are
those devices which allow unidirectional
flow of current you will study about
diodes also in class 12 but for now you
should I mean this is just for an
information that when you plot the VI
characteristic that is the when you plot
the voltage current graph for diodes you
see that instead of a straight line you
get a curve like this so this shows that
Ohm's law is not true for values similar
entities of this compound called gallium
arsenide it is seen that the Ohm's law
Ohm's law does not hold true again for
water voltammeter it is seen that the
when the voltage this this graph is
often known as VI characteristic this is
all often known as the name of this plot
is VI characteristic so when when the VI
characteristic of water Volta method is
also plotted it is seen that it doesn't
follow Ohm's law so in case of water
voltammeter
it is seen that when you plot this
voltage versus current you get a graph
like this instead of a graph
this so the Ohm's law is not true in
this case also so that these are these
by some of the limitations of Ohm's law
that it was not true for everything
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