Hello, I'm Dr.

George and I'm going to be helping you with this lesson.

It's called Current Through an Electromagnet and it's part of the unit Magnets and Electromagnets.

The outcome of the lesson is I can plan and experiment to test the strength of an electromagnet and explain how different factors affect the strength of an electromagnet.

And I'll be reminding you what an electromagnet is first.

Here are the keywords for the lesson.

I'm not going to go through them all now because I'll introduce them as we go along, but if you need to come and check the meanings anytime, you can just come back to this slide.

Here are the parts of this lesson.

Factors affecting the strength of an electromagnet, increasing the strength of an electromagnet and a soft iron core.

Let's get started.

So how do you make an electromagnet? Well, really you just need a coil of wire to start with, and you could wind that around something that is round like a boiling tube, which you can usually find in a science lab.

And that now makes an electromagnet if a current flows in the coil.

And if current flows in this coil, there'll be a magnetic field around the coil and it will behave like a magnet.

If you increase the number of turns in the coil, it's how many times the wire wraps around, then you increase the strength of the electromagnet.

So on the right you have a stronger electromagnet because it has more turns.

And a stronger electromagnet can pick up more paperclips.

This electromagnet here might look a bit different, but it's still a coil of wire.

It's just wrapped around a different material, an iron rod.

So more turns pick up more paperclips because there's a stronger magnet.

If you spread the turns out more, you actually get a weaker electromagnet.

So in the second picture, we have the same number of turns, but because they're more spread out, that will be a weaker magnet.

In fact, there are several factors that you can change.

Several things about an electromagnet that you could change.

You could change the type of wire, we've already seen.

You could change the spacing of the coils, you could change the diameter of the coil, you could wrap the wire around something thinner or something wider.

You could change the core material.

This core is actually mostly made of air and also a bit of glass or whatever the boiling tube is made of.

And you could also change the size of the current in the coil.

So here are five different things that you could change if you were trying investigate what affects the strength of an electromagnet.

And now a question.

Which of the following factors will not affect the strength of an electromagnet made with a boiling tube? The diameter of the tube, the number of turns, the spacing of the turns or the length of the boiling tube.

I'll wait for five seconds, but if you need longer, press pause and then press play when you're ready with your answer.

And the correct answer is the length of the boiling tube.

It doesn't matter, it's just the spacing of the coils and the diameter of the coils.

As you can see here, we've made a coil just up one end of the boiling tube.

It doesn't matter how long the rest of the boiling tube is.

Well done if you pick that one out.

Now electromagnets behave like magnets.

So if you bring another magnet close to an electromagnet, the two magnets attract or repel depending on which way they're facing.

If like poles are facing, there'll be repulsion.

And if unlike poles are facing, there'll be attraction.

Here, we've actually got repulsion going on and the size of that force of repulsion depends on how strong the two magnets are.

Now, here's a way we could use that to investigate how current affects the strength of an electromagnets magnetic field.

And what we have here is an electric balance, something that normally measures mass, and we have an ordinary magnet just standing up on the balance.

And now we've clumped our electromagnet so that it's partly over the top of the magnet.

They're not actually touching, but the magnet is slightly going up inside the boiling tube.

And then with no current in the coil, we can reset the balance to zero.

We press the tare button and that sets it to say zero.

And then when we switch on the current in the electromagnet, and these two magnets repel, if we set it up so that they repel.

We'll actually have a way of measuring the magnetic force by how hard the magnet resting on the balance presses down onto the balance as the electromagnet repels it.

So this is a clever way of measuring in a way the size of the force between these two magnets.

And by doing that, we're indirectly measuring the strength of our electromagnet.

So if you were doing this experiment, you would change the current and try out different values of the current, and that would be the independent variable, variable, thing that changes or can change.

And the independent variable, the thing that you deliberately change and you choose the values of it.

And then you would look at the reading on the balance each time you'd record that.

And that's called the dependent variable, the variable that you'll measure to see how it's affected by the changes you make to the independent variable.

And there are other variables that could affect the strength of the electromagnet, and you don't want to confuse things by changing two or three different things at once.

So these variables, you actually wouldn't allow them to vary, you'd keep them constant.

They're called control variables.

So other things about the electromagnet, the number of turns in the coil, the diameter of the coil, the position of the coil where you've clamped it.

You wouldn't change them in this experiment because the thing you're changing is the current.

So these things that you don't change, they're called control variables.

And if you make sure that they stay the same, then you've made this a fair test.

So fair test is one way you only change one variable, the independent variable.

And now a question, what will happen if another magnet is brought close to the magnet on the balance from above as shown? So we're going to just bring down a magnet in the direction of the arrow.

If you need longer than five seconds to think, press pause and press play when you're ready.

And the correct answer is the reading will either increase or decrease.

I didn't say which way round this other magnet is.

It could end up repelling the magnet on the balance, pushing it down, in which case the reading on the balance will increase or it could attract the magnet that's on the balance pulling upwards on it, and that would make the reading decrease.

Well done if you realise that.

And now for a task, the equipment shown in the diagram can be used to test how the current through a coil affects the strength of an electromagnet as I've been telling you.

I'd like you to name the independent and dependent variables and list the control variables and explain how you would carry out a fair test.

Pause the video.

Take as long as you need to write down your answers.

And when you are ready, press play and I'll show you example answers.

Okay, let's look at some answers now.

The independent variable is the current in the coil.

That's the only correct answer here.

And the dependent variable is the force on the balance.

Although what you actually read is a reading on the balance which shows you a mass.

You know that if you press your hand down onto a balance, it will make the reading go up.

And so if the magnet gets pressed down, it will make that reading go up.

Now list the control variables and explain how you would carry out a fair test.

Control variables are the number of turns on the coil, the type of wire, the spacing of the turns in the coil, the diameter of the coil and the material inside the coil, which we call the core.

Any of these are things that you could change about the electromagnet, but you wouldn't want to change them because you've already decided on the one thing you're going to change, the current.

So all of these control variables should be kept the same during the experiment, and that's what makes sure it's a fair test.

So well done if you've got most or all of those points.

Let's move on to part two of this lesson.

Increasing the strength of an electromagnet.

So it turns out that when the current inner coil is increased, the force exerts on the other magnet also increases.

You get a graph actually something like this.

So as current increases, the force increases.

Now let's think about how a coil works as an electromagnet.

The loops of wire, the turns of wire are a bit like separate wires.

They are joined together, but each one of them is carrying a current.

So it's a little bit like what we have on the right.

Here, we have three separate wires carrying a current, and on the left, we have three turns carrying a current.

And what happens in both cases is having three instead of one increases the total current that is flowing around.

So a coil with one turn carrying three amps would have the same effect, the same magnetic effect as a coil with three turns where each turn carries one amp.

There's still only one amp in the coil on the right, but the total current going past any point is actually three lots of one amp.

So it's three amps.

And it's that total current that's important for deciding the strength of a magnetic field around the coil.

And now question about that.

Which wire or group of wires in these pictures causes the strongest magnetic field? And make sure you read the labels because they're important too.

Press pause if you need longer than five seconds.

And the correct answer is B.

Did you get that? It's not the one with the most wires, C, because in C, each wire carries one amp.

So there are actually five wires there, that's a total of five amps going up page.

But in B, we have three amps in three wires that adds up to nine amps.

And on the left, we just have four amps.

So B, causes the strongest magnetic field around it.

Now electromagnets are actually very useful and various different devices use them including an electric drill.

If you need to change the strength of the electromagnet, which is needed in some devices when they're on different settings, the easiest way to do that is to change the current in the coil.

Otherwise, if you wanted to change the coil itself, perhaps change the number of turns, you'd have to open up the device and replace the coil with a longer coil or a shorter coil, and that would be difficult to do.

So most appliances, most devices that use electromagnets change the field strength, change the magnet strength by changing the current.

Now, which of the following would not change the strength of an electromagnet? The colour of the coil wire, the current in the coil, the number of turns in the coil or the diameter of the coil.

If you need longer than five seconds, press pause and play when you're ready.

The correct answer, the colour of the coil wire wouldn't change the strength of an electromagnet.

I hope you pick that one out.

And now a written task for you.

I'd like you to list the factors that can change the strength of an electromagnet.

What things can you change to make the strength of the magnet change? And then I'd like to imagine you have a wire and you can only have a low current for some reason.

Maybe that's to do with the power supply you're using, but you want to make a strong electromagnet.

Explain how you could make a strong electromagnet using a wire that's only carrying a low current.

Take as long as you need, press pause while you're writing, and when you're ready, press play and I'll show you example answers.

Okay, here are some example answers.

The factors that can change the strength of an electromagnet.

Well, here are really all of them.

The current in the coil, the number of turns in the coil, the spacing of the turns in the coil, the diameter of the coil, and the material of the core.

If you remembered most or all of those, well done and now explain how a wire carrying a low current can be used to make a strong electromagnet.

The wire can be coiled around a core to make an electromagnet.

If many turns are wound, the low current adds up once for each turn and so will cause a strong magnetic field.

So increasing the number of turns, having a large number of turns in the coil can give you more strength in your electromagnet.

And also what you choose as a material for the core has an effect on the strength.

And let's talk more about the core of an electromagnet.

So this part is called a soft iron core.

So you've already seen that a coil of current carrying wire has a magnetic field around it.

And here's a picture representing that magnetic field.

These lines with the arrows on them are called magnetic field lines, and they're showing us the shape of the field.

If you add an iron core into the middle of the coil, which is just an ordinary piece of iron, an iron rod, that actually makes the magnetic field much stronger.

Can you see that in this picture? The magnetic field lines are drawn closer together than they were before.

This is representing a stronger field, and this electromagnet will be a stronger magnet.

What happens is the iron core gets magnetised itself, and this adds to, this strengthens the magnetic field that you're getting.

That's actually because iron is a magnetic material.

Now we call it a soft iron core, that doesn't mean that this iron is squashy or easy to shape.

It's a different meaning of soft.

It's made of pure iron.

And it turns out that pure iron is easily magnetised but easily loses its magnetism.

So when the current stops flowing in the electromagnet, the iron core immediately stops being magnetised.

And that's the meaning of soft here, something that easily loses its magnetism.

So pure iron is known as a soft magnetic material.

And that's useful because if you want your electromagnet to be a magnet that's easily switched on and off, then you don't want a core that hangs onto its magnetism after you've switched off.

If you use steel, it does that.

It stays magnetised even when you switch off the current.

Steel, by the way, is an alloy, a mixture of iron and carbon.

It's a magnetic material too because it's made mostly of iron.

If you have an iron core to strengthen your electromagnet, you can wind the wire directly around it if your wire is insulated, that would usually be by coating it with plastic.

And the insulation stops the current flowing into the core.

We actually don't want that.

Here, we have wire that's not insulated.

There's no plastic coating on it, and what will happen is the current will pass straight through the core.

So instead of going round and round the turns, which we want, because that gives us the stronger field, the current's just going to flow in a straight line, straight down the core.

And so you won't get a strong electromagnet.

Also, if the bare wire turns touch each other, that's also a problem because the current will take the shortest route through instead of going round each of the turns.

So here's a question.

Three electromagnets or they're supposed to be electromagnets.

The one on the left, A has a wooden core, B and C have an iron core, and C is an insulated wire.

The other two are not.

Which of the wire and core combinations shown will make an electromagnet that does not work? Pause if you need longer than five seconds to think and then press play.

And the one that does not work is B.

The other two will work, although one of them will work better than the other, but B has bare wire on an iron core.

The current won't go round and round the coils, they'll just go straight along the core.

And another question, similar but not the same, which of these wire and core combinations shown will make the strongest electromagnet? So we have two wooden cores in A and B, an iron core in C, and we have bare wire for A, an insulated wire for B and C.

So think about which will make the strongest electromagnet and press pause if you need longer than five seconds.

And the strongest electromagnet is C.

All of these will make an electromagnet, but in A and B, we have a wooden core.

Wood is not a magnetic material, so the wood doesn't strengthen the magnetic field.

C has an iron core and that strengthens the magnetic field of the coil.

Well done if you pick that out.

And now a longer written task giving you a chance to show what you've learned.

Explain how the materials of the core and the coil can make an electromagnet stronger.

Now, pause the video while you write your answer, and when you are ready, press play and I'll show you an example answer.

So here's an example answer.

The core of an electromagnet should be made of soft iron that will become magnetised.

This will add to the magnetic field and make the magnet stronger.

Soft iron should be used so that the electromagnet loses its magnetism when the current is switched off.

The coil should be made with insulated metal wire so that the current does not flow through the core, but goes through the turns around the core.

Now of course, your answer doesn't have to be in exactly the same words as that, but did you get the main ideas? Use a soft iron core because that becomes magnetised and makes the field stronger, and then it becomes unmagnetized, demagnetized when you switch off the current.

And also use insulated wire so that the current doesn't flow through the core instead of the turns.

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

So here's a summary.

An electromagnet can be made by coiling wire around a core and running current through the turns of the coil.

The factors that affect the strength of the electromagnet include the current in the coil, the number of turns in the coil, the length and diameter of the coil and the material of the core.

A soft iron core increases the strength of the electromagnet because it becomes magnetised.

Current flows through the coil, but not through the core.

So well done for working our way through this lesson about electromagnets.

I hope you enjoyed it, and I hope to see you again in the future lesson.

Bye for now.