Hello, I'm Dr.

de Mello and I'll be teaching you today's lesson.

Today's lesson is called Static Electricity and it's part of the series circuits unit.

So let's begin.

The outcome today is I can describe how objects with a positive or negative charge attract or repel other charged objects.

These are the keywords for the lesson.

The first set are static electricity.

This is when charge builds up, usually due to friction.

The second sets of keywords are positive charge and negative charge.

There are two sorts of charges and we divided them up into positive and negative.

Third set of keywords are electrostatic force.

When objects are charged, they exert forces on each other and we'll be looking at these sorts of forces.

Here are the keywords defined.

Pause the video now if you'd like, have a read of the keyword definitions and look out for them in the coming video.

This lesson has two parts.

The first part is about static charge, and the second part is about the electrostatic force that charges experience.

So let's start with static charge.

When a ruler is rubbed with a cloth, it can make pieces of paper stick to it.

A ruler is able to attract the pieces of paper without actually touching them.

The ruler is exerting an attractive force on the pieces of paper.

Because the pieces of paper are quite light, they're able to be lifted by the ruler.

Heavier objects would not be able to be lifted.

This balloon can also make the pieces of paper attract to it when it is rubbed.

If you take a balloon at home and you rub it against your head, your hair, you'll feel it attracting the hair on your head.

Let's do a check for understanding.

Which two of these things will allow a plastic ruler to pick up some small pieces of paper? The choices are A, the ruler needs to be connected to a battery.

B, the ruler needs to be rubbed with a cloth.

C, the ruler needs to be brought near pieces of paper.

And D, the ruler must touch the pieces of paper.

Pause the video, make a couple of choices, and then come back to check your answer.

Welcome back.

So if you chose the ruler needs to be rubbed with a cloth, that's correct.

Secondly, if you chose the ruler needs to be brought near the pieces of paper, that's also correct.

Connecting the ruler to a battery would have no effect because the ruler is plastic and it's an insulator.

And also the ruler doesn't need to touch the pieces of paper, it can attract them without contact.

It's a non-contact force that is acting.

If you rub a balloon against a wall, it will stick to the wall.

The paper previously and the balloon are light, so it's easy to see this effect where they stick to another object.

The ruler, balloon and wall have become charged.

This is called static electricity or more scientifically, electrostatic charge.

These objects are all insulators.

Static charge can often build up on insulators.

Let's do a check for understanding.

What is the reason that static charge can often build up on some objects? Is it A, they are insulators, or B, they're conductors, or C, they're magnetic.

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

Welcome back.

If you chose they're insulators, that's correct.

Conductors wouldn't allow the charge to build up.

The charge would flow away, and magnetic forces are a different sort of force.

We cannot see static charge on the ruler, but we can detect it because it exerts a force on the paper.

There are two sorts of charge, positive charge and negative charge.

Again, we can't see these charges.

Let's do a check of understanding, how many sorts of charge are there.

The choices are A, one sort of charge, or B, two sorts of charge or C, three sorts of charge.

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

Welcome back.

If you chose two sorts of charges, that's correct.

There's positive and negative charge.

These three balloons are neutral where there's no charge, negatively charged and positively charged.

Most things are usually neutral, but they sometimes have a positive or negative charge, as shown by the plus and minus symbols.

Here's a check for understanding, what is the charge on this ruler? Is it A, negative, B, positive or C, neutral? Pause the video, make your choice and then come back when you're ready to check your answer.

Welcome back.

If you chose the positive charge, that is correct.

You can see the positive or plus charges on the ruler.

Remember, in real life you wouldn't see these charges.

Charged objects exert a force on other objects, so this negatively charged balloon is attracting this positively charged balloon, which is also attracting the negatively charged balloon.

This force is called an electrostatic force.

Here is a check for understanding what is the name of the force holding the balloon to the wall? Is it A, magnetic, B, gravitational or C, electrostatic? Pause the video, make a choice, and then come back and check your answer.

Welcome back.

If you chose the electrostatic force, that's correct.

That's the force that's causing the balloon to stick to the wall.

Here's another question.

Which of these balloons will not cause an electrostatic force? Is it A, a positively charged balloon, B, a neutral balloon, or C, a negatively charged balloon? Pause the video now, make your choice and then come back to check your answer.

Welcome back.

If you chose answer B, that's correct.

The neutral balloon will not cause an electrostatic force.

The other two balloons are charged and they'll cause an electrostatic force.

Well done if you got that right.

We've come to the end of the section and now's a chance for you to practise what you've learned.

Here are four experiments.

Carry them out and write down what you see each time.

The first experiment is you charge a balloon by rubbing it and put it near some pieces of paper.

Second one is you charge a balloon by rubbing it and then put it near an empty drinks can without actually touching the drinks can.

The third experiment is to charge a balloon again by rubbing it and then hold it near a thin stream of water.

Make sure you don't actually touch the water.

And the fourth experiment, hold two thin polythene strips together at the top and then rub them downwards with a cloth.

What happens after you've done this? Pause the video now, try out these four different experiments and then come back to see how you've got on.

Welcome back.

This is what you're likely to have observed.

In experiment one, the balloon after rubbing will pick up the pieces of paper.

Some of the pieces of paper might fall off, but they'll be picked up again.

In experiment two, the can will be pulled towards the balloon without actually touching it.

If the balloon is brought a bit closer, the can will move faster because the force is stronger.

In experiment three, the stream of water is pulled towards the balloon.

The balloon needs to be quite close to be able to pull the water and it does need to be quite a thin stream of water.

And finally, experiment four, when you rub the strips, they'll move apart and stay as if they're pushing against each other.

They're repelling each other.

Well done if you managed to do those experiments and found similar results.

We've now come to the second part of this lesson, electrostatic force.

The can we see over here is uncharged and is attracted to the charged balloon without it touching.

The electrostatic force is a non-contact force and acts over a short distance.

These two balloons do not have the same charge.

Positive and negative are opposite charges.

The red balloon is charged negatively and the blue balloon is charged positively.

Opposite charges attract so the balloons are attracted to each other.

They will try to move together.

When this balloon is rubbed against the wall, the wall becomes positively charged and the balloon becomes negatively charged.

The charges make the balloon and the wall attract, so the balloon will stick to the wall.

The charges on the wall and the balloon stay where they are for a short time because both the wall and the balloon are insulators.

This is why it is called static electricity.

The electricity stays still.

It doesn't flow.

We've learned in current electricity the charges are pushed and pulled round a circuit and can move through conductors, like this wire and the filament of the bulb.

In static electricity, the chargers can also push and pull each other, but they're stuck on the insulators and so cannot move.

Let's do a check for understanding.

Why is the charge seen when insulators are rubbed called static electricity? The choices are, A, the charge is negative, B, the charges remain in place, or C, the objects are held by the force? Pause the video now, make a choice and come back when you're ready.

Welcome back.

If you chose, the charges remain in place, that's the correct answer.

Well done.

Answer, A, is a true statement, but it doesn't actually answer the question about why static electricity is called static.

Answer C, the objects are held, but we're talking about the charges that remain in place.

The same types of charges will repel or push away from each other.

These two balloons are negatively charged.

Negative charge repels negative charge, so these two balloons will push away from each other.

Similarly, these two balloons have the same charge.

They're positively charged and they will also repel each other.

Positive charge repels positive charge.

So these two balloons will push away from each other as well.

Let's try a check of understanding.

What will happen to this ruler and balloon? Will they, A, repel, B, they will attract, or C, there will be no force? Pause the video, make your choice, and then come back to check your answer.

Welcome back.

If you chose, they will repel, that's correct.

Both the ruler and the balloon are positively charged.

Like charges, these are positive in this case, will repel.

B, they will attract as not correct because they would need to be oppositely charged.

They're the same charge so they don't attract.

And with answer C, there will be no force is not correct.

There will be an electrostatic force because the objects are charged.

Increasing the distance between these balloons reduces the size of the electrostatic force.

It gets smaller as the balloons are moved apart.

Let's do a check for understanding.

Which three of these are true about the electrostatic force? The choices are A, it does not require contact, B, it increases with distance, C, it can be attractive, D, it can be repulsive? Pause the video and then choose three of these statements and come back to check your answer.

Welcome back.

If you chose, it does not require contact, that's correct.

The electrostatic force is a non-contact force.

If you chose it can be attractive and it can be repulsive, that's also correct.

Those two are the types of forces that act with the electrostatic charges.

Well done if you've got those right.

We've come to the end of this section and we can do a practise task to see how you're getting on.

This plastic strip has been rubbed with cloth and the two parts are repelling each other.

The strip is suspended with a piece of plastic rod.

Explain what is happening in this case.

Pause the video now, write out your explanation and then come back to check how you've got on.

Welcome back.

So your explanation could have had something like this in it.

The plastic strip has become charged when it was rubbed with a cloth.

The charge on both ends of the strip is the same.

This is because it's the same material.

This causes the two ends to repel by the electrostatic force, since light charges repel.

And that means the two ends will stay apart.

Well done if you managed to get some of those points right.

Let's summarise what we've done so far.

So static electricity or static charge builds up when objects are rubbed by others.

There are two types of electrostatic charge, positive charge and negative charge.

Charged objects can exert a non-contact electrostatic force on other objects.

Like charges repel and opposite charges attract, the electrostatic force decreases with distance.

Well done.

You've come to the end of this lesson.

I hope to see you soon again.