Hello, my name's Dr.

George, and this lesson is called Electric fields.

It's all about what they are and how they affect objects.

The lesson is part of the unit Electric fields and circuit calculations.

The outcome for this lesson is: I can describe the properties of electric fields.

I'll be introducing these keywords during the lesson, but if you want to come back and check the meanings anytime, you can come back to this slide.

The lesson has three parts called charging insulators, electric field lines, and electric field rules.

Let's get started by looking at how we can charge insulating objects.

Two balloons or any two objects that are oppositely charged will attract each other.

So here we have a positively charged and negatively charged balloon, and they experience forces towards each other.

The force between charged objects is called the electrostatic force.

Static is related to the word stationary, meaning not moving.

The electrostatic force acts on charges even when they're not moving.

Now, if two objects such as balloons have the same charge, they'll repel each other.

So negatively charged balloons repel each other, and positively charged balloons repel each other.

If you increase the amount of charge on the balloons, the force becomes larger.

Now, how do you charge a balloon? Well, you can do it by rubbing it against a wall.

Friction between the surfaces that rub together transfers electrons, which are negatively charged, and this leaves the balloon and the wall oppositely charged, and it makes them attract each other.

In fact, we know that it's the balloon that gains electrons, and so it becomes negatively charged, and the wall has lost electrons, so it's left with a positive charge of equal size.

So now these two attract each other.

So here's a question about that.

Which of the following statements about a balloon rubbed on a wall are correct? The balloon repels the wall.

The wall attracts the balloon.

There are more charges on the balloon than the wall.

And the number of charges on the balloon is equal to the number of charges on the wall.

On short questions like this, I'll wait five seconds, but you may need longer.

In which case, press pause, and press play when you've chosen your answer.

One of the correct statements is the wall attracts the balloon.

The balloon also attracts the wall, but that's not in the statements here.

And the other correct statement is that the number of charges on the balloon is equal to the number of charges on the wall, and that's because the electrons that left the wall ended up on the balloon, the same number of electrons.

Now it turns out that if you rub a polythene rod with a duster, it makes the rod gain electrons and that gives it a negative charge.

And the duster has lost electrons, so it's left with a positive charge of equal size.

If you rub the rod more against the duster, you'll increase the charge.

You'll increase the number of electrons transferred.

Static charge is often produced when two insulators like this are rubbed together.

So friction can give an outer electron in an atom energy and allow it to leave the atom.

Insulating materials don't allow the electron to flow back to its original atom, but the positive nucleus of the atom always remains in place.

The nucleus is never transferred from one material to another when you rub them together.

In fact, it isn't always necessary to rub two materials together.

Sometimes just touching them together can cause electrons to transfer from one to the other.

And the reason this transfer can happen is because some materials are better at holding on to or taking electrons than others.

And we can use a model to help us understand how the rod and the duster get charged.

Imagine we have a ruler with double-sided sticky tape on it, and we have a tray of tennis balls and ping-pong balls, and the ruler represents the charged rod.

And the tennis balls represent atoms that are in the duster.

The ping-pong balls represent electrons that are in the duster.

Now, double-sided sticky tape is sticky on both sides, and it's about as sticky as any other sticky tape.

So what do you think will happen when the ruler with the double-sided sticky tape on it is rubbed over this box of tennis balls and ping-pong balls? Press pause, and press play when you've chosen your answer.

The correct answer is the ping-pong balls will be picked up, but the heavier tennis balls won't be.

So of course this isn't what really happens, but this model helps us get an idea of what's going on.

The force that the sticky tape can exert is large enough to pick up a ping-pong ball but not a tennis ball, and the force that the charged rod exerts is large enough to move an electron but not an entire atom.

Now here we have two polythene rods and one of them is resting on a watch glass.

It's a kind of curved glass.

And the reason for it being here is it lets that rod easily rotate.

And if we bring a charged polythene rod near it, the two rods, which have the same kind of charge, repel.

The rod on the watch glass rotates around.

And these charged objects don't have to touch for a force to act between them.

So they're somehow influencing each other from a distance.

And we can think of charged objects as having a kind of field around them that affects other charged objects that come into that field.

So now here's a question.

Which of the following statements about the electric field of the rod on the watch glass are correct? So press pause while you read the statements and choose your answers, and press play when you're ready.

One of the correct statements here is the nearer to the rod, the stronger the rod's electric field.

You can get an idea of that by the fact that the closer charged objects are to each other, the greater the forces they exert.

Also, adding more electric charge to the rod by rubbing it makes its electric field stronger.

We've already seen that objects with more charge on them affect other charged objects more strongly.

A is not true, any electric charge in the rod's electric field is repelled, because it will actually attract a positively charged object.

And D, at more than about five centimetres from the rod, the rod has no electric field.

Well, we have no reason to think that.

Now a lot of schools have a device called a Van de Graaff generator, and it's the large object shown here.

And when you switch it on, the dome at the top becomes charged.

And the reason schools have these is because they can be used to study the effects of charge on objects around.

The way the generator works is when it's switched on, there's a rubber belt, a loop of rubber that goes around rubbing against rollers inside, and that causes charge to build up on the belt.

And the belt then drops that charge onto this hollow metal dome, which collects the charge.

The charge can't then escape from the dome because the dome is resting on an insulating support.

Now here are two charged balls labelled X and Y hanging from threads near a Van de Graaff generator that's switched on.

Which ball experiences the larger force from the Van de Graaff generator? Press pause, choose your answer, and press play when you're ready.

The correct answer is that ball X experiences more force and that's because it's closer to the charged dome and it will experience a stronger electric field.

And now for a longer question.

Here you can see two charged balls hanging close to the generator.

One of them has a charge of plus two, and the other has a charge of plus one.

So the ball on the left has twice as much positive charge.

Describe what will happen to the two polystyrene balls and explain why this happens.

So press pause for as long as you need to write your answer.

Press play when you're ready and I'll show you an example answer.

So here's an example answer.

Describe what will happen.

The two polystyrene balls will move away from the dome of the Van de Graaff generator.

The polystyrene ball with a larger charge will be moved more.

Now how could we explain that? The polystyrene balls both have a positive charge, which is the same type as that on the dome, so they will be repelled.

The polystyrene ball with a larger charge will be repelled with a larger force, so it will move further away from the dome.

So well done if you included most or all of those points in your description and explanation.

And now to move on to the second part of this lesson, electric field blinds.

Now take a look at this video.

We have pie tins made of metal placed on top of the dome of a Van de Graaff generator.

Now the video reruns.

And the generator is switched on, the dome is charged, and we see these pie tins fly off one by one, starting with the top one.

The pie tins are made of metal, they're conductors.

And so when they sit on a positively charged dome, they also become positively charged.

The charge is shared between the dome and the pie tins.

So now we have positively charged pie tins sitting on a positively charged dome, so they repel each other.

So we have this repulsive electrostatic force from the dome upwards on the pie tins, but the pie tins also have weight, the gravitational force acting downwards on them.

So they will only move when the electrostatic force becomes greater than the gravitational force.

Then there's a resulting force upwards and the pie tin accelerates upwards.

So here's a question about that.

When will a pie tin lift off the dome of a Van der Graaff generator? Press pause while you think about this, and press play when you've chosen your answer.

And this is all about comparing the upwards and downwards forces.

The pie tin will lift when the electrostatic force, which is upwards, is greater than the gravitational force, which is downwards.

Let's experiment in a different way with a Van de Graaff.

Here we have a positively charged polystyrene ball, and it will be repelled by the dome.

We find if we look at the direction of the force that it's horizontal here and it's due to the electric field of the dome.

So this ball is in the dome's electric field.

And if we put balls in different places and see which direction they're repelled, we find that any positive charge near the dome is repelled away in the direction shown by these arrows.

And so we can use these arrows to show to represent the electric field due to the charged dome.

And this field we could describe as a radial field.

And the reason we call it that is the field lines are like the radii of a circle.

You can see that the field lines point along different radii of this circle.

So if we put a positive charge here, it will actually be repelled in the direction shown by the arrow, away from the dome, along a radial line.

Positive charge here will be repelled in this direction.

And down here will be repelled in this direction, along a radial line.

Now, in this picture, which green arrow correctly shows the force on the charge due to the Van der Graaff generator? Press pause if you need more than five seconds to decide.

The answer is B.

The arrow for B is pointing along a radial line, and that's what we expect for a radial field.

So we see that positive charges are repelled radially in this field.

They're repelled in the directions of the field arrows.

And what about negative charges? Well, negative charges, negatively charged objects will be attracted by the positively charged dome.

And we find that they're attracted radially in the opposite direction to the field arrows.

Now just a point about electrostatic fields and magnetic fields, which you've probably heard about.

They are different.

They have some similarities.

They're both called fields.

They're both areas of influence on other objects and they can cause attraction and repulsion, but they are different kinds of field.

If you put a magnet in an electric field, nothing happens.

There's no force between them.

If you place a charged object in a magnetic field, nothing happens, there's no force between them.

So different kinds of field.

And when we draw an electric field, the lines and the arrows show the direction in which the field pushes a positive electric charge.

The lines in a magnetic field show the direction a magnetic north-seeking pole will be pushed or pulled.

So different types of objects are affected by electric and magnetic fields.

Now, which of the following statements about electric field lines is incorrect? Press pause while you're thinking, and press play when you're ready.

And the incorrect statement here is they are the same as magnetic field lines.

They're not.

They may look similar, but they're representing different types of field.

Now let's just check why each of the other statements is correct.

B, we've already seen that electric field lines around a charge are radial, at least as long as the charge is spherical.

They do show the force on a small positive charge.

That's the direction the arrows show.

And so if we had a negative charge and we drew the electric field around it, the arrows would point inwards towards the charge because positive charges would be attracted towards it.

And now a written task for you.

I'd like you to try to explain why pie tins lift off a Van de Graaff generator starting from the topmost one, followed by the next, until they all lift off.

So take as long as you need to write your answer.

Pause while you do, and press play when you're ready.

I'll show you an example answer.

Yours doesn't have to be exactly the same as this, but it shows you the key ideas to include.

The pie tins gain a positive charge from the Van de Graaff generator because they can conduct electricity.

This causes a repulsive electrostatic force between them and the dome.

When this electrostatic force is greater than the gravitational force on the pie tin, it will lift off the dome.

The uppermost pie tin has only its own gravitational force.

The pie tins below have additional force from the pie tins on top holding them down.

This means the uppermost pie tin will lift off before the ones below, and so they will lift off one at a time starting from the top.

Now that was a fairly challenging task.

It's not that easy to explain, but well done if you did include some of those key ideas.

And now let's go on to the final part of this lesson, electric field rules.

We're going to look at characteristics of electric fields and how they affect charged objects.

Here we have a large positive charge and the arrows show the field around it.

And if there's a small positive charge nearby, it's repelled.

That same small positive charge would be attracted towards a large negative charge.

Now, of course, the small charge also has its own electric field, so that could complicate things.

We then have two electric fields in this same region.

So what we're going to do is assume that the small charge is small enough that we can ignore its electric field, that it doesn't affect the overall electric field in this space.

And that's what I mean when I talk about small charges.

So now we have a small negative charge.

It will be attracted to a large positive charge.

That same negative charge would be repelled away from a large negative charge.

So it's doing the opposite from the small positive charge.

Now the force with which a positive charge is repelled by a large positive charge depends on the distance between those two charges.

If we move the small charge closer to the large one, it's repelled with a larger force.

And if we move it further away, it's repelled with a smaller force.

So have a look at the diagram.

There's a large positive charge in the centre and four positions for a small positive charge.

Where would the small charge experience the largest force? Press pause if you need to and press play when you've decided.

The correct answer is the position where the charge is closest to the larger charge, which is B.

How do we represent stronger or weaker electric fields? The strength of the field at a point depends on the number of field lines in a given area on the page, or in a given volume if we actually have a 3D representation of an electric field.

So look at these two crosses.

If you draw a circle around the top one there, there are two field lines within that circle reasonably close to that point.

Same size circle around the other cross, there are four field lines within that space.

That represents a stronger field.

The force due to an electric field depends on the distance between charges.

Look at this charge here.

There's a smaller force at a larger distance, a larger force at a smaller distance.

And we can see that the larger force is happening in a region where the field lines are closer together.

And another question for you.

There are four small charges here.

They're not all the same type of charge.

Which of them will experience the largest attractive force due to the large negative charge? Press pause if you need to, and press play when you've decided.

Now, only C and D will experience an attractive force because they're positive, and C is closer, so it will experience the larger attractive force.

And now here's a longer question for you to try.

I'd like you to describe what will happen to the small charge in each case and explain the reasons why.

And in four, there are two small charges for you to write about.

Assume that the large charge doesn't move, it's held still somehow.

So press pause while you're writing your answers and press play when you're finished.

So here are the answers to the questions.

First one, the small positive charge is repelled horizontally to the right, away from the large positive charge, with a large force.

And there's the picture, repulsion to the right, and it's close, so there's a large force.

And now for two, the small positive charge is attracted horizontally to the left, towards the large negative charge, with a small force.

So it's attracted to the left, but it's further away, so the force is relatively small.

Now look at three.

The small negative charge is attracted upwards, towards the large positive charge, with a large force.

And they're close, so that's why the force is large.

Now take a look at four.

We'll think about each of the small charges in turn.

The small negative charge is repelled in a direction that is radially away from the large negative charge, with a large force.

And the small positive charge is attracted in a direction that is radially towards the large negative charge, with a small force.

We're assuming that these two small charges are too small to affect each other.

And that's all I have to tell you about electric fields for now.

So we're at the end of the lesson, and here's a summary.

Insulators can be charged positive or negative by friction.

The charge is caused by the movement of electrons, which are then trapped on insulators.

Like charges repel, and unlike charges attract.

There is a radial field around a small or spherical charged object shown by radial lines.

The field lines show the direction of force on a positive charge.

The field is stronger the closer the lines are together, so it decreases with distance.

Electrostatic and magnetic fields are different.

So well done for working through the lesson.

I hope you found it interesting, and I hope to see you again in a future lesson.

So bye for now.