Hello, I'm Dr.

de Mello and I'll be guiding through your learning today.

Today's lesson is about the thickness of a wire and it comes from the resistance and parallel circuits unit.

Let's begin.

The outcome that we are looking at today is, I can describe how a battery pushes current through wires of a different thicknesses.

These are today's keywords.

The first keyword is atom.

An atom is the smallest particle that can exist as an element.

We'll be looking at atoms and how they behave in metals.

Atoms also have inside them electrons.

Electrons are small negative charges.

The next set of keywords are metal ion.

A metal ion is formed when an atom looses an electron.

Next we have resistance and resistance tells us how hard it is for current to pass through a component.

Finally, we have proportional, which is used to relate two factors.

When two factors are proportional, they will change as a multiple of each other.

The multiple will stay the same.

These are the keywords with their full definitions.

If you like, pause the video now and read through them, but do look out for them in the rest of the lesson.

There are two parts to thickness of a wire lesson.

The first part is current through wires of different thicknesses and in the second part, we look at a model for electric current.

So let's start with current through wires of different thicknesses.

Copper wire is usually used for most electrical devices, mainly because it's a very good conductor of electricity.

We don't often see the copper metal wires, because they're normally covered with insulation to stop us getting electric shocks.

You can see the outer white plastic insulation.

These also insulation on the inner wires in blue, brown and green and yellow.

These are colour coded to tell people what the wires are connected to.

The copper wires themselves are on the inside, protected in this case by two layers on insulation.

Let's do a check for understanding.

Why are metal wires carrying mains electricity covered in coloured plastic? The choices are A, to enable the electricity to flow faster, B, to prevent heat loss, C, to prevent electric shocks and D, to help electricians identify the wires.

Pause the video now, make your choice and then come back to check your answers.

Welcome back.

If you chose to prevent electric shocks, that's correct.

If you also chose to help electricians identify the wires, that's also correct.

The plastic is an insulator and won't help electricity to flow faster.

Also, the wires should not get hot, so answer B, to prevent heat loss, isn't applicable.

Well done if you got those right.

Copper wires can be used in many devices and can be found in many different thicknesses.

Here's a bunch of copper wires ready for recycling.

You can see at the top right there's some very fine, very thin wires.

On the bottom right you can see a medium wire all on its own.

And you can also see some thick wires on the left.

They'll have been used in different applications.

Inside the metal itself, when an electron leaves an isolated copper atom, it leaves behind a positively charged metal copper ion.

In a metal, the electrons do not fully leave the atoms, but stay relatively close.

In the copper wire, there are copper ions with electrons that are loosely held.

You can see the electrons as these small blue dots.

The electrons can move from one ion to another and between them and the copper ions are shown over here in the large spheres.

Note that this model doesn't show the charges for simplicity.

The copper ions remain stuck in position.

It's only the electrons that can move.

Let's try a check for understanding.

What charge will a copper atom have if it loses an electron? Will it be A, positive, B, negative or C, have no charge i.

e.

neutral.

Pause the video now, make your choice and come back to check your answer.

Welcome back.

If you chose positive, that's correct.

The copper ions lose the negative electrons and so they become positive.

Well done if you got that right.

If a battery is connected across the wire, the ends of the wire become charged and the electrons drift through the wire, moving from ion to ion.

So on the right, we have the negative end of the wire and on the left we have the positive end of the wire.

Connecting the battery will cause the charge to flow and only the electrons can flow.

So this sea of electrons can drift through the wire from the negative to the positive end of the battery, causing a current.

Here's a check for understanding.

Which of these causes current to flow in a metal? Is it A, the drifting sea of electrons, B, the moving ions or C, the atoms carrying their electrons.

Pause the video now, make your choice and then come back to check your answer.

Welcome back.

If you chose the drifting sea of electrons, that's correct, well done.

Only the electrons can move and they drift in between the ions.

The ions are stuck in place, so they don't move and the atoms don't actually move carrying their electrons.

Metals, such as silver, copper and gold are the best conductors and allow the sea of electrons to move most easily.

This is due to the way that the metal atoms are arranged, how tightly the electrons are held and how pure the metal is.

You can see these samples over here look very pure and they'll be very good conductors, however, silver and gold are very expensive.

Some metals, such as constantan, nichrome or steel are also good conductors of electricity, but they're not as good as silver, gold or copper.

We usually use these metals when investigating resistance.

So we often use constantan or nichrome wires since their resistance is high enough to measure more easily.

It'll also be cheaper.

Here's a check for understanding.

Which of these wires, with the same thickness, shape and length, has the highest resistance? The choices are A, the gold wire, B, a constantan wire, C, silver wire of D, copper wire.

Pause the video now, make a choice and then come back to see how you've done.

Welcome back, if you chose a constantan wire, that's correct.

Constantan has a relatively high resistance compared to the other three materials.

Well done if you got that right.

These two copper wires are the same length.

The thicker wire has space for more copper atoms and so it has more electrons that can carry a greater current.

So the thicker wire will have a lower resistance.

Here's a true or false check for understanding.

In these two wires shown on the right, the thinner wire has a smaller resistance.

Is this true or false? Pause the video now, make a choice and then come back to see how you did.

Welcome back.

If you chose false, that's correct.

The thinner wire does not have a smaller resistance.

Now is the chance to justify your answer.

The choices are, A, there is less metal for the charge to move through or B, there are fewer metal ions in the way of the charge.

Read the statements carefully, examine the wires, pause the video and then come back to check your answer.

Welcome back.

If you chose there is less metal for the charge to move through, that's the correct answer.

Well done if you got that right.

We've come to the end of this section.

Here's a practise task to see how you've done.

You're going to test how much current can flow through different thicknesses of wire.

You're going to use three thicknesses of nichrome wire.

Use an ammeter to measure the current through 1.

0 metres of each wire.

Measure the current through each wire three times and calculate the mean current through each one.

You put your answers in a table like shown, you'll then describe what you found and then you'll also describe how you ensured your results were accurate and reliable.

Pause the video now and then come back when you finished your investigation.

Welcome back.

Here's some example results.

We have three wires, a thin wire, a medium wire and a thick wire.

Wires are usually classified with an SWG, a swage number.

So the thin wire was SWG 30, the medium wire was SWG 36 and the thick wire was 38.

The currents were as follows.

In the thin wire, there were 0.

477, 0.

472, 0.

481, which gives a mean of 0.

477.

In the medium wire, there were 0.

179, 0.

180, 0.

175, which gives a mean of 0.

178.

In the thick wire, there were 0.

111, 0.

055, 0.

113.

The 0.

055 measurement seems to be anomalous, so we'll put a line through it and not use it to calculate the mean.

This gives a mean of 0.

112, using the two good results.

Describing what we found.

The results showed that the thinner the wire was, the lower the current.

Thinner wires have a higher resistance to the flow of current.

I ensured my results were accurate and reliable by taking repeat readings and measuring the current to three decimal places.

I looked for anomalous measurements and did not use them in the calculation for the mean.

When measuring out the length of the wire to one metre, I ensured the wire was stretched out straight and the crocodile clips were exactly at the 0.

0 centimetre and 100.

0 centimetre points.

To avoid the wire overheating, I switched on the current, took the reading and immediately switched it off.

Well done if you completed that practical and got similar results and similar descriptions.

We'll now start the next section, a model for electric current.

We cannot see electrons flowing in wires, so we use different models to help us understand electricity.

In this model, electrons are large spheres orbiting a nucleus.

In this model, the electrons are spread around the metal ions and are free to drift.

Remember, we called them a sea of electrons.

In this third model, the electrons in the wire move around the circuit.

These are all models, not the real thing.

They're there to help us understand how electricity works.

Let's do a check for understanding.

Which statements are correct about this model? The choices are A, the blue spheres are electrons, B, the blue spheres are protons, C, the red sphere in an electron or D, the red sphere has a positive charge.

Pause the video now, make your choices and then come back to see how you've done.

Welcome back.

If you chose the blue spheres are electrons, that's correct.

If you also chose the red sphere has a positive charge, that's also correct.

Answers B and C are not correct.

The blue spheres are electrons, not protons and the red sphere is the nucleus of the atom.

Well done if you got those right.

Here's another check for understanding.

Which statement is correct about this model for copper wire? The choices are A, there are more free electrons than ions, B, there are equal numbers of free electrons and ions, C, there are more ions than free electrons.

Pause the video now, make your choice and then come back to see how you've done.

Welcome back.

If you chose, there are equal numbers of free electrons and ions, that's correct.

Every atom will lose one electron in this case, so there'll be equal numbers of metal ions and electrons.

Well done if you did that right.

Another model for current is some people walking between seats, like shown over here.

The moving people, from this bird's eye view, are like a flowing current of charge.

They're walking in a classroom between stools.

The wooden seat acts like an ion and the person is like an electron.

This is similar to another model you've seen where the electrons flow down a wire.

In both these cases, the charge flows downwards.

In a narrow space, only a few people can get through at a time.

The gaps are small and the seats get in the way.

So this is like a high resistance and a low current in a thin wire.

In a wider space, more people can get through.

This is like a lower resistance and a higher current in a wider wire.

Here's a check for understanding.

Which statements are correct about the people and the seats model? The choices are A, the seats represent the flowing charge or B, the seats represent ions or C, the people represent electrons, or D, there are more people than seats.

Pause the video now, make your choices and then come back to see how you've done.

Welcome back.

If you chose the seat represent ions, that's correct.

And if you also chose the people represent electrons, that's the other correct answer.

The seats are stuck in place so they don't represent flowing charge and the number of people should equal the number of seats.

So A and D are not correct.

Well done if you got those right.

These models of electricity can be useful, but they can also be misleading.

We look at the advantages and disadvantages of them.

So they help us to visualise something that we cannot see, for example, electrons.

However, we often use spheres for electrons, but electrons are not like that.

Another advantage is they use things we understand to explain abstract ideas.

The difficulty is, they can misrepresent the true idea.

Lastly, they simplify complex ideas.

And the trouble is, they can sometimes over simplify those ideas.

As long as we bare in mind the disadvantages, as well as the advantages, we're okay to go.

Different models can be used to explain the same idea.

A different model uses road and cars to represent electric current.

A narrow road like this is like a thin wire, only allowing a few cars through.

A wide road, like this motorway, is like a thick wire, allowing more cars through at a time.

Let's evaluate the road and cars model.

The advantages are that we use roads all the time and they're quite familiar to us.

The width of the roads is like the wire thickness and we can treat cars like they're electrons.

Car drivers avoid other cars, electrons also repel.

The disadvantages are that copper ions in wires are not represented.

The cars can easily drive off the road whereas electrons will stay in the wires.

There are many fewer cars than there are electrons in the wires and finally, car drivers choose where to go, electrons are pushed in one direction by the battery.

They have no choice.

Here's a check for understanding.

Which statement is an advantage of the road and cars model? The choices are A, car drivers can choose destinations, B, cars can go off-road, C, there are fewer cars than electrons and D, roads are familiar to us.

Pause the video now, choose an advantage and then come back to check your answer.

Welcome back.

If you chose roads are familiar to us, that's correct.

That's the advantage of this model.

The other three are disadvantages.

The cross-sectional area of this wire is A.

If the area is doubled to 2A, it can carry twice as much current.

The resistance will be halved.

Let's check your understanding so far.

What is the resistance of the thick wire shown? We have a thin wire with a resistance of eight ohms and then we show a thick wire, what is its resistance? Look at the wires carefully, pause the video and then make your choice and come back to see how you've done.

Welcome back.

If you chose two ohms, that's correct.

The thin wire is a quarter of the thickness of the thick wire, so a quarter of eight ohms is two ohms. The thick wire only has a resistance of two ohms. Well done if you got that right.

That was quite a difficult question.

If we're using the same voltage, the current depends on the area of a wire.

So if the area increases by two times, the current also increases by two times.

We've got two wires.

One wire carries one amp and the wire with double the thickness will carry two amps.

If the area increases by three times, then the current also increases by three times.

The current is proportional to the area.

It goes up by the same factor.

Let's do a check for understanding.

What is the current in the thin wire if the same voltage is applied across it? The thick wire has a current through it of 16 amps.

The choices are A, eight amps, B, four amps or C, two amps.

Pause the video now, make your choice and then come back to see how you've done.

Welcome back.

If you chose answer B, four amps, that's correct.

A quarter of 16 is four.

The correct answer is four amps.

We've reached the end of the section, let's do a practise task.

So we're looking at the model for electric current that is people and seats.

Give three advantages of this model.

And then secondly, what are the three things that are not quite right about it? Pause the video now, write out your answers and then come back to see how you've done.

Welcome back.

Here are three advantages of the people and seats model.

The first one is people are familiar with seats in a room and they can easily use them.

Secondly, the seats get in the way of the people, like atoms in the way of electrons.

And thirdly, during the role play, they can try out different scenarios to see the effect.

For the disadvantages, there are many fewer pupils than electrons, so it's not an accurate representation.

Secondly, pupils have choices where they go.

The electrons can only go from negative to positive.

Thirdly, there are large spaces between seats.

In wires, the spaces and electrons are much, much smaller.

Finally, the room will have walls keeping the pupils in.

The wire doesn't need walls, the electrons just stay within the metal.

Well done if you got similar answers.

We've come to the end of the lesson, let's summarise what we've learnt.

A sea of electrons can drift through a wire, between the metal ions, from a negative end to the positive end, causing a current.

Increasing the thickness of a wire increases the cross-sectional area, which increases the number of electrons that can carry current.

The current that can be carried is proportional to the cross-sectional area of the wire.

So doubling the area will double the current.

Various models are useful in explaining how current behaves.

You've done well to complete that lesson.

I hope to see you again soon.