Lesson video

Hi, I'm Mrs. Hudson, and today we're going to be looking at a lesson called "Loops of a Parallel Circuit".

This is a key Stage 3 physics lesson and it comes under the unit called "Resistance and Parallel Circuits".

So let's get going.

The outcome of today's lesson is, I can divide a parallel circuit into a set of nested series circuits.

There will be some keywords that are used frequently in today's lesson, and they are series circuit, battery, parallel circuit and potential difference.

So let's look a little bit closer at what each of those mean.

A series circuit is an electric circuit with one complete loop from one end of a cell or battery to the other end.

A battery is two or more cells connected in series to form a battery.

Parallel circuit, an electric circuit with more than one complete loop from one end of a cell or battery to the other end.

And finally, potential difference is a more formal term for voltage.

They can be used interchangeably.

If you want to pause the video to make a note of those keywords, then please do, but we're going to move on now to look at how this lesson is structured.

So today's lesson on loops of a parallel circuit is going to be split up into two parts.

In the first part of the lesson, we're going to look at parallel circuit loops, and then we're going to move on to look at cells in a parallel circuit.

But let's get going first of all, with parallel circuit loops.

The circuit shown here is a series circuit and it consists of a cell and a lamp.

And we can see that there is a single loop connecting the cell and the lamp together, and it's connected together using wires.

A second series loop can connect another lamp to the same cell.

So we can see now that there's a second loop that's directly connecting the lamp to that cell.

And a third series loop can connect a third lamp to the same cell, and we can see here we've got the first two loops that we've already made, and now there is a third loop, which is connecting the third lamp directly to the same cell.

These three wires can be replaced.

A single wire and branches connect to the lamps, so we can see there that we've replaced the three individual wires with one wire coming out of the cell, and then there are three branches that connect to each of the individual lamps.

And then those three wires can also be replaced with another single wire, and branches connect to the lamps.

The original circuit looked like this, where each individual lamp was connected with three individual loops.

But it can be simplified to look like this, where the cell has got one wire coming out of it and then there's a junction, and then there are three loops connected to each lamp, and then they connect back up at a junction with an individual wire going back into the cell.

So these two circuits are showing the exact same thing.

It's just that the one on the right-hand side is simplified.

We could make that diagram even simpler by representing it as a circuit diagram that looks like this.

We can see here that we've got one cell and three lamps, and that the circuit is representing each of those lamps in an individual loop connected to the cell.

Some important facts about parallel circuits like this are that each individual lamp is connected directly to the cell, and what that means is they all receive the same voltage from the cell, as long as there aren't any other components within that branch.

It also means that if one of the lamps would be broken, that the other lamps would remain lit because they are still connected directly to the cell.

Let's check our understanding of this so far.

So we can see here that we've got a circuit, and the components within it are set up in that image.

And the question is, all of the lamps and the circuits shown are identical.

Which of the following statements are correct? A, the circuit has three loops coming from the cell.

B, if one lamp blows, the others will go off.

C, the lamp closest to the cell will get the highest voltage.

And D, each lamp is connected directly to both ends of the cell.

So which statements there are correct? Hopefully here we went for A.

The circuit has got three loops coming from the cell, so well done if you got that right.

And also D is correct.

Each lamp is connected directly to both ends of the cell.

B is incorrect, because if one lamp blows then the others will stay lit, because each lamp is individually connected to the cell.

And then C is incorrect.

The lamp closest to the cell will get exactly the same voltage as the other two lamps.

There is no difference.

So well done if you remembered that.

Each of the diagrams below show connections to a cell that are effectively the same.

So we can see that we've got a cell here which has got three wires coming out of it.

We could also represent this like this, where there's one wire which then branches into three different wires.

Or it could be represented like this, again, where there's one wire that branches into three wires but not up to the same point.

If there are no components along the wires, the connections can be anywhere, as long as they lead to the cell.

Let's check our understanding of that.

Which of the following circuits is the odd one out? Is it A, B, or C? Well, the odd one out in this case is B.

We can tell that because there's a cell with three lamps, but two of the lamps in the first loop are connected in series, whereas A and B are showing three lamps, each one within a different loop.

So each one is directly connected to the cell through its own branch.

Well done if you managed to get that right.

Each loop in a parallel circuit is like a separate series circuit, which we can see represented here by the purple lines.

So in our circuit diagram we can see the first loop containing the first lamp, and we can also see that visualized on the other circuit.

The second loop is represented in the same way here, both connected to the same cell.

And then finally the third loop, represented with the purple line there, each of the individual lamps though are directly connected to the same cell.

Let's check our understanding of that with a true or false question.

True or false, the two circuits shown are the same.

So have a look at those two circuits and are they the same? You pick an answer, true or false? Hopefully here we recognize that it's true, those two circuits are the same.

Now let's justify our answer.

A, each lamp is connected on its own loop to the cell.

Or B, the wires are in different positions, so they are different circuits.

This is A.

Each lamp is connected on its own loop to the cell.

Now, B is sort of correct, the wires are in different positions, but that doesn't make them different circuits.

So well done if you realized that wasn't quite right.

Now if you look again at this circuit here, it's a parallel circuit that contains a cell and three lamps, and each lamp is connected to the cell with a direct loop.

This lamp in the middle here can be moved without changing the connections to the cell.

So we could just move it above that cell, but it's still the same circuit.

The new loop position does not change the way the circuit works.

This circuit here, the original one we looked at, is the same as this circuit.

You've just moved the position of the second lamp to be above the cell, but it still works in the same way.

The circuits can be shown as circuit diagrams. So rather than using this image, if we change that into a circuit diagram, it would look like this.

This is the first loop, where the cell is connected to the first lamp.

This is the second loop.

The position and size of the loop is not important, but the lamp is still directly connected to the cell.

And then finally, this is the third loop.

A really important factor to consider with parallel circuits is the idea that the cell or the battery is pushing the current around the circuit.

And within each branch, the current which is being pushed will be pushed with the same strength through each branch.

And because the cell is pushing current with the same strength through each branch, that means that the voltage is going to be the same as well.

Let's check our understanding of that concept.

So we've got the same circuit here that's been set up, a cell with three different lamps, each connected directly to the cell in parallel.

Which of the following statements about the current in the loops of the circuit shown is correct? A, the current in all loops is pushed with the same voltage.

B, the current is pushed most in the loop closest to the cell.

Or C, the current is pushed most in the loop furthest from the cell.

So we should have got here A.

The current in all loops is pushed with the same voltage, so well done if you got that right.

In parallel circuits, the positions of the wires and components are not so important.

It is how the loops or branches are connected to the cell or battery that matters.

A good way to analyze the circuit is to follow each side of the loop back to the two sides of the cell or battery.

So for example, if we look here at the first loop, we can see that the red line is showing you how the lamp is connected to one side of the cell, and the blue lines are showing you how it connects back to the same cells.

That's the first loop.

The same way looking at the second lamp now, this is the second loop, we can see the red line's showing us how it's connected to the cell and then back to the same cell.

And then the same thing for the third lamp, which is the third loop.

If there are no other components in the way, the component will get the full voltage of the cell or the battery.

So if the cell had nine volts, then the voltage that would be going through each of the lamps would also be nine.

Let's check our understanding with another true or false question.

So there are two circuit diagrams shown in these images, so just have a quick look at those.

Now answer this question.

True or false, the two circuits shown are the same.

This is true, they are the same.

Now let's justify that answer.

A, the loops were in different positions.

Or B, each lamp connects directly to the cell.

The answer here is B.

Each lamp connects directly to the cell.

Now A is correct, the loops are in different positions, but that doesn't matter here because as long as each lamp directly connects to the cell, then it will have the same voltage reading that the cell has.

So really well done if you recognize that.

We're ready now to move on to the first task of the lesson.

And in the first part, you're going to build circuits A and B and check that they work.

Write down what happens to the lamp and the motor.

And you can use the circuits given there, A and B, to guide you.

Then secondly, you're going to predict what will happen to the lamp and the motor, if you build circuit C using the same 3-volt battery, and explain why you think this is the case.

Then you're going to actually build circuit C.

And then for number four, you're going to describe what happens to the lamp and what happens to the motor.

And then number five, were your prediction and explanation in two correct? If not, explain why and or improve them.

I'm sure you're gonna do a really good job of this and have lots of fun at the same time.

Pause the video and then come back to me when you're ready to get to the answers.

Let's see how we did.

So starting with question one, when you built those circuits, you should have seen that the lamps lights brightly and the motor spins first.

Now, if you were going to predict what would happen to the lamp and the motor if you built circuit C here, what we've done is we've put the lamp and the motor in parallel with the same cell, rather than having them in two separate series circuits.

So you might have predicted that the lamp will light but won't be as bright as before, or the motor will spin but more slowly than before.

That's often what students think when they build these circuits.

You might not have made that prediction, but if you did, let's see how that compares to what actually happens.

So for question four, what actually happened was that the lamp lights as brightly as it did in circuit A, and the motor spins as fast as it did in circuit B.

Now you might have made that as your prediction as well, and that's absolutely fine.

Your prediction is your prediction, it's not really right or wrong.

And then for question five, were your prediction and explanation in two correct.

If not, explain why and or improve them.

So if your prediction was incorrect, then you might have said, my predictions for circuit C were not correct.

The lamp did not get dimmer and the motor did not get slower.

And then if you did get your prediction correct, then you should have also this information as well.

I should have remembered that the lamp and motor are each connected directly to the cell in separate loops, so they will get the full voltage.

This means they will work as well as if they were in a single circuit, a series circuit like A and B.

Really well done if you managed to get that right 'cause that was quite a complicated task.

If you need to pause the video to add anything into your answers and make them a little bit better, then please do.

But now we're gonna move on to the second part of today's lesson.

So well done, we know about parallel circuit loops now.

Let's have a look at cells in parallel circuits.

When cells are connected, they form a battery.

When cells are connected in series, their voltages add together.

So we can see here we've got one cell, and if you added that to two more cells, it would make a battery.

And the individual voltages of each cell added together would make the total voltage of that battery.

So in this case, there are three cells, each with a 1.

5 volt rating, which would equal 4.

5 volts overall.

Potential difference is another name for voltage.

It is similar to the energy in the gravitational store.

So the potential difference or the voltage likewise could be looked at as being the energy in the gravitational store due to the height.

We can use the analogy, which is a model of height, to explain what is meant by potential difference.

So we can see here we've got a rock which is quite high, and there's a ball at the bottom of it.

Lifting a ball up a cliff transfers energy to the ball.

Energy is transferred to the gravitational store.

Lifting a ball twice as high transfers twice the amount of energy to it.

So another way we can look at this is by saying, well, the energy in the gravitational store is very similar to the potential difference provided by the cell.

Increasing the height increases the energy in the gravitational store.

Likewise, adding a cell in series increases the potential difference supplied by the cell.

So now you would have twice the potential difference.

These two 1.

5 volt cells are connected in series to form a battery.

So you've got two 1.

5 volt cells.

They give a combined potential difference or voltage of three volts, and that's because they're connected in series.

1.

5 volts plus 1.

5 volts equals three volts.

Adding a cell in parallel does not raise the height of the potential difference.

So unlike the previous example where the cells were added in series, if you add cells in parallel, it doesn't increase the height of the potential difference.

So you can't add the voltages together.

The two cells shown, when connected in parallel, can only give a maximum potential difference of 1.

5 volts.

So even though there are two cells, because they're connected in parallel, the maximum potential difference is just the individual voltage of one cell.

Even adding a third cell in parallel only gives a potential difference of 1.

5 volts.

However, three cells contain more chemicals than one, so will work for longer.

So although the voltage supplied by the battery, which is connected in parallel, will only be the minimum of one cell's value, because there are more chemicals within the three cells together, then it means that the battery will last for longer than if there was just one individual cell.

This simulation below shows that the voltage is the same across the three cells.

So as the probes of the vault meter move across each of the cells, we can see that the voltage stays the same.

It's only going to be the value of one of those individual cells, which is nine volts in this case.

The voltage across the cells remains the same.

Let's check our understanding of that concept.

So have a look at the two circuit diagrams here.

Notice the difference really is that the bottom circuit has got two cells connected in parallel, with a volt meter around them.

The question is, what will the reading on the volt meter in the bottom circuit be? A, three volts.

B, 1.

5 volts.

C, 0.

75 volts.

Or D, zero volts.

For this question, we should have selected B, 1.

5 volts.

It's the same voltage as the top circuit.

It doesn't matter that there's an extra cell.

Because it's being connected in series, then the voltage across the battery is only going to be the same as one individual cell, which was 1.

5 volts.

Really great job if you recognized that and got that right.

Using our knowledge of this then, this lamp will light brightly.

So we can see here there are three cells connected to form a battery, but they're connected in parallel, and that lamp is going to light brightly.

But the lamp will light just as brightly as this setup.

So here you've got two cells that are connected in parallel to one lamp as well.

It doesn't matter that there's one less cell, the voltage supplied is just going to be the same, so the lamp will still be as bright.

And that will be the case, even if you just had one cell connected directly to that lamp.

Because the cells are connected in parallel, it is not changing the voltage that is going to be supplied to that lamp.

The voltage across a cell, or cells in parallel, always stays the same.

However, if I asked you which one of these lamps will stay lit for the longest, what do you think the answer would be there? Well, this would be the circuit on the left-hand side, and that's because there are more chemicals across the three individual cells, and therefore that lamp will stay lit for the longest.

Let's just quickly check our understanding of that concept.

Which of the following lamps will stay lit the longest? A, B, or C.

Should here have gone for A.

There are three individual cells, and even though they're connected in parallel, so therefore the voltage will be the same as circuit C, because there are three individual cells, there are more chemicals and therefore the lamp will stay lit for the longest, but it will have exactly the same brightness as the circuits A, B and C lamp does.

Well done if you got that right.

We're ready now to move on to the final task of the lesson, Task B.

And here there are three different circuits.

Sam and Andeep are building some circuits and making predictions.

Sam has said the lamps will be brightest in circuit three and the dimmest in circuit one.

Andeep has said the lamps will be the same brightness in all of the circuits.

Your job is to identify whether each pupil is correct or incorrect, and explain why.

So I'm sure you'll do a really great job of this.

Give it your best go, pause the video, and then press play when you're ready for me to give the answers.

Let's see how we did.

So in this instance, Andeep is correct and Sam is incorrect.

The lamps will all have the same brightness because the voltage across each cell is the same.

This means that the voltage across each branch of the circuits is also the same.

If the voltage across each branch and each lamp is the same, then all of the lamps will have the same brightness.

Really great job if you managed to get that question right.

If you need to pause the video to add in any extra detail, then please do.

But we're going to summarize what we've learned now.

Well done on today's lesson.

So we've been learning about loops of a parallel circuit.

And first of all we said a parallel circuit is made of loops that branch off at junctions.

We said each loop goes from one end of a cell or battery to the other end of the cell or battery, like a separate series circuit.

But we did talk about the fact that we could simplify those circuits.

We also said the voltage across each loop of components is the same as the voltage across the cell or battery.

And finally we talked about the fact that cells in parallel will give the same voltage as a single cell, but will last longer as there are more chemicals to react.

I've really enjoyed today's lesson, I hope you have too, and I look forward to seeing you next time.