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- So, right over here depicted

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a square loop of a conductor.

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Let's say it's a wire and it's stationary

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and it's sitting in a magnetic field.

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And I've drawn a few
vectors that represent

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the magnetic field and
you can see at least on

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the surface that is defined
or that is contoured

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by the wire that the magnetic
field looks constant.

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So, if we just had this scenario

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nothing too special going on

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but it becomes interesting
if I were to actually change

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the flux going through the surface.

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So, both of these pictures right over here

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they actually show the same scenario

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where we have increased the flux.

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We have increased the flux at these points

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on the surface defined by
the wire at every point

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the magnetic field has
now gotten stronger.

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So, we have increased the flux.

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So, let me write that, the flux.

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We use the Greek letter
phi used to denote flux.

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The flux of the magnetic field.

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The flux of the magnetic
field has gone up.

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And we know from Faraday's law

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that when you have a change in your flux

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that that's going to induce
a current in the loop.

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And so an interesting question is

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what direction is that current
actually going to go in?

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We have two options.

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The current could go.

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The current... Let me find a
nice color for the current.

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The current could go in
the clockwise direction

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or it could go in the
counter clockwise direction.

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So, which of these two do you think

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the current will actually go with?

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Well, let's think a little bit about it.

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We know that a current flowing through

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a wire actually on it's own
will induce a magnetic field

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above and beyond a magnet
field that's already there.

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So, let's think about the
type of magnetic field

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that this orange current
would actually induce.

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So, if it's going in the
counter clockwise direction

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remember we use the right hand rule.

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I can take my right hand

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point my thumb in the
direction of the current.

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So, let me see if I can do that.

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So, my thumb in the
direction of the current

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and then my hands are going to loop

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or I should say my
fingers are going to loop

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in the direction of the magnetic field.

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So, when I do that with my right hand,

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so my right hand looks
something like that thumb

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in the direction of the
current right over here

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we see that it shows the
magnetic field being induced

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that would wrap around like that.

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Or if we were to show
it, if we were to sample

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it points right on the surface

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the magnetic field that would be induced

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would look something like this

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and I'm just doing it
at some sample points.

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So, notice it would be additive

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to the existing magnetic field.

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In fact, it would increase
the flux even more.

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In fact, at these points these vectors

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would increase even more.

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These vectors, let me
see if I can draw that.

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These vectors would increase even more.

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Well, what would that do?

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If the flux increases even more

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then the current is going to increase.

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The current is going to increase even more

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which is going to make the
flux increase even more

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and which makes the
current increase even more

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and you would have this never ending cycle

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where the current keeps increasing,

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the flux keeps increasing
and you would have

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this energy that's
appearing out of nowhere

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which would violate the law
of conservation of energy.

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And so that's a pretty good argument

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for why you would not...

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That's a pretty good
argument for why we would

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not expect the counter clockwise scenario.

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We would not want the
current that's been induced

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to induce a magnetic
field that goes in the

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same direction as our increase in flux.

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So, just by deductive reasoning we know

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that this is going to be the scenario

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and let's see if we think
about what happens here.

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We can do the right hand rule again.

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We take our right hand

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point our thumb in the
direction of the current

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point our thumb in the
direction of the current

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and we see if we do that
with our right hand.

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Well, now this would
induce a magnetic field

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that would decrease the flux.

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So, it would produce a magnetic field.

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The current right over here would produce

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a magnetic field that's going downward.

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The current in that direction would also

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when you take your right hand

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and you were to put it along here.

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Once again if you're going
in the counter clockwise,

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or sorry if you're going
in the clockwise direction

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over here that too when you do your

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right hand rule right over here

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your fingers would coil around that way.

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And so once again when
you look at the surface

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it would produce, it would
induce a magnetic field

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that is going in that direction.

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And so it would have the
induced magnetic field

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from the induced current will go against

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the change in flux.

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And this makes sense because
we won't go into this

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never ending positive feedback loop where

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the current keeps getting
strong and stronger

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and the flux keeps increasing

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and increasing and increasing.

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And this idea that the orientation

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of the current that is induced

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will produce a magnetic field

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that counter acts the change in flux.

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This is Lenz's law.

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Lenz's Law.

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And once again if that wasn't true

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then you would have a violation

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of the conservation of energy.

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So, in general if someone says okay well,

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this is the change in flux

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is happening in a certain direction.

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Well, to think about which way

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the current would flow
you just have to say

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well what type of a magnetic field

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would each direction
of the current produce?

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And that magnetic field
should go in a direction

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that goes against your change in flux.

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So, if the flux is increasing

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the magnetic field that's induced

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by the induced current should
make the flux decrease.

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If the flux was decreasing

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then the induced magnetic field

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by the induced current should make

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the flux decrease less

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or should be additive to the flux.