Electromagnetic Induction
Students, this is one of the deep topics of physics
which has very deep applications at different places,
but before understanding this,
we will see
What is Galvanometer?
Students, you will see there is a one simple instrument
one device. What this does is
let me tell you
You can connect this device to any electric circuit.
 
suppose, you connect it.
This is the symbol for it,
this makes a circle from it, and named as G.
Now, you have connected it with one electric circuit.
maybe there is some current flow from the electric circuit
If the current flow from that electric circuit
then what happens
as you can see there is a one needle
This needle will deflect
if current flow like this
then the needle deflects like this.
Normally it is straight, deflect like this.
If you connect it with any circuit
and current flow like this
then you will understand that needle deflects like this
and after the connection, if it doesn't flow any current
then what happens?
then you will understand that needle keep straight like this.
So that is the main thing about this instrument.
so see that, the work of this instrument
It will tell you that the current is flowing or not
If you have connected it and if the current does not flow from it then needle doesn't deflect
and if the current is flowing then it will deflect.
so we can write as,
Galvanometer tells us if the current is flowing or not.
Besides that, it also tells us
if the current flow is flowing then in which direction it flows? isn't it.
Look here, from these two it describes the direction of current
so we can write, Galvanometer also tells us a direction of the current.
Now, let's see What is Electromagnetic Induction?
So, students, we take one coil-like this
there is one conducting wire, which we wind like this
you can also think of it as a solenoid.
Now at both ends of it,
between them, we connect the galvanometer like this.
You can understand that right now neddle of the galvanometer is straight,
because there is no current is flowing from it.
Now, we take one magnet.
it has two poles, a north pole, and a south pole.
and right now the magnet is on rest.
also, look at the needle.
There is no current is flowing,
but, you can see one amazing thing,
when I move this magnet.
By moving this magnet, you can see that the needle will start deflecting.
I will show you,
as we move the magnet here, the needle deflects.
What's the meaning of it?
It means that when the magnet move here,
then the current is flowing from this circuit,
that's why the needle deflects.
The deflection of the needle describes that current is flowing from it.
once again see, right now the magnet is on rest,
so the needle is not deflecting so there is no current.
But when you move it here,
the needle start deflects means the current is flowing from it.
Why it happens, students? why?
I will tell you, just wait for it,
few more things to be observed here.
Now students, let's move this magnet in this direction,
now, what's happening student?
now the needle is deflecting in other direction,
means the current is still flowing but in a different direction.
have you understood it?
Students, one more thing we can do,
now invert this magnet.
so its south pole comes here and north pole comes here.
now you can see all things are inverted.
If you move magnet here, then the needle deflect there,
now move magnet there then the needle deflects here.
Means the point is,
whenever the magnet moves,
the current starts flowing in the loop.
And the direction can also be changed depending on what are you doing here.
So, first of all, we will write these observations,
after that, we will see the reason behind it.
So let me write these students,
when the magnet moves,
current flows in the coil.
The direction of the current can be reversed
By changing the direction of motion of the magnet
or by changing its polarity.
Right students, this is what we see.
Now we will see that, why does it happen?
So you can see,
the magnets will have its field lines.
all the mystery is hidden in these field lines.
Now, these field lines are on the coil
also, cross the coil.
So we mark out the region of the coil with little circle mark,
because we have to focus on it.
Now see, when I move this magnet here,
you have to observe what happens to the region of the coil,
and how the field lines changed.
So we move the coil here, it's moving
now see here,
Is the magnetic field lines are changing in this region?
Is the magnetic field changing in this region?
obviously, it's changing.
Let's try once again.
Watch it, when it moves here,
in this region, the magnetic field changes
filed lines move ahead.
Means, in that region
the value of the magnetic field changes in reference to the time.
When you move it,
then the magnetic field lines changed in that region.
right, students.
What happened is,
There is a law in physics,
When a magnetic field changes with time through a coil,
then current flow in that loop.
are you understand it?
If in any loop or in any coil,
because of any reason the magnetic field changed,
then current start flows in that coil.
That's why the current flow in this coil.
So when you move it,
the magnetic field changes in this coil,
that's why the current flows.
When you move it far from it,
see, is that the magnetic field is still changing?
Yes, obliviously it changing.
That's why the current flow at that time.
I hope you understand this.
So, students, we can write it as,
when magnetic field
changes with time through a coil,
current flows in that coil.
This phenomenon is called Electromagnetic Induction.
In short EMI.
So, students, this phenomenon is called Electromagnetic Induction.
In short EMI.
EMI
E for Electro
M for Magnetic
I for Induction.
I hope you get this.
Now, I tell you one more interesting thing about it,
see,
this time I fix the magnets.
it's on a rest. I will not move the magnet.
This is a coil,
supposed that I move this coil here,
then you tell me what happens?
What do you think?
What will happen, if I move this coil here?
I did not move the magnet
even if in this coil, magnetic field changes?
See, we move it from here
then still magnetic field changes.
So the needle is deflected and current flow in this case.
Do you understand?
So if you rest the magnet and move the coil,
it still flows the current
because still the field still changes
you can also try by doing it reverse,
if you move it here, the field still changes.
so the current is also flowing in this case.
So, students, the point is,
If you move any from these two,
the magnetic field changed from this coil in both cases.
So, in both cases
the current flows from it.
So, students, we can write it as,
Current flows through the coil,
either move magnet or the coil itself.
right,
I am sure students,
you understand the electromagnetic induction which I explained to you.
You will understand it.
If there is any doubt in your mind, then rewind the video and look back.
Now I explain the one more example for electromagnetic induction,
Once again we have that coil with a galvanometer,
and here, we take one more coil
but in this case, we are connecting one battery,
one switch.
Now the switch is open,
current is not flowing from it.
here, a very interesting observation is to be done,
now watch, focus on this needle
What I m doing is, turn ON the switch.
So what happens
I turn ON the switch.
You observed what happens?
The needle is deflected,
but it backs to come to zero.
Once again I do it,
turn it ON,
can you observe that the needle is deflected and come back to zero?
What was the meaning of it?
It deflects means current flows,
and came back to zero, means current is zero.
It means that when you turn ON the switch,
the moment when you turn ON it, current flows from it
and then it becomes back to zero.
That's why the needle back to zero.
What happens here?
First of all, why the current flow and after that why it become zero?
Did you understand what I want to say?
See, I describe you,
Suppose the switch is OFF right now,
again I make the same situation.
There is our switch,
if the switch is ON, current flows here
because of that current, there are magnetic field lines available
which also overpass the coil
right, now the thing is to understand
that when the switch is OFF,
there is no magnetic field available.
As you turn ON the switch,
the magnetic field arrived.
So you can see that the magnetic field changed from coil
It is zero in starting, and then it gets higher.
so from it, the magnetic field changed.
so as you turn ON the switch,
the magnetic field changed.
according to the concept of electromagnetic induction,
current should flow because the magnetic field changed,
that's why the needle is deflects
Now why it does not deflect and came back to zero?
Because you keep the switch ON,
the field remains the same
current keeps flowing and field remains the same.
So the magnetic field on the region of the coil,
the magnetic field is the same as it goes higher,
it does not change.
also, keep in mind that you are not moving anything,
so the magnetic field which you can see in the coil, it remains the same.
It means it does not change.
so if magnetic fields do not change, then current will not flow from it.
that's why needle came back to zero.
The electromagnetic induction says that,
if the magnetic field changed from the loop,
then current will flow.
If the magnetic field is only there in the loop but it does not change, then current will not flow.
it should be changed, rising or falling
then current will flow.
So when you turn ON the switch,
there is no magnetic field and then suddenly it arrives
means it changes that's why current flow.
But if you keep it ON,
it does not change and remains the same.
that's why current is not flowing.
I hope you understand this.
Now it might be reversed,
supposed that, if I turn OFF the switch,
means opened it,
say as turn OFF from ON
then what happens?
In this case, the needle will deflect and then back to zero.
In this case, the deflection is in the opposite direction.
You will understand why it happens,
There is seen where the magnetic field is available,
as you turn OFF the switch,
magnetic filed goes away.
so there is a change in a magnetic field.
Earlier it's there and now it is zero, so it is changed.
so the current will flow because of electromagnetic induction,
and the needle will deflect.
But after that, the needle will again back to zero,
because after that you keep it turn OFF,
so there is no changed happens
and the needle remains on zero and there is no current is flowing.
So, I hope you understand this student,
If you don't understand then try a little bit harder,
sometimes student gets confused in this,
but actually, it is the same as this.
Now I write it
move it a little bit
so that we have space,
when switch is turned ON, current flows for a moment.
right,
when switch is turned OFF
then also current flows for a moment but in opposite direction.
right.
 
 
 
 
 
 
 
