EMF : Electro Motive Force - Current Electricity Video Lecture - Class 12

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ahead with part 3 we will try to define
EMF that is electro-motive force EMF is
nothing but the maximum work done per
unit charge to take a charge from one
point to another
because electro-motive force is nothing
but potential right it is super take
whatever towards potential if you look
at our previous chapter where we talked
about potential potential difference and
work we saw that work per unit charge is
nothing but potential difference so that
is why work per unit charge to take a
charge from one point to another is
known as EMF so alternatively it can
also be defined as the maximum potential
difference between electrodes of the
cell in an open circuit this is very
important what is an open circuit and
what is a closed circuit are you aware
of that okay let me tell it quickly let
us suppose if you have a circuit which
is a circuit like this is called a
closed circuit so current always flows
in a closed circuit closed circuit means
you are strict if the switch is on so
there is a path throughout so the
circuit is closed and current can flow
when I talk of open circuit that means
it is open somewhere and if it is open
somewhere that means that somewhere
doesn't have any conductive path so
current cannot flow through an open
circuit right so when I talk about EMF
it is basically the difference in
potential between the positive electrode
and the negative electrode when it is
not connected to a circuit that means
when it is in an open circuit open
circuit system so when I talk of EMF
that means the circuit is open so there
is no current which is drawn from this
cell so now as I told that
is not a force but it is a potential
difference so how is EMF different from
potential difference because we often
talk of potential difference between two
points in a circuit you often say that
the potential difference across this
resistor is two volts and things like
that right so how does EMF vary from
potential difference the most important
point here is when I talk of EMF I
always refer to an open circuit that
means EMF basically represents the
maximum potential difference between
electrodes in open circuit because once
the circuit becomes closed in that case
what happens the circuit will also draw
some current from that cell from the
battery itself therefore its potential
difference will reduce so the potential
difference between its electrodes will
decrease right so when we talk of EMF
EMF talks about the maximum potential
difference between the electrodes so
that potential difference will be
maximum only when no current is drawn or
no current flows through the circuit
that means when we are talking about an
open circuit whereas when I talk of
potential difference
I don't precisely talk about potential
difference in case of a cell or in case
of a resistor it is basically the
difference in potential between any two
points in a closed circuit so potential
difference comes into picture when we
talk about the closed circuit EMF is
always for an open circuit so EMF is
always the potential difference between
the electrodes of the cell whereas
potential difference is nothing but the
difference in potential between any two
points at random
EMF also represents the source of
electric current because we talk of EMF
only for the battery or the cell right
so it which is nothing but the source of
the current whereas potential difference
is not at all related to source of
current it is just measured between any
two points in the circuit EMF is
independent of resistance
in a circuit where as potential
difference is affected by resistance in
a circuit what do I mean by that for
example let me draw a simple circuit let
us suppose we have a cell here right now
the circuit is open okay so this is an
open circuit and the difference between
the potential of these two electrodes is
nothing but EMF the circuit is open so
there will be no current flow in this
circuit now even if I add a resistance
in this circuit if I add two or three
resistances in the circuit that really
doesn't matter to this EMF the EMF of
the cell will still remain the same
because adding resistances to this
circuit doesn't make any difference to
the potential difference between these
two electrodes because the circuit is
anyways open so even though there are
resistances in the circuit there is no
current flow taking place across the
circuit whereas when we talk about
potential difference in case of a closed
circuit what happens let us consider a
similar circuit but the only difference
now would be the switch is now on that
is the circuit is closed so now this is
the switch which is closed now so the
current flows through this circuit so
there is a voltage drop which will take
place across this resistance right let
us suppose if I consider these two
points a and B so what will happen there
will be voltage there will be a voltage
drop across this resistance let us
suppose that voltage drop as V so when I
talk of the potential difference between
points a and B it is nothing but the
voltage drop across the resistance R so
that means if we add more resistances
more voltage drop will take place across
the resistances so the potential
difference between two points will get
affected due to the addition of
resistance
and the circuit right so when I talk
about potential difference it is it gets
affected by resistance in the circuit
because it is a closed circuit and there
is flow of current in a closed circuit
because of which drop of voltage takes
place across the resistance whereas when
I talk of EMF EMF is the potential
difference between the electrodes of the
cell in an open circuit so in open
circuit there is no current flow so it
really doesn't matter how many
resistances are present in the circuit
it even doesn't matter whether
resistance is there or not right so EMF
is concerned only about the cell it has
nothing to do with what is happening
outside whereas potential difference is
related to the entire closed circuit it
can be measured between any two points
in that closed circuit so it gets
affected when some resistance is added
or the value of resistance is increased
or decreased in a circuit thank you
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