Potentiometer (Part - 3) - Physics, Class 12 Video Lecture

hello friends this video on potential
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not let us have a quick review on EMF
and internal resistance of a cell a cell
is nothing but a battery so when I talk
of EMF what is EMF the full form of EMF
is electro-motive force it is basically
a mismo misnomer it is not a force
actually it is the potential difference
between two electrodes when there is no
current in the cell that means this is
how the internal structure of a cell
looks like it consists of two electrodes
made up of two different metals one of
which act as a cathode the other one
acts as anode so electron flow happens
between these two electrodes and that
constitutes current so now the potential
difference which exists between these
two electrodes when there is no current
in the cell that means there is no
external circuit connected it is just an
open circuit there is no current flow
through the circuit still there is some
potential difference between these two
electrodes and that potential difference
there is known as EMF that is why every
cell whether you connect that cell to a
circuit or not but every send will have
an EMF that is because of the potential
difference between its electrodes it is
always present inside the cell right
when I talk of internal resistance even
inside the cell like how we connect
resistance externally to a circuit to
oppose current similarly even inside the
cell there is a resistance so what is
the purpose of this resistance it will
oppose the excess the flow of current
inside the cell as I said inside the
cell also there is some potential
difference between the two electrons
therefore there will be flow of charges
taking place inside the cell so there
has to be something which actually put
some check puts a check to the flow of
charges so for that there is an internal
resistance as well so that is why if you
look at the working of a cell in detail
it is seen that a lot of heat energy is
wasted inside
the cell itself that is because of this
resistance right so now we can write
that if e denotes the EMF of the cell so
we can say that E is equal to I into I
that means plus I so small are always
denotes the internal resistance of the
cell capital R is the external
resistance in the circuit now let us
suppose if this battery or this cell is
connected to a circuit so and there is
an external resistance capital R in the
circuit and if some current flows
through the circuit so what do we say we
say that EI EMF is equal to I into small
R plus capital R so this internal
resistance varies from cell to cell it
is not constant for all cells depending
on each cell it will have a different
value it is negligible when E is very
very greater than I I that means if this
EMF is very very greater then I add so
in that case we can at many times we
actually neglect the internal resistance
if the EMF of the cell is very high so
we can neglect redundant resistance let
us now also look at the difference
between EMF and terminal potential
difference this also I have discussed
before but still just to give a quick
review so as I mentioned just now eat
whenever we talk of the term EMF it is
always related to the open circuit that
means when there is no current drawn
from the said so in open circuit we talk
about put EMF that is the potential
difference between the two electrodes
right when I talk of a closed circuit
closed circuit means a current is
flowing through the circuit so the cell
is connected in a circuit and at the end
a current is flowing through them santak
so that potential difference between the
two ends is known as the terminal
potential difference of a cell
so when I talk of an open circuit so in
an open circuit we talked about the EMF
E whereas in case of a closed circuit
there is a current I flowing through the
circuit right so here in a in a closed
circuit there is some letter suppose
that there is some external resistance R
which is connected to the circuit so we
can say e is equal to I capital R plus
small R that is this is the EMF of the
cell so this will be equal to the
current flowing externally plus the
current flowing internally so that will
be equal to I R plus IR what is this I R
this is nothing but this capital a that
is the current flowing through the
external resistance that means the
current flowing through the external
circuit
so whenever a current flows through a
cell feed that means there is a
potential difference existing across
that circuit so here also since the
current flows through this external
circuit that means there is a potential
difference existing in that external
circuit so this potential difference is
known as the patek this is the potential
difference across the terminals of the
cell so you understand the difference
right when I talk of EMF it is the
potential difference between electrodes
of the cell it is something which is
internal to the cell when I talk of
terminal potential difference it is the
potential difference between the
terminals of the cell that means the two
terminals through which the end points
of the cell through which it is
connected to the external circuit so
this is the terminal potential
difference therefore we can say that
terminal potential difference we will be
denote by V and EMF we denote by e so
this is the relationship between
terminal potential difference and EMF so
looking at this we can see that terminal
potential difference is always less than
this
now how much less it is less than the
EMF that depends on the value of the
internal resistance so terminal
potential difference of a cell is less
than EMF of the cell but there is an
exception to that charging of a set
during the charging of the cell terminal
potential difference is greater than EMF
to know about it in detail you can refer
the videos on current electricity right
okay so I think with this you got a
review of EMF internal resistance and
terminal potential difference thank you
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