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 this lesson is I can describe how different factors affect the size and direction of an induced current or potential.

And don't worry if you don't know what an induced current or potential is 'cause I'm going to explain.

These are the key words for the lesson.

I'm not going to read out the definitions now because I'll introduce them as we go along, but if you want to come back to this slide, you can anytime to check the meanings.

The lesson has three parts, they're called induced current, induced potential, and a coil battery.

So let's start on the first part.

Now, it turns out that if we place a wire in a magnetic field, as shown here, and then pass an electric current through the wire, there'll be a force on the wire that pushes it up, and we call this the motor effect.

The word motor originally means movement.

But if we change things slightly and now push a wire through a magnetic field, we can generate a current through the wire as long as it's part of a complete circuit, as shown here, and we call this the generator effect.

So in both of these effects, there's a magnetic field present, but in the motor effect, we need to put a current through the wire and it moves.

In the generator effect, we move the wire and a current appears in the wire.

Now, the current may be very small, so to measure it, we might need to use a milliammeter that measures milliamps, thousandths of an amp, or we might even need a microammeter, which can measure microamps, millionths of an amp, but there will be a current there.

Now, any current that's generated in this way is called an induced current.

Induced means made to happen or brought into existence.

But a current is only induced when the moving wire crosses magnetic field lines, and that's going to happen here.

If we move this wire upwards, there will be magnetic field lines crossing it.

So notice that we're getting a current here, even though there's no battery and there's no power supply in this circuit.

Now here's a question to check if you're following along.

Which of the following is the most accurate definition of an induced current? So read these three and decide which of them is the best definition.

On short questions like this, I'll wait five seconds, but if you need longer, press pause and press play when you've decided your answer.

And the correct answer here is A, an induced current is a current that is caused to flow when a wire is moved through a magnetic field.

So here's that diagram again.

When we push the wire up in this magnetic field, we induce a current and it flows in one particular direction, the direction shown there.

Now, if we push the wire down, you can perhaps guess what might happen.

Is the current going to flow in the opposite direction? And it turns out, yes, it does.

If the wire doesn't move at all, there is no induced current, so just having a wire in a magnetic field doesn't cause a current to flow.

But if we do move the wire, the quicker we move it, the greater the induced current.

And now a short question for you.

Which of the following wires has the greatest induced current? The one that's moving quickly, the one that's moving slowly, or the one that's not moving? Press pause and press play when you've decided.

And the correct answer is the one that's moving quickly.

As I said, moving the wire faster induces a larger current.

Now, instead of moving the wire, we could also induce a current by moving the magnetic field.

How do we do that? Well, we can do that by moving a magnet.

And we can make that easier by coiling up the wire, and then we could just move the magnet in or out of the coil.

So what seems to matter is that the magnetic field lines are cutting through the wire, or the wire is cutting through the magnetic field lines, that's what seems to induce a current.

Now, if we push the magnet into the coil, we induce the current to flow in one direction, but then if we pull the magnet out of the coil, so we move it in the opposite direction, we induce a current in the opposite direction.

Another thing that affects the induced current is the speed of movement.

If we push the magnet into the coil at a higher speed, we get a greater induced current.

The induced current in the coil turns the coil into an electromagnet.

That's what an electromagnet is, a coil with a current flowing in the wire.

And it turns out that the coil then pushes back on the magnet as we try to push it in.

And the faster we move the magnet, the greater the force pushing back.

That actually means we have to do work to push the magnet in, we exert a force over a distance, and it's that energy that we put in that goes into the current.

So we're not getting something for nothing here.

And now, which of the following pairs of ammeters shows the induced current as a magnet is moved in and then out of a coil of wire? Press pause and press play when you've chosen your answer.

And the correct answer is B, that's the only pair that shows the current in one direction when the magnet is moving in and in the other direction when the magnet is moving out.

Well done if you picked that one.

And now a longer question.

A magnet is moved in and out of a coil of wire that is connected to a sensitive ammeter.

What happens to the size and direction of the induced current when the magnet is pushed in three times faster, a magnet that is two times stronger is pulled out at the same speed, and the magnet that is two times stronger is pushed in at two times the speed? You may not know the answers for sure, but think about what you've seen and use logic to deduce what might happen.

Press pause while you are deciding your answers and press play when you're ready.

And here are some example answers.

When the magnet is pushed in three times faster, it's going in again, so the current will be in the same direction.

But three times faster means that magnetic field lines will cut the wire at three times the rate, and that induces a current that's three times greater.

Now if the magnet's two times stronger, but pulled out at the same speed, the original speed, the induced current is two times greater because you're going to have field lines cutting the wire twice as often, but the induced current now is going to go in the opposite direction because we're pulling the magnet out, so it's moving in the opposite direction.

And if the magnet is two times stronger and it's pushed in at two times the speed, the induced current will be doubled by doubling the strength of the magnet, but doubled again by doubling the speed, so the induced current will be four times greater.

And if the magnet is going in, it will be in the same direction as the original current.

Well done if you've got some or all of those right.

And this kind of thinking is quite common in physics, where you use relationships to predict what's going to happen when something changes.

Now let's go on to the second part of the lesson, induced potential.

If we push a magnet into a coil of wire that this time is not connected to a circuit, we can't induce a current to flow because current can't flow unless there's a complete circuit.

But something does happen, we induce a potential also called a potential difference across the ends of the coil, and that means that one end of the coil becomes positive and the other becomes negative like the ends of a battery.

If we push the same magnet in at two times the speed, that actually increases the induced potential by two times, it doubles it.

And that's because the wire of a coil is cutting across magnetic field lines twice as quickly as before, so twice the number of field lines cutting the wires in the same amount of time.

What if we double the strength of the magnet? If it's two times stronger at the original speed, that also doubles the induced potential.

And again, it's because the wire of the coil is cutting magnetic field lines twice as quickly.

If the magnet is twice as strong, then there's half the distance between the field lines.

And now here's a question about that.

What would happen to the size of the induced potential if a magnet of three times the strength was pushed into a coil three times as quickly as before? Think about that.

Press pause while you're thinking and press play and I'll show you the answer.

The correct answer here is that it would be nine times greater because tripling the strength of the magnet triples the induced potential, but doing it three times as quickly triples the potential again.

And if you multiply something by three and then by three again, it's nine times greater than it was before.

Well done if you work that out.

Now, in a single coil of wire, what you have is one wire wrapped around in many turns, each connected loop we could call a turn.

And each turn of wire cuts across the magnetic field lines when a magnet is pushed into the coil.

So the induced potential across each turn adds up to give a total induced potential across the whole coil.

So if we double the number of turns of wire for the same sized coil, so we pack more turns into the same length, the same potential is induced across each turn as before, but because there are two times as many turns along the same length of the coil, the induced potential across the coil is doubled.

So, what would happen to the size of the induced potential if a magnet of two times the strength was pushed into a coil of wire of the same size as before, but with twice the number of turns? Press pause while you're thinking and press play when you have your answer ready.

And as we've seen, if we double the strength of the magnet, we double the induced potential.

But if we double the number of turns in the same length of coil, we also double the induced potential.

And if you double something twice, then you multiply it by four.

So the induced potential is four times greater than before.

Now here's a longer task for you.

Explain the difference in induced potential when a magnet is pulled out of a coil of wire compared to when it is pushed into a coil.

And the new coil is the same size, has two times as many turns, and the magnet is moved at two times the speed.

So it's illustrated in the two pictures here.

So not just describing the difference in potential, although it may be helpful to start with that, but explaining it as well.

So press pause, take as long as you need to write your answer, and press play when you are ready.

So here's one way that you could write your answer.

The induced potential will be in the opposite direction because the magnet is moving in the opposite direction.

Having two times the number of turns on the coil will double the size of the induced potential, and moving the magnet at twice the speed will double it.

Again, overall the induced potential is four times greater and in the opposite direction.

So well done if your answer included those same points.

And now onto the last part of this lesson, a coil battery.

Now, we've already seen that if we move a magnet through a coil, we induce a potential across the coil.

So during that time, the coil is actually behaving like a battery.

And we could connect it into a circuit, for example, with a bulb, and it will induce a current in the circuit when we move the magnet.

And we can measure the induced current with an ammeter and the induced potential with a voltmeter.

So really this is just a very simple series circuit containing the coil and a bulb, but where also making measurements with an ammeter and a voltmeter.

If we add a second bulb into the circuit, we change the circuit, but we haven't changed the coil or the magnet, and we're not going to change the speed we move the magnet either.

The induced potential is the same in both circuits, it has nothing to do with what's connected to the coil, it's just about the coil itself and the magnet and the way we move it.

But in the second circuit here with the extra bulb, the induced current will be different.

In fact, it will be smaller because the bulb adds more resistance to the circuit.

And that's true with batteries as well.

A battery has a voltage written on it, its potential.

And it doesn't matter what circuit we put the battery in, that doesn't change, but the current that we get depends on what is in the circuit.

So now which of the following statements is not correct? So read these four, press pause while you do, and press play when you've chosen your answer.

The only statement here which is not correct is B.

It says, induced potential depends on the circuit the coil is connected to, but it doesn't in the same way as the potential across a battery does not depend on the circuit that you connect the battery into.

It's worth taking a look as well at the other statements and seeing why they are correct.

So a says, induced potential depends on the movement of a magnet on the coil used.

And we know that's true, induced potential is determined by the rate of field lines that are cutting across the wires of the coil.

C and D say that the induced current depends on the movement of the magnet and on the circuit, and that's true, because the potential across the coil is determined by the movement of the magnet, and it's the potential that determines the current, is potential that drives a current in a circuit.

Now, here's a task for you.

An induced potential is generated across the ends of a coil of wire by pushing a magnet into the coil.

Compare this coil to a battery used to push a similar sized current around an identical circuit.

And remember, if you're asked to compare, it's useful to think of aspects that are the same and aspects that are different between the two.

So press pause while you write your answer and press play when you're finished.

Let's take a look at an example answer.

There is a potential or voltage across the ends of the coil and across the end of the battery.

They both push current round the circuit.

The battery pushes current continuously, but the coil only does this when the magnet is moving into it.

When the magnet is pulled out, the current is pushed in the opposite direction.

Changing the circuit in both cases would not change the potential, but it would change the current.

So well done if you made a lot of those same points.

Let's take a look at a summary of the lesson now.

A magnet moving relative to a wire in a circuit can generate a current around the circuit.

This is an induced current.

When a magnet is moving relative to a coil of wire, it can cause an induced potential across the ends of the coil, even if it is not in a circuit.

The induced potential is proportional to the speed of the magnet into or out of the coil, the number of turns per centimetre on the coil, and the strength of the magnet.

The size of the induced current around a circuit depends on the size of the induced potential and on the resistance of the circuit.

Now, you may be wondering how much you care about this funny little effect.

So okay, if you have a coil and you move a magnet in or out of the coil, you get an induced potential, an induced current if it's part of a circuit.

But so what? But it turns out that life today absolutely depends on the generator effect because we use this effect to generate most of our electricity.

Whenever you plug anything in, you are benefiting from the generator effect.

And now that you understand it, you are ready to learn about generators and how they work.

So well done for working through this lesson.

I hope to see you again in a future lesson.

Bye for now.