L5 : EMF of cell - Electric Circuits, Physics, Class 12 Video Lecture

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please make sure that you have watched
all the videos till part 4 before going
ahead with part 5 this week now what
happens to this EMF when you when the
cell is connected to an external circuit
as soon as the cell is connected to an
external circuit there is a current flow
as I desire there is some voltage drop
across the resistor present in the
circuit so because of that voltage drop
the potential difference between these
two electrodes will also reduce now we
will study about that in our next slide
that what happens when this what happens
to the EMF when the cell is connected to
an external circuit
but for now you understood what is EMF
right so now is the turn to talk about
internal resistance again just in the
previous slide I told you what is
internal resistance now I will tell you
how does internal resistance play a role
in case of a cell so now let us suppose
that I connected to the external circuit
let us say I have connected it to an
external resistor i I connected the two
electrodes so what will happen current
will flow some current will start
flowing through this resistance so now
what will be the direction of the
current so the current will flow in this
direction I am showing the direction of
conventional current this is the
direction of in electronic current so we
talk about conventional current right so
the current is flowing from cathode to
anode right so what will happen let us
see what is the role that internal
resistance will play here now let us say
I have a closed circuit right so in this
case what will happen to the potential
difference between the two electrons let
initially when it was an open circuit
the potential difference between the two
electrons was this
II so now what will be the potential
difference the potential difference will
be V 1 plus V 2 that is the initial
footage initially whatever potential
difference was there that potential
difference will be there but there will
be some potential drop across the
internal resistance I mean as I told you
that this electrode and the electrolyte
will offer some resistance when the
current flows through it
till now we assumed that there was no
current flow so when there was no
current flow there will be no resistance
offered by the electrolyte but now when
there is current flow
this internal electrodes and electrolyte
will offer some resistance so let us say
that in that internal resistances R so
what will be the voltage drop across
this internal resistance that will be I
into R right so that means your earlier
potential difference between the two
electrodes will get reduced by the
potential drop or the voltage drop
across the internal resistance when
current flows that means your potential
difference between the two electrodes
will now become EMF minus the voltage
drop across the internal resistance now
what is this V this V is nothing but the
potential difference between these two
electrodes that is the potential
difference these two points so basically
this capital V is the voltage drop
across the external resistance at if you
look at this circuit these two points
denote the voltage across that is the
voltage drop across the resistance
capital R as well as the potential
difference between these two electrodes
in a closed circuit so we can also write
this capital v as I into capital R is
equal to e minus I smaller right so you
understood the ro and which internal
resistance plays in a circuit so
internal resistance will not play
internal resistance will not play any
role as long as you are talking about an
open circuit because in an open circuit
you don't I mean there is no current
flowing
at all so when there is no current
flowing at all
there will be no resistance as well
because when current is not flowing who
will resist the flow of current right so
that time the potential difference
between the two electrodes is nothing
but the EMF of that cell so EMF of any
sin is already defined whenever you are
given the same you are told that this
cell is of 6 volts that means 6 volt is
the EMF of that cell but when you
connect that cell in a circuit then the
potential difference between the two
electrodes decreases now again there is
an exception to this it is not that
always the potential difference will
decrease whenever you connect a cell in
a flow circuit the exception here is
again the charging of a cell during
charging of a cell the potential
difference between the two electrodes
becomes more than the EMF of the cell
but in all other cases the potential
difference in a closed circuit between
the electrodes is lesser than the EMF I
hope it is clear to you ok so the
internal resistance is something which
is a property specific to any kind of
cell so it depends if van is constrained
to cell because the internal resistance
depends on the type of electrolyte and
the type of electrodes we are using
right so it has to be different for
different cells it is negligible when e
is very very greater than I R so if you
look at this expression your internal
resistance will be negligible when this
EMF is very very greater than the
voltage drop across the internal
resistance so if the EMF of the cell is
say 500 volts and when that 500 volt
cell is connected in a circuit the
voltage drop across its internal
resistances point 0 0 0 0 0 1 volt in
that case we can neglect
internal resistance because the voltage
drop across that internal resistance is
very less so internal resistance
basically plays a role in the fact that
there is a voltage drop across the
internal resistance when the cell is
connected in a circuit or when there is
a current flow in the circuit thank you
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