Lesson video

Hello, my name's Dr.

George, and this lesson is called The Generator Effect.

It's part of the unit Electromagnetism.

The outcome for the lesson is I can describe how an alternator works to generate AC, and how a dynamo works to generate DC.

And there may be several key terms in there that you don't know yet, but don't worry, you will learn about them during the lesson.

And here are all the key words for the lesson.

I'm not going to go through all the definitions now, I'll introduce them when they come up, but you can come back to this slide anytime to check the definitions.

There are three parts to the lesson called rotating a loop of wire, DC generator or dynamo, AC generator or alternator.

Let's get started on the first part.

If we take an ordinary piece of wire, and we move it in a particular way, in a magnetic field, something rather strange happens.

So here we're pushing this piece of wire upwards in a uniform magnetic field that's a magnetic field where the field lines are all evenly spaced and parallel.

And when we push this wire upwards, a potential is induced.

So what happens is that one end of the wire becomes positive and the other end becomes negative and it now has a potential across it in the same way that a battery has a potential across it.

And if we connected this wire to make it part of a complete circuit loop, there would be a current flowing because a potential will cause a current to flow, and we call it an induced current.

Induced means made to happen.

Something that you've brought about.

So you've made a current happen by moving this wire.

You can actually predict the direction of the current.

Which way will it flow along the wire using a rule called Fleming's Right Hand Rule.

You might have heard of Fleming's Left Hand Rule, which is used when you are predicting the direction of motion in the motor effect.

But here you need to use your right hand.

So what you do is you make your first two fingers and your thumb at right angles to each other.

Like this as I'm showing you now.

You need to point your first finger in the direction of the magnetic field, the direction that the field line arrows are pointing.

And you point your thumb in the direction of the movement here that's upwards.

And then your second finger is pointing in the direction of the current.

Now you can see already here there's only two possible directions for the current along this wire.

So your use of Fleming's rule needs to be clear enough that it shows you which of those two directions it is.

And in this case, the current is flowing from left to right.

To remember whether to use your right hand or your left hand for this, you could call it the generightor rule because this is all about predicting the direction of a current of electricity that you generate.

Now use Fleming's Right Hand Rule to find the direction of the current induced in the wire shown below.

And if you realise that you can't remember how to use it, go back to the previous slide on the video and remind yourself before you try the question.

On these short questions I'll wait for five seconds, but when you need longer, press pause and then press play when you're ready, The correct answer is right to left.

If you didn't get that right, you may want to go back and check how to use Fleming's Right Hand Rule.

Now instead of a straight piece of wire, we have a loop of wire and it's being turned in a uniform magnetic field.

We have these two magnets and they have unlike poles facing each other that gives us a uniform field.

And we are making this loop turn.

Perhaps it's got a handle attached and we're turning the handle.

If we look at the left hand side, it's being pushed down in this uniform magnetic field.

And that will induce a current in a particular direction depending on which faces north seeking and which is south seeking on these magnets.

But let's say the current comes towards you.

The right hand side is being pushed up.

The induced current on that side will be in the opposite direction if you use Fleming's Right Hand Rule and you switch the direction of movement you'll find that it switches the direction of the current.

So we get an induced current that's moving away from you.

And so what we're going to have is a current that simply goes around this loop.

There'll be a current in the back side of the wire going from right to left.

As the loop turns, if we keep on turning it, it'll then become vertical.

And now the movement of each side of the wire is in the same direction as the magnetic field lines.

The magnets in this picture look rather small, but let's imagine that they're large enough that the whole loop is in a uniform magnetic field.

Look for example, at the top side of the wire it's just going to be skimming along a field line at this moment not cutting through one.

And so when the wire doesn't cut across field lines, no current is induced.

While we're turning this loop at this moment, we stop getting any induced current.

And now a question.

Which of the following is the correct reason for current being induced in a rotating loop of wire in a uniform magnetic field? You'll need to refer to the diagram for this.

Pause the video if you need more than five seconds.

And the correct answer is B.

In side one current is induced in the opposite direction to side two.

And that's because those two sides are moving in opposite directions in the magnetic field.

And now here's a written task for you.

When a loop of wire is pushed round in a uniform magnetic field, a current is induced around the loop.

Use Fleming's Right Hand Rule to add arrows that show the direction of current inside A and B in each diagram and then explain why current is greatest.

When the coil is horizontal, take as long as you need, pause the video while you're writing and press play when you're finished.

Let's take a look at the answers.

So here are the current directions.

What you should find if you try to use Fleming's Right Hand Rule for the third diagram, you can't because your fingers need to be at right angles to each other.

But in the third diagram, the movement is actually parallel, not at right angles to the magnetic field direction.

So Fleming's Right Hand Rule can't be used, and in fact, there is no induced current.

Now to explain why current is greatest when the coil is horizontal, when the coil is horizontal and being pushed round, it is cutting across magnetic field lines at the greatest rate.

When the coil is vertical is not cutting across any field lines.

And in that middle picture you see it's cutting field lines, but it won't be doing it as quickly as in the first picture.

Now for the second part of this lesson, a DC generator or dynamo.

So this is a way that we can use this generator effect that we've been seeing.

Imagine taking a motor.

You might have built one like this if you've studied motors, and instead of connecting the motor to a power supply so that the coil turns, you're going to attach a handle and just rotate the coil of the motor yourself.

And what you have here is a rotating coil in a magnetic field of the two magnets.

And so you induce a potential and there's a potential induced across each loop of the coil.

And the induced potential across each loop adds together so that you get a much bigger potential across the ends of the whole coil.

So we'll get a bigger potential here than if we just had a single loop turning.

If you double the number of turns of wire on the coil, you'll double the induced potential across the ends of the coil.

So if you wanted a large potential, you would make sure there were lots of turns in the coil.

Now when a coil of 40 turns is rotated in a magnetic field, the induced potential is three volts.

What is the induced potential across a coil of 80 turns rotated at the same speed in the same magnetic field? Press pause if you need longer than five seconds.

The correct answer is C.

We've doubled the number of turns, but we've kept everything else the same.

Doubling the number of turns, doubles the induced potential, the two are directly proportional to each other.

If we take this same motor, the kind of motor that is often made at school.

And if we connect it to a circuit and then we rotate the coil, we'll get an induced current because the potential is what causes the current to flow.

Now take a look at what happens when this coil rotates.

Whichever way up the coil is, the right hand side as you look at it, is always moving upwards.

So these red curved arrows show the direction of rotation it's anti-clockwise here, and that right hand side always moves upwards.

When the coil is horizontal, the right hand side of the coil is always connected to the right hand connecting wire, the right hand contact that's sticking up touching the end of the coil.

And so current will always be pushed in the same direction because that side of the coil, it's always moving upwards.

It's always in the same magnetic field.

Magnetic field direction doesn't change.

And so you're always going to get the same current direction.

And this type of current that always goes in the same direction is called a direct current or DC for short.

Notice how the same device, the same piece of kit, can be a motor or a dynamo and it just depends how we are using it.

So if we connect the coil to a power supply, we are putting in current in a sense, and what we're getting out of it is movement, it's a motor.

But if we take the power supply away and instead we turn the coil, we are putting in the movement and what we get out of it is a current and it's a dynamo.

As the call turns around, the direction doesn't change, but the size of the current does change.

Here we get a medium current.

The coil is cutting magnetic field lines not at its highest rate, but it is cutting them.

Here we get no current because at this moment the way the coil is moving is not cutting through any magnetic field lines.

Now here on the right we get the largest current, we've got the highest rate of cutting through field lines.

So we have this current that varies in size.

So this type of generator, it makes a direct current because it's always going in the same direction, but it's a varying direct current.

And this type of generator is called a DC generator, but it has another name which is a dynamo.

Now which of these graphs show a direct current? Think about what the meaning of direct current was.

Press pause if you need longer than five seconds.

And A shows a direct current because the potential is always the same way around.

It's always a positive potential and that's going to always make current want to go in the same way round.

B is also going to cause direct current.

We have a potential that is always negative, it's always in the same direction.

So current will be in the same direction opposite from what it was in A.

In C this is a direct current because again, the potential is always in the same direction.

Here it's always positive.

So although the size of this current changes, it's all in the same direction and that's what counts as a direct current.

So well done if you spotted all three of those.

Now I'd like you to do a comparison.

Compare the output of a DC generator, what we get from it with the output of a battery.

And when you compare, it's a good idea to try to think of similarities and differences.

Take as long as you need.

Press pause and press play when you're ready.

And I'll show you things you could have mentioned Starting with common to both, potential across the contacts.

So we have a potential also called a potential difference between the two ends of the coil and between the two ends of the battery.

And they both push current around a circuit in one direction only.

So they're both causing a direct current.

Now the DC generator only produces a varying direct current, a current that changes in size.

And it only produces a current when it's being turned.

If we look at the battery, a fully charged battery, it produces an unchanging direct current, the same size current all the time, and it produces a current that all the time as it's part of a circuit.

You don't have to be moving it, you don't have to be doing anything with it it's just steady.

If you missed any of those, take a look and check that you understand them.

Now let's look at a different type of generator, an AC generator or alternator.

Here's a picture of a simple alternator, and at first glance it might look the same as the dynamo that we've just been discussing.

But there is a difference in the way the contacts touch the ends of the coil.

It's arranged so that as the generator turns, each side of the coil always connects with the same contact leading to the circuit.

Imagine we painted one side of the coil pink.

Let's say the side that's on the right at the moment.

The way this is arranged is that pink side will always be connected to one particular contact leading to the circuit and never to the other one.

I'll show you how that works.

So here we've just drawn in the rest of the circuit.

And one side of the coil is marked with a dot.

And right now because the coil is turning clockwise, the side with the dot is moving downwards through the magnetic field.

And we don't know which way the magnets are facing here, but let's say it generates a current displaying anticlockwise around the circuit, like this.

Now if we zoom in, we can follow the way the current goes between the coil and the contacts.

There are two rings, as you can see here on the rod that's attached to the coil.

And those rings are made of conducting material, they're made of metal.

Now if we look at the end of the coil, it's coming from the right hand side.

That wire passes under the first ring but isn't making an electrical contact with it it's insulated still at that point.

So it passes onto the first ring without making electrical contact, but then it touches to the second ring or in fact attached to the second ring where it's bare.

And so that wire coming from the right hand side of the coil is making electrical contact with the second ring.

That is making contact with the left hand contact leading to the circuit.

And as the coil spins, that dot side of the coil will only ever make contact with that second ring and that left hand contact to the circuit.

Half a turn later, the side of the coil with a dot is now on the other side and it's now moving upwards through the magnetic field.

And actually going to generate a current that flows clockwise around the circuit the other direction.

And I'll show you why.

So if you look again at the dot side of the coil, it's now on the left.

Its wire is as always passing through the first ring, but it's insulated.

So there's no electrical connection there.

And the end of that wire is into the second ring making contact and that's touching the left hand contact leading to the circuit.

So no contact here, but electrical contact here.

So again, the dot side of the coil is connected with the left hand contact leading to the circuit.

Now which contact in this picture is connected to the side of the coil marked with a dot? You'll need to look quite closely at the diagram.

Press pause if you need longer than five seconds.

Correct answer is contact two.

The side of the coil marked with a dot has its end wire sticking into the first ring where it's bare and that ring is touching contact two.

So current can flow between the dot side of the coil and the first ring, the inner ring and contact two leading to the circuit.

So well done if you've got that, this can be a little bit tricky and if you're finding it hard, you could always go back and watch the explanation again.

So what we see is every half turn, the current pushed around the circuit by this generator switches direction.

The current in the dot side of the coil switches direction each time it switches between moving up and down in the field.

Because it's always in contact with the same side of the circuit it makes the current in the circuit change direction.

And this type of current, a current that changes direction is called an alternating current or AC for short.

So current that goes in the same direction all the time is direct current, DC, and this is AC.

And so we call this an AC generator, but we can also call it an alternator and that should be quite easy to remember because an alternator generates alternating current.

Now which of these graphs shows alternating current? Press pause if you need more than five seconds, It's B, because only B has the current switching direction.

We have here a current that goes between positive and negative, and that represents current going in opposite directions.

In A and C, we have a varying size of current, but in A, it's always in the positive direction, it's always in one direction, and in C, it's always in the other direction.

So that still counts as a direct current.

Now a task for you, you're going to arrange these sentences in an order that explains how an AC generator that is connected to a circuit works.

Take as long as you need, press pause, and press play when you're finished.

And let's look at the correct order.

The coil is rotated in a magnetic field.

As one side of the coil moves down through the magnetic field, the other side moves up.

Current is induced around the rotating coil.

Every half turn he opposite side is moving down through the magnetic field.

The induced current changes direction through the coil.

An alternating current flows around the circuit.

That's not the only order that could work.

If you've chosen a different order, look carefully at yours and see if it's telling the same story as here.

And now we're at the end of the lesson.

So here's a summary of what it was about.

Rotating a loop of wire in a uniform magnetic field induces a potential across the ends of the loop.

An electrical generator can be made in the same way as an electric motor.

A DC generator, a dynamo, generates a varying direct current, DC, always in the same direction.

The connections in an AC generator, an alternator, are different to those of a dynamo.

An alternator makes an alternating current, AC.

That repeatedly swaps direction every half turn of the coil as it spins around.

Well done for working your way through this lesson and learning about the two types of generator.

I hope you've enjoyed it, and I hope to see you again in a future lesson.

Bye for now.