Hans christian ørsted, a Danish
scientist had discovered that when a
conductor carries current, it produces a
magnetic field around it. And we have
studied about the properties of the
magnetic field that a current carrying
conductor produces around it.
Now as you can see in the picture, the
current-carrying wires have a certain
magnetic field around them which has
been depicted by the magnetic field
Lines. Now the direction of the magnetic
field lines can be obtained with the
help of the right-hand thumb rule as you
will recall. Now let us see an
interesting video in order to find out
the behavior of this current carrying
wire in the presence of another magnetic
field.
In this video you will find that a wire
has been placed in between the poles of
a another magnet.
Now this magnet, as you can see is a
horseshoe shaped magnet and it is an
external magnet.
So we can say that this magnet has been
kept and a wire has been kept in between
its poles.
Now notice what happens when we allow
current to flow through the wire.
The moment current will flow through
this wire, it will have a magnetic field
around it which the current will generate.
Now there will be a certain behavior of
the wire in the presence of the external
magnetic field.
As you can see, there is some sort of an
invisible force that is acting on the
wire causing it to get displaced from
its original position.
You will find if we reverse the
direction of current, the direction of
force is also reversed.
So initially we found that it got
deflected to the left hand side.
Now it gets deflected to the right hand
side. So where is this force coming from?
Let us try to explain it with the
concepts of physics and magnetic field
Lines.
So initially when we had simply kept the
wire in between the poles of this magnet,
we found that there was no force that
acted on the conductor,
Why? because initially we had considered
no current is flowing through the conductor.
Now let us see what happens when current
starts flowing through the conductor. As you
can see the moment current starts
flowing indicated by the highlighted
wires, there is a force that acts on the
conductor causing it to get deflected to
the left hand side.
A similar thing happens if we reverse
the direction of current but in the
opposite direction.
So over here we have reversed the
direction of current and as we can see
the force is now acting on the conductor
in the opposite direction. So initially
the force acting on the conductor took
it inwards or inside the magnet.
Now the force acting on the conductor is
bringing it outside the magnet.
So now let us try to explain with the
help of magnetic field lines, how this
force is exactly acting on the conductor.
Firstly  we consider the magnet. Now this
magnet will obviously have magnetic
field lines which will be moving from
north to south.
So as you can see I have marked out the
magnetic field lines from north to south.
In a similar manner if we consider the
current carrying conductor,
it will also have a magnetic field
around it given by the right-hand thumb
rule.
So over here as you can see the
direction of current is
in this manner,
all through the wire and back to the
battery. So if I apply the right hand
thumb rule, my thumb will indicate the
direction of current that is this
direction and my fingers  encircling
the wire or the conductor will give me
the direction of magnetic field lines. So
with the help of blue arrows,
I have marked out the direction of
magnetic field lines. So we find that the
current carrying conductor creates a
magnetic field around it.
Now notice what happens when the current
carrying conductor is kept in between
the poles of the magnet. Under such a
situation the two magnetic fields, one of
the external magnet and one of the
current carrying conductor interact with
one another.
Now you will notice that at this
particular point,
the two fields are in the same direction
that is the field due to the external
magnet as well as due to the current
carrying conductor and at this point they
are in opposite direction.
So when the field lines are in the same
direction it means that there is the presence
of an excess amount of force than the
case where the field lines are in
opposite directions. At that point the
force cancel out one another. Or in other
words the force due to the external
magnet cancels out the one due to the
current carrying conductor.
So this means that there is an addition
of field due to co-directional magnetic
lines on this particular side and there
is a cancellation of field due to
opposite direction lines on this
particular side. So this means the force
that will act on the wire will be from
this point to this point. As you can see
that due to the current carrying wire, the
magnetic field of the permanent magnet
or the external magnet have been
stretched like a rubber band. And it is
due to this stretching that there is an
interaction taking place between both
the magnetic fields. And this interaction
is causing the force to act from this
side to this side because over here
the field lines are co-directional
whereas on this particular side they are
in the opposite direction.
Now if the current carrying conductor is
placed parallel to the external magnetic
field,
no force will act on it. In the previous
case we saw that if this was the
magnetic field
from north to south, the current carrying
conductor was placed perpendicular to
the external magnetic field.
However if the current carrying
conductor is placed parallel to the
magnetic field, then no force will act on
the conductor.
Why? Because in the previous case where
the two were perpendicular,
in that case there was a scope for the
addition of magnetic field and the
cancellation of magnetic field because
they were in the same plane. Or in other
words there was a time when they were in
the same direction and in the opposite
Direction. But if this wire is kept
parallel then you will find that the
magnetic field generated around the wire
will not be in the same plane as that of
the external magnetic field.
So this will imply that there is no
scope for addition neither cancellation.
So it is due to this reason that there
will be no interaction between the two
magnetic fields and thus no force will
act on the wire.
So this means that there will be no
displacement of the conductor
if the conductor is placed parallel to
the external magnetic field.
It has also been experimentally found
out that the displacement of the conductor
is maximum or the magnitude of the force
is highest
when the direction of current is at
right angles to the direction of
magnetic field. Or in other words if the
current carrying conductor and the
external magnetic field are placed at
right angles or perpendicular to one
another, under that situation
the force is maximum because the
interaction between these two fields
will be maximum.
 So this leads us to a very important rule
known as lemmings left-hand rule.
Now the scientists Fleming had given
this rule in order to find out in which
direction the force will act. As you saw
in the previous cases certain elaborate
experiments were carried out in order to
determine in which direction the force
acts, With the help of this simple rule
we can also find out the direction in
which force acts and we do not need to
carry out any elaborate experiments.
So let us say we have the magnetic field
in this particular direction and we
place our index finger pointing towards
that direction.
We place our middle finger pointing in a
direction at which current is flowing
and if we placed the thumb perpendicular
to both the index finger as well as the
middle finger,
we will find that the direction of the
thumb gives us the direction of force
acting on the conductor.
So if I place my fingers in this manner
with the index finger denoting magnetic
field direction and middle finger
denoting the current direction then the
thumb will denote the direction of force.
So as you can see firstly we have to
stretch the fore finger, middle finger
and thumb of the left hand in this
manner such that they are perpendicular
to one another. In that situation if the
forefinger points in the direction of
magnetic field and the middle finger
points in the direction of current then
the thumb will give us the direction of
force.
So let us demonstrate this rule.
Over here we can see the current is
flowing in this manner all the way
through and back to the battery.
So now let us apply Fleming's left-hand
rule in order to demonstrate whether the
force is indeed proper. So over here
current which is the middle finger close
in this direction and the magnetic field
is from bottom to top.
So this gives us the thumb pointing
inwards. So as you can see the conductor
is getting  displaced inwards.
Similarly in this case current is
flowing in the opposite direction or as
you can see from here to here. So I place
the index finger in the same manner,
Why? Because the direction of magnetic
field does not change and I place my
middle finger in the direction of
current that has reversed and is from
this end to this end.
So now if I stretch out my thumb I will
find that the thumb is stretched out in
the outward direction and as you can see
the direction of displacement of this
particular conductor is also in the
outward direction.
Thus we found out that Fleming's left
hand rule gives us the direction of
force acting on the current carrying
conductor very easily.
