- So, right over here depicted
a square loop of a conductor.
Let's say it's a wire and it's stationary
and it's sitting in a magnetic field.
And I've drawn a few vectors that represent
the magnetic field and you can see at least on
the surface that is defined or that is contoured
by the wire that the magnetic field looks constant.
So, if we just had this scenario
nothing too special going on
but it becomes interesting if I were to actually change
the flux going through the surface.
So, both of these pictures right over here
they actually show the same scenario
where we have increased the flux.
We have increased the flux at these points
on the surface defined by the wire at every point
the magnetic field has now gotten stronger.
So, we have increased the flux.
So, let me write that, the flux.
We use the Greek letter phi used to denote flux.
The flux of the magnetic field.
The flux of the magnetic field has gone up.
And we know from Faraday's law
that when you have a change in your flux
that that's going to induce a current in the loop.
And so an interesting question is
what direction is that current actually going to go in?
We have two options.
The current could go.
The current... Let me find a nice color for the current.
The current could go in the clockwise direction
or it could go in the counter clockwise direction.
So, which of these two do you think
the current will actually go with?
Well, let's think a little bit about it.
We know that a current flowing through
a wire actually on it's own will induce a magnetic field
above and beyond a magnet field that's already there.
So, let's think about the type of magnetic field
that this orange current would actually induce.
So, if it's going in the counter clockwise direction
remember we use the right hand rule.
I can take my right hand
point my thumb in the direction of the current.
So, let me see if I can do that.
So, my thumb in the direction of the current
and then my hands are going to loop
or I should say my fingers are going to loop
in the direction of the magnetic field.
So, when I do that with my right hand,
so my right hand looks something like that thumb
in the direction of the current right over here
we see that it shows the magnetic field being induced
that would wrap around like that.
Or if we were to show it, if we were to sample
it points right on the surface
the magnetic field that would be induced
would look something like this
and I'm just doing it at some sample points.
So, notice it would be additive
to the existing magnetic field.
In fact, it would increase the flux even more.
In fact, at these points these vectors
would increase even more.
These vectors, let me see if I can draw that.
These vectors would increase even more.
Well, what would that do?
If the flux increases even more
then the current is going to increase.
The current is going to increase even more
which is going to make the flux increase even more
and which makes the current increase even more
and you would have this never ending cycle
where the current keeps increasing,
the flux keeps increasing and you would have
this energy that's appearing out of nowhere
which would violate the law of conservation of energy.
And so that's a pretty good argument
for why you would not...
That's a pretty good argument for why we would
not expect the counter clockwise scenario.
We would not want the current that's been induced
to induce a magnetic field that goes in the
same direction as our increase in flux.
So, just by deductive reasoning we know
that this is going to be the scenario
and let's see if we think about what happens here.
We can do the right hand rule again.
We take our right hand
point our thumb in the direction of the current
and we see if we do that with our right hand.
Well, now this would induce a magnetic field
that would decrease the flux.
So, it would produce a magnetic field.
The current right over here would produce
a magnetic field that's going downward.
The current in that direction would also
when you take your right hand
and you were to put it along here.
Once again if you're going in the counter clockwise,
or sorry if you're going in the clockwise direction
over here that too when you do your
right hand rule right over here
your fingers would coil around that way.
And so once again when you look at the surface
it would produce, it would induce a magnetic field
that is going in that direction.
And so it would have the induced magnetic field
from the induced current will go against
the change in flux.
And this makes sense because we won't go into this
never ending positive feedback loop where
the current keeps getting strong and stronger
and the flux keeps increasing
and increasing and increasing.
And this idea that the orientation
of the current that is induced
will produce a magnetic field
that counter acts the change in flux.
This is Lenz's law.
Lenz's Law.
And once again if that wasn't true
then you would have a violation
of the conservation of energy.
So, in general if someone says okay well,
this is the change in flux
is happening in a certain direction.
Well, to think about which way
the current would flow you just have to say
well what type of a magnetic field
would each direction of the current produce?
And that magnetic field should go in a direction
that goes against your change in flux.
So, if the flux is increasing
the magnetic field that's induced
by the induced current should make the flux decrease.
If the flux was decreasing
then the induced magnetic field
by the induced current should make
the flux decrease less
or should be additive to the flux.