Lesson video

Hello, my name's Dr.

George, and this lesson is called Moving Electric Charge.

It's all about why current flows in electric circuits.

It's part of the unit Electric Fields and Circuit Calculations.

The outcome for this lesson is I can explain how an electric field causes electric current to flow in a circuit.

Here are the key words for the lesson.

I won't go through these definitions now, but I'll introduce them as we go along.

Come back to this slide anytime if you want to check the meanings.

There are three parts to this lesson called jumping charge, constant charge flow, and calculating the flow of charge.

Here's a picture of a Van de Graaff generator, which a lot of schools have for demonstrating the effects of electrostatic charge.

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

And if that charge builds up, which it will over time, you see sparks coming from the dome.

And what's happening is that the dome is discharging, that means it's losing its charge or some of it.

And you can get something called a discharge ball, which is just a metal sphere on a conducting support.

And you can connect that to the generator, the base of the generator using an electrical lead as shown here.

And that lead allows any buildup of charge to flow through it from the discharge ball to an earth connection which is in the base of the dome.

That connection is literally connected to planet earth, which is so large that it can exchange as much charge as the Van de Graaff in this discharge ball can give or take.

So what we have here is charge building up on the dome, jumping across in sparks to the discharge ball and then travelling down to the lead and into the base and into the earth.

And now a question for you.

Which statement about charge flow between the discharge ball and the Van de Graaff generator when switched on is correct? Electrons flow along the lead from the ball to the generator.

Electrons flow along the lead from the generator to the ball.

Positive charges flow along the lead from the ball to the generator or positive charges flow along the lead from the generator to the ball.

Press pause while you're thinking and press play when you've chosen your answer.

And the correct answer is electrons flow along the lead from the ball to the generator.

The charge on the dome is negative and we know that negative charge is jumping across to the ball, negative charge is carried by electrons.

And we know they're flowing away from the dome down from the ball along the lead and into the base of the generator, so the answer is A.

Well done if you picked that out.

The negative charge on the dome is what repels electrons on it and that causes electrons to jump from the dome to the discharge ball.

And that's actually what we're seeing when we see sparks.

More sparks will carry more bunches of negative charge across the gap.

Now, Jun is holding a fluorescent tube, that's a kind of bulb that glows when a current flows through it.

And he's also holding in his other hand a wire lead which is connected to the base of the Van de Graaff generator.

And the tube lights up because charge flows through it from the dome, through his body and back through the lead.

It can pass through Jun because humans are reasonably good conductors of electricity.

That's why we have to be careful around high voltages.

And now instead of charge flowing in bunches, in sparks, we have it flowing continuously.

Now let's look at why charged particles experience forces.

In this picture, we have two plates.

Plate is just a flat sheet, but the plate on the left is positively charged and the one on the right is negatively charged, and between them there is a small positive charge.

This small charge is gonna be affected by both of the plates.

So it experiences a propulsion from the positive plate, so a force to the right, but also an attraction towards the negative plate, so also a force to the right, so the overall effect is a force to the right.

And this is happening even though this little charge is not in contact, it's not touching either of the two plates.

And we say that it's in the electric field created by these charged plates.

An electric field is an invisible field, an invisible influence on charged particles in the area.

And the force on that charge is to the right.

So we could draw field lines going from left to right and the arrows on field lines show the direction of force on a positive charge in that place.

So here we have this field caused by both plates, it's caused by the positive and the negative charges.

And electric fields are used to explain how charge particles move.

By the way, in this diagram, only part of this electric field is shown.

And now here's a question.

Which of the following is the correct reason why the particle moves to the right? Is it because the positive surface pushes it? Is it because the negative surface pulls it? Or is it because there's an electric field between the two plates? Press pause and press play when you've decided.

The correct answer is because there's an electric field between the two plates.

It's electric fields that exert forces on charged particles.

So back to the Van de Graaff generator, there is an electric field between the dome and the discharge ball, and that field pushes electrons from the negatively charged dome across the gap.

Similarly, the current in the fluorescent tube is caused by an electric field, and that field passes all the way from the dome through the tube and through gen back to the earth connection at the base of the dome and it's what's making electrons flow around this circuit.

And which of the following statements about the flow of electrons when the Van de Graaff generator has a negative charge on the dome is correct? Press pause and press play when you've decided.

One of the correct statements here is that in the fluorescent tube, electrons flow towards Jun.

We know that the dome becomes negative and that repels electrons, so move along the tube towards Jun.

There's a second correct statement here.

And that's in the electrical lead, electrons flow away from Jun.

They're going from him to the base of the generator.

So now can you explain how electrons move from the negatively charged dome of a Van de Graaff generator to a discharge ball and then along a wire to an earth connection at the base of the dome? Press pause while you're writing your answer and press play when you've finished.

Now I'll show you an example answer which explains this correctly.

When the Van de Graaff generator is switched on, negative charge is moved to the dome where it collects.

When enough charge builds up on the dome, electrons are pushed by the electric field onto the discharge ball, which is the nearest conductor to which the electrons can move.

The electrons moving to the discharge ball still repent each other and can spread out by moving along the wire connected to an earth connection.

In this way, a negative charge does not build up on the discharge ball and there remains a difference in charge between it and the Van de Graaff generator's dome.

Don't worry if you didn't make all of those points, but check that you understand this answer.

And now let's move on to the second part of the lesson, constant charge flow.

Let's think about how we can get a steady flow of charge.

An electrical cell uses chemical reactions inside to build up charge on the ends.

It's interesting to learn about how this works.

It involves both chemistry and physics, but I'm not going to go into it now.

In everyday life, by the way, we would probably call this a battery, but strictly speaking, it's a cell.

It ends up with a negatively charged end and a positively charged end.

Those will be labelled on the cell, and negatively charged electrons would be repelled by the negative end of the cell and attracted towards the positive end.

And it's actually because there's an electric field around the cell because of its charge ends and electrons experience forces in that electric field.

Now, which of the following have an electric field? The dome of a Van de Graaff generator when it's turned on, the end of a battery or cell, or the end of a magnet? So press pause if you need to think about that and press play when you're ready.

Well, the Van de Graaff generator dome is charged when it's switched on and so there will be an electric field around that and the end of a battery are charged, there are electric fields around those.

The end of a magnet is not charged, it's a pole, so it does not produce an electric field.

And another question.

Which of the following statements about electrons and the electric field due to a cell is correct? Press pause and press play when you've decided.

The correct answer is that a battery with a larger voltage has a stronger electric field.

How would you know that? Well, from any work you've done on circuits or from everyday life, you'll know that a battery with a larger voltage has a stronger effect and it's more strongly positive and negative at its ends.

Let's take a quick look at the other statement.

A, the electric field around the positive end of the battery repels electrons.

Electrons are negatively charged so it would attract them.

B, the electric field around the negative end attracts electrons.

No, it would repel them.

And D, the arrows of the field show the direction electrons are pushed.

That's not true because the arrows show the direction of force on a positive charge.

Now if we put a cell in a complete circuit, the cell makes an electric field and that field is carried all around the circuit by the wires and that electric field is what pushes all of the electrons in the circuit in the same direction.

It would be tricky to draw the shape of that field accurately around the circuit, so field lines haven't been drawn on this diagram.

Now which statement about the electrons in the circuit shown is correct? The electric field around a circuit pushes on all of the electrons in the wire.

The electric field around a circuit pushes on the electrons with different forces.

The electrons that are further from the cell get pushed with a smaller force.

Press pause and press play when you've chosen your answer.

The correct answer is the electric field around a circuit pushes on all of the electrons in the wire.

Now, Aisha and Alex are both trying to explain why electrons in the circuit move so that current flows.

And Aisha says, "Electrons everywhere in the circuit wires experience the same electric field, so move with the same force." And Alex says, "The electric field decreases the further the electrons are from the cell so the force is smaller with distance." Which of these statements is correct? And can you explain your answer? So while you're thinking about that, press pause and press play when you've written down your answer.

Let's take a look at how you could explain your answer.

So Aisha's statement is correct and Alex's is incorrect.

So Aisha said that the force would be the same everywhere and Alex said that the force would be greatest closest to the cell.

So the explanation says in circuit wires, the electric field pushes all the electrons with the same force.

The conducting wiring allows the field to push each electron equally.

This means that the current everywhere in the circuit is equal, if it's a series circuit.

So well done if you used some of those ideas in your answer.

Now let's move on to the third part of the lesson, calculating the flow of charge.

A charge is something we can measure.

We don't just say it's positive or it's negative.

We can say how much charge there is.

And when we do that, we use units of coulombs with symbol capital C.

One single electron has a very small charge in coulombs.

In fact, this many electrons are needed to make up one coulombs of charge.

That's 6.

25 billion billion electrons.

A Van de Graaff dome can collect up to about this many electrons, that's 10,000 billion, before it releases a spark.

Now that's less than two millionths of a coulomb.

But as you can see, even that amount of charge has quite an impressive effect.

Now current, not the same thing as charge, current is the number of coulombs of charge that flow past a point in one second, so past any point in a circuit in one second.

So if one coulombs of charge flows in a circuit in one second, we say the current is one ampere, or we might say one amp for short.

So take a look at the table.

We've got the charge, the time and the current.

And if we have one coulombs in one second, we have a current of one ampere.

Double the charge in the same amount of time, we say that we've got double the current.

Two coulombs in a second, that's a current of two amps.

We've doubled the charge again, but we've also made it take twice as long.

So four coulombs in two seconds is the same rate of flow of charge as two coulombs in one second, so it's still a current of two amps.

Now let's say we have twos coulombs in half a second.

That's a bigger current.

So when we had two coulombs in one second, that was two amps.

Two coulombs in half a second, that's four amps.

It would be four coulombs per second.

And now perhaps you can see why one coulombs has been chosen as a unit to be so many electrons worth of charge because actually a current of one amp is nothing unusual.

It's not a particularly large current and that's one coulomb per second.

If we measure current in number of electrons per second, then ordinary current would be enormous numbers, and so a coulombs works out to be a sensible size for working out numbers of amps.

Now what is the current, if eight coulombs of charge flows in four seconds? Press pause while you're thinking and press play when you're ready.

The answer is two amps.

It's two coulombs in each second if it's eight coulombs in four seconds.

Here's a way of thinking about that.

Eight in four seconds, four coulombs in two seconds.

Halving again, two coulombs in one second, which is two amps, so well done if you've got that.

Now we can write an equation that relates current charge and time.

Here it is.

Current is charge divided by time.

In symbols, we actually use the symbol I for current and Q for charge, so I equals Q over t.

This relationship in a way is similar to the relationship for speed.

Speed is distance divided by time, so how many metres covered in each second.

Current, how many coulombs go past in each second.

So current is measured in amps.

Charge is measured in coulombs.

And if we want this equation to work, we need to use time in seconds.

And we may need to work out charge given a current and a time how much charge flows, so we can rearrange the equation.

And remember, when you rearrange an equation, you need to do the same thing, the same operation to both sides to get a new version of the equation.

Here it would make sense to multiply both sides by time.

Then on the right, we have charge divided by time, but then multiplied by the time.

If you divide by a number and multiply by the same number, then overall, you haven't made any difference.

We could say they cancel out and we can cross those out.

So we're left with current times time equals charge or we could write that the other way round as charge is current times time.

When you're rearranging an equation, you don't have to write this many steps, just write as many as you need to be confident of what you're doing.

So how much charge flows if there's a current of three amps and it flows for four seconds? Press pause while you're thinking and press play when you have your answer, The correct answer is 12 coulombs.

And the reason why is charge is current times time, as we saw.

And so that's three times four, which gives us 12 coulombs.

Now what if we want to calculate the time? If we know that a certain amount of charge flows and we know what the current is, we could ask how long would that take? Again, we can do the same thing to both sides of the equation.

Let's multiply both sides by time.

So again, multiplying by time undoes dividing by time.

And so we have current times time equals charge.

We're not there yet, but if we divide both sides by current, we get this.

So on the left, we have time multiplied by current but also divided by current.

Overall, that's not doing anything.

So on the left, we just get the time.

And so time is charge over current.

And again, you don't have to write this many steps.

Write as many steps as you need to be confident of what you're doing.

So how long would it take for six coulombs of charge to flow if the current is 0.

2 amps? Press pause and press play when you're ready.

The correct answer here is 30 seconds.

And the reason is because time is charge divided by current so that's six divided by 0.

2, which is 30.

And the unit will be seconds because we've used coulombs for charge and amperes for current, as we should in this equation.

And now a longer written task for you to try.

So first question asks you to write down the equation for current in terms of charge and time and then rearrange the equation in two different ways, as shown.

Then there are two calculations for you to do.

So press pause while you're doing that.

Take as long as you need and then press play when you're finished.

So here are the correct answers.

Current is charge divided by time.

If you've written that in symbols, that's fine.

I equals Q over t.

And rearranging, if we want charge to be the subject, it's charge equals current times time.

And if we want time as a subject, time is charge divided by current.

So you don't have to memorise all three arrangements of this equation if you're confident about rearranging using the same skills you use to rearrange formulas in maths.

Now for the calculations.

To calculate the current used in charging a car battery, if 16 coulombs flows in 20 seconds, current is charge divided by time, so we find 16 divided by 20, which is 0.

8 amps.

An LED bike light is used for 300 seconds and draws a current of 20 milliamps.

How much charge flows through it? Well, a milliamp is a thousandth of an amp and 20,000th of an amp is 0.

020 amps, as shown on the right.

And now we calculate charge, which is current times time, so 0.

020 times 300, that gives us six coulombs of charge flowing through.

So well done if you've got most or all of the answers to these questions right.

If you didn't, check that you understand them now.

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

So here's a summary.

Charge from a Van de Graaff generator can jump between the dome and a discharge ball.

It can also flow through a conducting lead or a fluorescent tube.

An electric field exerts a force on charges, making them move.

An electrical cell uses a chemical reaction to create an electric field that forces electrons round a circuit.

Positive charge has a force in the direction of the electric field, and negative charge has a force in the opposite direction.

Current is found using the equation current equals charge divided by time.

I hope you've enjoyed the lesson and I hope it's helped you to understand why current flows in an electric circuit.

I hope to see you again in a future lesson.

So bye for now.