Lesson video

Hello, I'm Dr.

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

This lesson is called Static Electricity and is specifically about charge.

It's from the series circuits unit.

Today's outcome is: I can describe what charges are and how they're transferred between some objects, giving them opposite charges.

Let's look at today's keywords.

The first keyword is atom.

This is the smallest particle that exists as an element.

We'll be looking at how atoms get charged or lose charge.

The next set of keywords are positive charge.

Positive charge is found in the middle of an atom in the nucleus.

It's carried by the protons.

Then we have negative charge.

Negative charge is found on the outside of an atom.

Specifically, negative charge is made up of electrons, which is the next keyword.

Electrons are tiny particles which carry negative charge.

These are the keywords defined.

If you like, pause the video now, have a read of what they are and then look out for them during the lesson.

This lesson has three parts.

The first part is about where charge comes from.

The second part is how charges move, and we finish with a third part, where we look at the uses and problems of static electricity.

So let's start with the first part, where does charge come from? An atom is sometimes shown using a model like this.

On the outside, we have electrons.

These are the blue circles with a minus sign in them.

On the inside we have the nucleus, and this is positively charged.

Here is the check for understanding.

What is the charge on the nucleus of an atom? The choices are A, it's neutral; B, it's negative; and C, it's positive.

Pause the video now.

Make your choice of the three words and then come back to check your answer.

Welcome back.

If you chose positive, that's correct.

The nucleus of an atom is positively charged.

The negative charges are on the outside.

We can draw this model in a more accurate way.

On the outside are tiny, negatively charged electrons.

They're absolutely tiny.

And on the inside there are positive charges in the nucleus, and the nucleus is much larger than the electrons.

In this atom, there are eight electrons with negative charges on the outside.

There are also eight positive charges in the nucleus.

The number of electrons and the positive charges should be equal.

Let's check your understanding.

How many electrons are there in the atom shown below? Are there two, four, six, or eight? Pause the video now.

Choose the number of electrons there are in the atom, and then come back to check your answer.

Welcome back.

If you chose six electrons, that's correct.

There are six blue circles with the minus signs in them, and they're on the outside of the atoms. So there are six electrons.

The eight positive charges in the nucleus cancel out the eight negative charges which are circling around the outside.

+8 and -8 add up to 0.

So this means that the atom is neutral.

So overall it has no charge.

Most atoms have equal numbers of positive and negative charge.

This means they're neutral.

It's relatively easy to force off a negative electron from the outside of the atom.

It's more difficult to remove a positive charge from the inside of an atom.

You could say that the positive charge is shielded by the negative charges on the outside.

Here's a chance to check your understanding.

What is the overall charge on the atom shown below? Is it A, +1; B, -1; C, -3; or D, +4? Pause the video now, make your choice, and then come back to check your answer.

Welcome back.

If you chose +1, that's correct.

There are four plus charges and three minus charges.

4 - 3 = +1.

Well done if you got that right.

Now is a chance to try a practise task to check your understanding further.

First of all, explain why most atoms do not have an overall charge.

And then secondly, explain why it's easier to remove negative charges from atoms than positive charges.

Pause the video now, write out your answers, and then come back and check how you've done.

Welcome back.

So for task one, most atoms have equal numbers of positive and negative charges.

This means that the overall charge is neutral as the positive charges cancel out the negative charges.

And then part two, it's easier to remove negative charges, the electrons, from atoms, than positive charges because the electrons are held on the outside of the atom and are relatively easy to force off.

The positive charges are much more tightly in the middle of the atom.

Well done if you got those right.

We are now ready to do the second part of the lesson, how charges move.

When an insulator, like this balloon, is rubbed against another insulator, like this jumper, some of the electrons can be forced off from one object to the other.

The negative electrons leave behind an equal number of positive charges on the jumper.

Let's try a check of your understanding.

Why does the negative charge remain on the balloon? You have a choice of three answers.

A, the rubber is sticky.

B, the balloon is an insulator.

And C, the positive charge holds it there.

Pause the video now, pick one of these answers, and then come back to check how you've done.

Welcome back.

If you chose the balloon is an insulator, that's correct, well done.

Because the balloon is an insulator, the charges can be rubbed off, but then they can't move anymore.

They'll stay in position for a short while.

If you chose the rubber is sticky, you are right that the rubber is sticky, but that's not the reason why the negative charges stay on the balloon.

If you chose answer C, the positive charge holds it there, the positive charge does in fact attract the negative charge, but it's not holding the negative charge onto the balloon.

The positive charge would actually cause an attraction between the jumper and the balloon.

Well done if you managed to get that right.

That was a difficult question.

We usually don't show all the charges in an object to avoid making the image overly complicated.

In fact, there are very, very many positive and negative charges in all objects.

Only a few of the charges are able to move from one object to another.

In most objects, the number of positive charges is balanced by the number of negative charges.

If you look at this diagram very carefully, you can see every positive charge is matched with a negative charge beside it.

This means that most objects are neutral.

The balance of positive and negative charges means there is no overall charge.

When objects are neutral, we usually don't show all the charges.

Remember, the charges are invisible to us, so they'd look just like this balloon stuck on a wall.

An object becomes charged when the negative electrons are moved.

Rubbing the balloon against the wall would cause the negative electrons to leave the wall and move onto the balloon, and this means the wall would become positively charged because the positive charges are left behind.

This would result in the negative balloon being attracted to the positive wall and sticking there.

Here's a chance to check your understanding.

What is the charge on a cloth if it is used to rub a ruler which then becomes positively charged? Is the charge going to be positive, negative, or neutral? Pause the video, make your choice, and then come back when you're ready to check your answer.

Welcome back.

If you chose negative, that's the correct answer.

The cloth would become negatively charged if the ruler was positively charged.

You may have seen a Van der Graaff generator in your lessons.

A Van der Graaff generator can be used to generate charge.

A motor drives a rubber belt loop round rollers.

As the rubber belt rubs the rollers inside, it causes charge to build up.

The hollow metal dome at the top conducts these charges and collects the charges on the surface.

The support is plastic and does not conduct, so the dome is insulated from other conductors.

This means the charge can collect there even though it is made of metal and is a conductor.

When the Van der Graaff generator is switched on, charge builds up on the surface of the dome.

The charges repel each other and spread out evenly on the surface of the dome.

The dome is insulated and surrounded by air, so the charge cannot flow away.

When too much charge builds up, the electrons can jump off the dome in a spark.

Touching the dome would allow the charge to flow to your hand and onto your body.

You'd get a small shock.

More often, the spark would jump to your finger or your hand and give you the shock that way.

Here's a chance to check your understanding.

Which of the following statements can explain why the charge on a Van der Graaff generator is spread evenly over the dome? The choices are: electrons attract each other, electrons repel each other, the dome is a conductor, or electrons stick to the metal dome.

Pause the video now, read through the statements very carefully, and then come back to check your answer.

Welcome back.

So if you chose the electrons repel each other, that's correct.

The other reason why is that the dome is a conductor and allows the electrons to move around and spread out.

Answer A, the electrons attract each other is not correct.

The electrons all have the same negative charge, so they repel each other.

And answer D, electrons stick to the metal dome, well, the dome is a conductor, so the electrons are free to move around on the dome.

Well done if you got that right.

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

Sofia touches a Van der Graaff generator and then switches it on.

Her hair spreads out.

Explain why her hair spreads out as shown.

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

Welcome back.

So here's the explanation why Sofia's hair spreads out.

When Sofia is touching the generator and it is switched on, charge moves onto her and spreads around her body.

All of her hair will have the same type of charge.

This makes the different hairs repel each other and push outwards, so the hairs are pushing away from each other and forming a ball around her head.

You may have seen this in your demonstrations in your class.

We've now reached the final section, uses and problems of static electricity.

Lightning is caused by a buildup of static charge.

Turbulence in a cloud causes charge to form.

When the charge builds up to a high enough level, charge jumps through the air as a bolt of lightning.

This can be very dangerous and destructive.

We can sometimes feel static discharges or sparks when we touch things.

Friction between our clothes or carpets is usually the cause of the buildup of static charge.

It also tends to happen more when we have dryer days and if we are wearing synthetic clothing.

You may have noticed a vacuum cleaner getting dusty.

The nozzle will develop a static charge due to the friction of the air passing through it.

The static charge attracts dust.

This is a little bit inconvenient because you're trying to vacuum up the dust.

Laser printers and photocopiers use static charge to attract the toner, which is powdered ink, to the correct places where the type is.

There are rollers inside that collect the pattern of static charge that attracts the toner.

These rollers then transfer the ink to paper, which is then sealed with a heater.

This is a useful example of static charge.

Let's check your understanding.

Which of the following is a useful effect of static electricity? A, dust sticking to appliances; B, lightning strikes; C, laser printers; or D, getting static shocks.

Pause the video, make your choice, and then come back when you're ready to check your answer.

Welcome back.

If you chose laser printers, that's correct.

Of these four, laser printers are the only useful effect of static electricity.

The other ones are either irritating, nuisances, or downright dangerous.

We've now come to the end of this section.

Here's a task to practise what you've learned.

The manager of a hotel wants to fit a new hardwearing carpet that's made of nylon.

Explain why this may cause a problem.

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

Welcome back.

So you could have written: when people walk on the carpet, there'll be friction between their shoes and the carpet.

A nylon carpet is an insulator and is good at causing static charge to build up.

The hotel guests will get static shocks due to the charge buildup.

Obviously this is a problem for people visiting the hotel.

Well done if you were able to make a good explanation there.

We've now reached the end of the lesson.

Let's summarise what you've learned.

Atoms usually contain equal numbers of negative and positive charges, making them neutral.

The negative charges, electrons, can be forced off the atom by rubbing.

Objects can become negatively charged by gaining electrons, or positively charged by losing electrons.

A Van de Graaff generator can generate charge by friction.

And finally, static electricity is often a problem, but it also can be useful.

You've done really well completing this lesson.

I hope to see you soon.