Video Transcript
In this lesson, we get to learn
about one of the most important and useful discoveries in history of physics. This is the discovery of the
phenomenon of electromagnetic induction. Electromagnetic induction is useful
in cell phone chargers, laptops, headphones, CD players, cooking, guitar pickups,
heating, welding, energy generating, transformers, graphics tablets, flow metres on
pipes, and electrocranial stimulation. Electromagnetic induction is just
about anywhere you look in any field or any industry.
And yet the idea behind
electromagnetic induction is really pretty simple. Say that we had a loop of wire and
a permanent magnet. At the moment, there’s no current
running through this loop of wire. That’s because there is no
electromotive force that could push charge around the loop. But let’s say that anyway we put a
device for measuring current called an ammeter into the loop just to see when
current is running through it.
Our ammeter — our current
measurement device — has a scale. And we see that at the moment the
needle on the scale is pointed to zero, no current flowing through the loop. That’s not too surprising of course
because we don’t have a cell or a battery or anything else to power current in our
loop. Let’s say though that we then take
our permanent magnet and we move it through the loop of the wire. If we did that, we might notice
very briefly a flicker in our ammeter. But then with the magnet at rest,
we see the ammeter needle is at zero. We do almost start to doubt
ourselves. Did we actually see the ammeter
reading change? Let’s try that again.
This time we move the magnet
through the loop in the opposite direction. This time ever so briefly we see
the needle deflect again, but in the opposite direction. Once more, when the magnet comes to
rest, the needle goes to zero. At this point, maybe, we can’t
explain what’s going on. But it definitely seems like
something is happening in terms of the current in this loop of wire when the magnet
passes through it. So we keep trying passing the
magnet back and forth through the loop one way and the other way and seeing how the
ammeter responds. Each time we do it, we notice the
needle on the ammeter flicker then come back to zero. Eventually, we’re able to make a
couple of conclusions which we can jot down in our lab notebook.
Our first observation is that when
the magnet moves through the loop, current flows in the loop. That seems completely odd. But it is what we’re seeing over
and over. The second thing we’ve noticed is
that when the magnet is not moving, no current flows to the loop. And lastly, we notice that when the
magnet moves in the opposite direction through the loop, so does the current in the
loop. What we’re uncovering here through
these three observations is that when our magnet moves through this loop, it induces
a current. And apparently, there’s something
about the motion of the magnet that’s very important. It’s only when the magnet is
moving, we’ve noticed that current is actually induced in the loop.
Let’s think for a moment about our
magnet. The way we’ve drawn it, we just see
the north and south pole of this permanent magnet. But we know that there’s more to
the story. In particular, that every magnet
creates a magnetic field around itself. That magnetic field looks something
like this. We show it using field lines that
point from the north pole to the south pole. We can recall from earlier that
this magnetic field is strongest where the field lines are most dense; that is,
where they’re closest together. That would be near the poles, here
and here. And then, as the field lines become
less dense, that indicates a weaker overall field strength.
So every time that we move our
magnet through this loop of wire, it’s not just the magnet that moving through, but
it’s also the field. And since the field of the magnet
has a different strength here as compared to here or here or here or farther out on
this line, we can say that as the magnet moves through the loop, the loop
experiences a changing magnetic field. Another way to say this is that if
we were to look at the cross-sectional area of our loop of wire, then as the magnet
moved through the loop, the magnetic field across that area would be changing. And apparently, based on our
observation, when that happens current flows through the loop. That is current is induced in the
loop of wire.
Here is one way we could write out
a concluding thought on this experiment then. We can say that when the magnetic
field through a loop of wire changes, current flows; that is, is induced in the
loop. This statement we’ve developed here
it turns out is a great summary of what electromagnetic induction means. It says that when we have a closed
loop of wire when the magnetic field through that loop of wire is changing, then
current is created or induced in the wire.
Now, this is really
interesting. And there are a lot of ways to
explore this idea further. There are plenty of changes that we
could make to our experimental setup to see what effect those changes have. For example, so far, we’ve just
used a single loop of wire. But what if we used lots of loops
of wire and pass the magnet through those? Or what if we used a stronger
magnet and passed that back and forth through the loops? Or what if we made the size of the
loops in our wire bigger or smaller? All these changes we’re talking
about will create changes in the amount of current that’s induced in our loop.
We can summarise those changes by
expanding a bit on what we’ve said here about electromagnetic induction. We find that the greater the change
in magnetic field through the total loop area, the more current is induced. So for example, when we talked
about adding more loops to our single loop of wire, whereas with a single loop, our
total loop area would be this area, when we add more loops, our total loop area will
go up because now all this area is included in our loop calculation. Or consider the question of using a
stronger magnet. In that case, we have a stronger
magnetic field.
And therefore, the change in the
magnetic field through the loop area — whatever that loop area is — would be greater
overall and therefore more current would be induced. And we can see that when it comes
to changing the loop size, either making them bigger or smaller, making them bigger
would increase the total loop area, therefore inducing more current. And making them smaller would
decrease that area, decreasing the current induced.
One last thing about this
experimental setup, we saw that when we have the magnet oriented the way it is with
the north pole to the right, then when we pass this magnet through the loops, the
needle on our ammeter move to the right. That movement in the direction of
what’s called positive current indicates a certain direction of the current as it
flows around the loop, either clockwise or counterclockwise. If we then flip the magnet around
so that now the north pole points to the left and passed it in this orientation
through the loops of wire, then we would see the needle deflect in the opposite
direction. In other words, current is induced
in the loops running the opposite way as before. We could say that this change in
the magnet orientation implies a change in the magnetic field through the total loop
area. In that sense, this statement here
really does summarise what we’ve seen in terms of the phenomena of this
experiment.
Now, so far, we’ve had a stationary
coil of wire and a magnet and therefore a magnetic field which is in motion through
it. That’s been the way that we
generated a change in the magnetic field through a total loop area. But there’s another way to create
this type of change. Let’s say we had this. What this is is a magnetic field —
we’ll call it B — that’s pointed out of the screen straight at us. We’ll say that this field has a
constant strength, that it has the same magnitude everywhere. Imagine then that into this field,
we place this U-shaped track. And we’ll say that this track is
made of a conductive material. It could be a wire.
We can see that as it is there’s no
way for current to flow along this track because it’s not a closed loop. But what if we take a straight wire
and we lay it across the track like this? In this case, thanks to this wire
across the track, we now do have a closed loop. It’s right here that current could
flow. But of course, in order for current
to flow, we need something to push it along, some electromotive force. Okay, let’s think creatively
here.
Electromagnetic induction tells us
that a change in the total magnetic field through a conducting loop induces a
current. We don’t have here a changing
magnetic field. The magnetic field in this case is
constant. But we could still change the
overall magnetic field experienced by this loop. How could we do that? We could do it by changing the area
of the loop. And we’ll do that by putting our
wire in motion along the track. Think about it. At first, this is the area of the
loop that we’re working with. But then as our wire moves along,
we’re adding area to our conducting loop.
So whereas before, the total
magnetic field passing through our loop was contained within this area, now since
our loop is bigger we’re enclosing more total magnetic field. In essence, the magnetic field is
changing through our loop. That’s because the loop area is
changing. Described in other way, we could
say that this magnetic field has a certain strength per unit area. As the area we’re considering grows
then, we’re adding more and more magnetic field strength together for the total
field strength. That means the total field strength
is changing as area grows.
What all this means is that if we
really did have this uniform magnetic field — this conducting track and the wire
across it in motion along the track — then as the wire moved, electrical current
would actually be induced around this closed loop. And here’s something interesting:
the faster that our wire moved along, the more current would be induced in the
loop. What we’re seeing overall is that
there are two important components to inducing current in a loop of wire. One is the magnetic field going
through the loop and the other is the area of the loop. What we’ve seen is that if either
one of these two things is changing, that’s enough to induce a current in the
loop.
Like we said, it’s a change in the
total magnetic field through a conducting loop that induces a current. And it doesn’t matter how that
change occurs whether by altering the field or altering the loop area. Now, let’s get a bit of practice
with electromagnetic induction through an example.
The diagram shows a permanent
magnet being moved through a loop of copper wire. This motion induces an electric
current of 0.5 amperes in the wire. If the magnet is moved through the
loop at half the speed, what will the current in the loop be? If the permanent magnet is changed
for one that is twice as strong and moves through the loop at the original speed,
what will the current in the loop be?
Alright, looking over our diagram,
we see our loop of copper wire, a conducting material, and the permanent magnet
which is moving through it. We’re told that when this happens,
when the wire moves through the loop, it induces a current of 0.5 amperes in the
wire. This motion of the magnet moving
through the loop occurs at some speed. We can call it 𝑆, even though it’s
not labelled in our diagram.
Our first question asks if we
change nothing about our setup, except the speed with which we move the magnet; we
make it half as big as it was before, then what will happen to the current induced
in the loop? To start figuring this out, it will
be helpful to draw in the magnetic field lines that show the magnetic field created
by this magnet. That field and the field lines
representing it look something like this. So initially, we move this magnet
and its magnetic field through the loop at the speed we’re calling 𝑆.
That means that the total magnetic
field through this loop is changing while the magnet moves. And the rate of that change — the
speed with which it occurs—has to do with the speed 𝑆. The higher 𝑆 is, the faster the
magnet is moving and therefore the faster the magnetic field through the loop is
changing. And this change is the mechanism
that induces current in the wire. The rate at which that change
occurs directly corresponds with the amount of current induced. In other words, the faster the
magnetic field through the loop is changing, the more current will be induced in the
loop.
That bit is important because we’re
told that the modification in this first question is that we no longer move the
magnet with our speed 𝑆. The original speed will be moving
at half that speed. Since we’re moving the magnet
relatively more slowly, that means the magnetic field experienced by the loop will
change more slowly. When that rate of change of
magnetic field through the loop goes down, so will be induced current.
We don’t know exactly what the
current will be when we move our magnet at half the original speed. But we just know that it will be
less than the original amount of 0.5 amperes. We’ll write that down as our
answer. And the explanation for it like we
saw is that the rate of change of the magnetic field through our conducting loop is
decreasing relative to what it was originally. Less change means less induced
current, which means that whatever the current is will be less than 0.5 amperes.
In part two of our question, we
asked if the permanent magnet is changed out for one that’s twice as strong, but
moves through the loop at the original speed — what we’re calling 𝑆 — what will the
current in the loop be. If we were to double the strength
of this magnet and therefore the strength of its magnetic field, while keeping the
motion of the magnet the same, then the question is what effect will that have on
the rate of change of the magnetic field through this loop.
With the field of the magnet being
stronger overall, that means we could expect the change in magnetic field strength
from moving from one pole of the magnet to the other to be greater than it was
before. That means if we pass this magnet
entirely through the loop, the change in magnetic field experienced by the loop
would increase. That increase will lead to an
increase in induced electric current.
Just like before, we can’t say
exactly what the current will be in this modified case. But we do expect that it will be
greater than what it was before, greater than 0.5 amperes. And we write that down as our
answer because we’ve seen that the rate at which the magnetic field through the loop
changes is increased in this case. And we expect that increase to
increase the current induced.
Let’s take a moment now to
summarise what we learnt about electromagnetic induction.
In this lesson, we saw what this
big long term electromagnetic induction means. Electromagnetic induction occurs
when a changing magnetic field through a conducting loop creates or induces current
in the loop. We saw there are different ways
this can happen. One is by taking a permanent magnet
and moving it through a stationary loop of wire. Another way is to set up a constant
magnetic field and then set up a conducting loop whose area changes over time. In both of these scenarios, we saw
that overall there is a change in the magnetic field through the area of the loop
and therefore an induced current in it.
Additionally, we saw that the
greater the rate of change of total magnetic field strength through a loop, the more
current is induced. And we saw this worked the opposite
way as well. The smaller that rate of change,
the less current is induced. These are the basic ideas behind
electromagnetic induction, a phenomenon so useful and so widespread we see it all
over in our everyday life.
