L3 : Direction of current - Electric Circuits, Physics, Class 12 Video Lecture

hello friends this video on electric
circuits part 3 is brought to you by
exam feel calm no move here from exam
please make sure that you have watched
part 1 and 2 before going ahead with
part 3 we will discuss the direction of
current flow in a bit more detail as I
told you that current always flow from
cathode to anode now in physics there is
no specific or no generalised statement
that cathode is the positive electrode
or cathode is the negative electrode
sometimes that word can behave as a
positive electrode sometimes that food
can even act as a negative electrode so
in physics we generally define the
current flow in terms of cathode and
anode we say that current will always
flow from cathode right no matter how no
matter that n is positive or negative so
how do we remember this here is a memory
tip for you for example this is your
battery so here you can see a group of
amps which are entering the battery so
that means the terminal of the battery
through which the ants enter that is
through which the currents enter is the
anode so a for ants and a for anode so
that means ants are entering inside the
battery and what are they doing they are
entering inside the battery and
forcefully pushing the cat to come out
of the battery from the other end so
that means the terminal of the battery
from which the cat comes out or the
current comes out is the cathode right
so there is no positive no specific
positive or negative let us not talk
about a specific positive and a specific
negative it is better to remember with
this way because when we talk of
charging and recharging charging and
discharging of a battery you again might
get confused that is why it is advisable
that you
remember it in this way that current
will always flow from cathode that is
the terminal of the battery from which
the current comes out is the cathode so
ants entering the battery that is a node
and that coming out of the battery that
is the cathode right so I hope this
memory tip will help you to remember
that current always flow from cathode to
anode right okay so now let us look at I
mean you must be wondering why is it so
confusing to talk about direction of
current flow from cathode to anode or
anode to cathode that's because when we
talk of charging of a cell and
discharging of a cell we see some weird
facts coming up which confuse people now
let us look at the charging and
discharging processes in case of a cell
so how how will we determine the
direction of current so we will take two
small examples but the universal
statement even now is correct always
flow from cathode to anode no matter
whatever that terminal mean so let us
first look at discharging of a cell what
do I mean by discharging of a say that
means you have a cell which you have
already charged before right so the same
has charge in it now you use that cell
in a circuit to provide EMF to the
circuit so to prove it now the cell is
charged now it will use it stored charge
to provide current in a circuit so let
us suppose I have a cell right this is
the battery say this is the positive
terminal and this is the negative
terminal okay so let us suppose that I
have connected this battery in a circuit
I have some external resistance I so how
will the current flow the current will
flow in this way
right so what is happening here the
current is flowing from the positive
terminal towards the negative terminal
so that means current is flowing from
cathode this is cathode to anode right
so what do you see here here your
cathode is positive but just now I told
you that sometimes it also happens that
cathode is not the positive terminal so
let us look at that scenario let us now
look at the charging of a cell what do
you mean by charging of a cell how do
you charge a cell you charge a cell with
the help of a battery charger so to
charge a cell you need a charger right
so what do you do you connect the
positive terminal of the cell to the
positive terminal of the battery charger
and you connect the negative terminal of
the cell to the negative terminal of the
battery charger so that is that is the
process of charging a cell now let us
suppose here instead of a battery
charger now here let let us suppose that
this cell was of 2 volts
it had an EMF of 2 volts now you must be
wondering what is EMF because I haven't
discussed that yet don't worry I will
talk about yem if in the next slide now
let us suppose I take the same cell of 2
volts now I take another bigger cell of
5 volts say this is the positive
terminal this is the negative terminal
this is the positive terminal and this
is the negative terminal now what do I
do I connect the positive terminal of
this 2 volt cell to the positive
terminal of the 5 volt cell and I
connect the negative terminal of the 2
volt cell to the negative terminal of
them 5 volt cell so now what will happen
how will the current flow in this case
so in this case what will happen since
this is a bigger cell right so therefore
this cell will dominate so the current
will flow from the positive terminal of
the cell and this current will go
to the cell and come out of the cell now
when you look at it when you look at
this look at this particular cell now
the two volt cell what is happening the
current is leaving the two volt cell at
this terminal and what did I tell you
that current always flows from cathode
that means the terminal at which the
current is leaving that terminal should
be cathode right so in this case what is
happening the cathode is the negative
terminal for the same cell so for the
same two volt cell here cathode was the
positive terminal from there the current
in was coming out in this case the
current is coming out of the negative
terminal why is it so because in this
case this two volt cell is not providing
current to the circuit this to hold cell
is getting charged with the help of the
current which is provided by the five
volt cell so in this case instead of the
battery charger I have used this bigger
cell that is the five volt cell
similarly when we use a battery charger
what do we do we connect the positive
terminal of the battery charger to the
positive terminal of the cell and the
negative terminal of the charger to the
negative terminal of the cell so in that
case the current flows from positive
terminal to the negative terminal
through the electrodes right so which is
opposite to what happens during
discharging right so you understood that
how the flow of current varies during
charging and discharging opposite this
is the reason why I told you that
because I have seen that ninety 90
percent of the students for the room
they memorize this fact that current
always flows from cathode to anode and
cathode is always the positive terminal
and anode is always the negative
terminal but this is not true always
because the exception to that always is
the phenomenon of the charging of a cell
so many people also learn that charging
of a cell is an exception to this but it
is better that instead of remembering it
in this way you understand it
this generalized form you understand it
in this way that current will always
flow from cathode to anode so the
terminal from which the current comes
out of the battery that is your platform
always whether it is discharging of a
cell or it is charging of a cell so
there is nothing like cathode has to be
positive always or cat hood has to be
negative always right so I hope I made
my point clear and also friends I would
like to tell you something which is very
important that do not get confused with
cathode anode in your chemistry with the
cathode anode which we study in physics
however cathode anode remains the same
in chemistry and physics but the way we
define a cathode and anode is different
in chemistry than in physics in
chemistry we define an electrode as an
as a cathode or anode depending on
whether which kind of reaction takes
place there whether it is an oxidation
reaction or a reduction reaction but in
physics we are not much bothered about
the reactions of the chemical reactions
which are taking place at the electrodes
we are bothered about electric current
so we define cathode and anode in terms
of electric current the terminal from
which the electric current comes out
that is the cathode and the other
terminal is the anode so so that was
another point so I hope that I am able
to explain you that what is how does
current flow in a cell right thank you
please visit exam fee or comm to watch
free educational videos try free online
tests get the best quality study
materials study from the best tutors and
mentors and much more thank you once
again