Lesson video

Hi, I'm Mrs. Hudson, and today we're going to be looking at a lesson called "Voltage in Parallel Circuits." This is a Key Stage 3 physics lesson, and it comes under the unit titled "Resistance and Parallel Circuits." So let's get going.

The outcome of today's lesson is: "I can describe the rule for voltages in a parallel circuit." There will be some keywords that are used frequently throughout today's lesson, and they are "voltmeter," "voltage," "potential difference," and "parallel circuit." So let's have a look at what each of those words mean.

A voltmeter is a device that is connected in parallel and measures the voltage supplied by a cell or battery or across a component.

Voltage is a measure of the push from a cell or battery that moves charge around a circuit.

"Potential difference" is a more formal term for "voltage," and they can be used interchangeably.

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

If you want to pause the video to make a note of those keywords, then please do, and then press play when you're ready to continue.

Today's lesson on voltage in parallel circuits is going to be split into two different parts.

In the first part of the lesson, we're going to be looking at measuring voltage in the parallel circuit, and then we're going to move on in the second part to look at a rule for voltage in parallel circuits.

But let's get going first of all with measuring voltage.

A voltmeter measures voltage in volts, and we use a capital V to represent volts.

And we can see here that there's an image of an actual voltmeter that you could use, and you can tell it's a voltmeter because there is a capital V on there telling you it's measuring the voltage.

And then underneath that, we can see the circuit component symbol that we use to represent a voltmeter, and that is a circle with a capital V inside with two wire lines coming out.

Now, notice on that image that there is a positive and a negative terminal of the voltmeter as well.

"Potential difference" is the scientific term for voltage.

So within science, we use the word "voltage," but also sometimes "potential difference" is used, and those two words can be used interchangeably.

Voltmeters are always placed across or in parallel with the component that they are testing.

So we can see there that you've got a diagram to represent that.

And there's a dotted line which is showing you the component that is to be tested.

So that could be a lamp, it could be a resistor, for example, and the voltmeter is connected across that component or in parallel with that component.

So you can see that there's a wired connection to the left and to the right of that component.

It's important to make a note here that the wire on the positive side of the voltmeter should be connected to the closest positive side of the cell or battery.

And likewise, the negative wire on the side of the voltmeter should be connected closest to the negative side of the cell or battery.

So key learning points from this slide are that a voltmeter is used to measure voltage, which we use a symbol capital V for.

"Potential difference" is the word used interchangeably with "voltage," and voltmeters are always placed across the component or in parallel with that component.

The voltmeter below is measuring the voltage across a lamp.

So we can see here that there's a series circuit which consists of a cell and a lamp, and if you wanted to measure the voltage across the lamp, you would place the voltmeter in parallel with the wires connected either side of that lamp, like that diagram shows.

The voltmeter is measuring the voltage across the cell in this diagram.

So here the voltmeter is across the cell, and you can see it's in parallel with the wires connecting either side of the cell.

This voltmeter, where the purple line is pointing, measures how hard the current is being pushed through the lamp by the cell.

To use a voltmeter, first, connect the circuit and ensure that it works.

So we can see here we've got a parallel circuit that consists of a cell, a resistor, and a lamp, and one of the loops has the resistor in, and the second loop has the lamp in.

If you wanted then to measure the potential difference or the voltage across the lamp, you would add the voltmeter in parallel to the component that you wanted to measure.

So in this diagram here, you can see that the voltmeter has been added across or in parallel to the lamp, so it will measure the voltage across that lamp.

Voltmeters should not be placed in series with a component.

If they are, they stop the current from flowing.

So if you look at this series circuit here, the voltmeter has been connected in series with the lamp, and what that will do is it will stop the current from flowing, and that lamp would not light up because the current would be stopped from flowing.

So that is wrong.

Let's check our understanding of that so far.

So here we've got an image of a circuit which consists of a cell, two lamps, and a voltmeter.

Have a look at that circuit, and then we're going to answer the question, what will happen to the lamps in the circuit shown? A, only lamp one will light; B, only lamp two will light; C, both lamps will light; or D, lamp one will be dim and lamp two will be bright.

Have a go at seeing which one of those you think is correct.

So here we should have B as the correct answer.

Only lamp two will light.

And the reason why that's the case is because the voltmeter has been added in series with lamp one, and that will stop the current from flowing through that branch.

So well done if you managed to recognize that and get that question right.

The voltmeter in the circuit below is measuring how hard the cell is pushing current through the branch with the lamp on it.

So we can see the circuit there is showing a parallel circuit that contains a resistor in one loop and then a lamp in the second loop, and there's a voltmeter that's being connected across the lamp.

And remember, the voltmeter is measuring how hard the cell is pushing the current through the branch with the lamp on.

So we're measuring the current going across that lamp there.

The voltmeter is now measuring how hard the cell is pushing current through the branch with the resistor on.

So the voltmeter has moved now, and it's measuring how hard the current is being pushed through that resistor, which that purple arrow is representing there.

Let's check our understanding of that.

So you can see here there are three pictures of three different circuits that each contain a cell and two lamps, and then there's a voltmeter that has been put into that parallel circuit.

Which of the circuits below correctly shows how to measure the voltage across lamp X? Is it A, B, or C? Now you need to look really carefully here at where the leads for the voltmeter are connected onto the circuit.

If you need to pause the video to work this out, then please do, and then press play ready when you've got your answer.

Hopefully, here we've recognized that A is the correct answer.

In A, the voltmeter is connected in parallel with the lamp X.

B is wrong because then the voltmeter is connected in series with the cell, and C is sort of a combination of series and parallel across, but the voltmeter is not connected in the correct way to measure the voltage across lamp X.

So well done if you manage to get that question right and said it was A.

Great job so far.

We're ready now to move onto the first task of the lesson, task A.

And to do this task, you need to set up the four parallel circuits shown here, represented by the letters A, B, C, and D.

And if we just have a look at each of those circuits, A consists of a cell, a lamp, and a resistor.

B is a parallel circuit that contains a cell, a lamp in one loop, and then a motor in the second loop circuit.

Circuit C is a circuit that contains a battery made up of two cells, and then the first loop has a resistor in, and the second loop has a motor.

And then finally, circuit D has a battery consisting of three cells, and in one loop, there is a buzzer, and in the second loop, there is a resistor.

Now your job is, first of all, to set up circuit A as shown in the diagram.

You're then going to measure the voltage across each of the branches.

So this is going to be across the lamp and the resistor for A.

And then thirdly, you're going to record all of your measurements.

Then, fourthly, you will repeat that process for the remaining circuits B, C, and D.

So you will have a reading for the voltage across each of the two components within the circuit.

I'm sure you're gonna have a great job doing this and lots of fun.

Give it your best go and then come back to me ready to go through the answers.

Let's see how we did with that.

Now these are just sample voltage measurements.

You might have slightly different readings, which is fine as long as they follow the same trend.

So for circuit A, we had a voltage reading across the lamp of 1.

4, and it was the same across the resistor, so they had the same voltage.

The voltage in each branch was the same.

Then for B, that was the same again.

The voltage across each of the components was 1.

5, which was roughly the same as the previous circuit.

But the important thing there is that the voltage is the same in each branch with circuit B.

Then for circuit C, the voltage across the resistor we had as 3.

1 volts, and the voltage across the motor was 3.

1 volts.

So again, the voltage in each branch of the circuit is the same, but what's important here is that because there are two cells, the voltage should be roughly double what the voltage was in circuits A and B.

And then for circuit D, this time there are three cells.

So the voltage within each branch should be roughly three times the amount of what you got in circuits A and B, but the voltage across each branch within the circuit is going to be the same.

So, really big well done if you manage to get those readings.

If you need to pause the video to correct anything, please do.

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

So great job; we know how to measure voltage in a parallel circuit.

So let's look now at a rule for voltage in parallel circuits.

Looking at this circuit here, which consists of a cell, a lamp, and a resistor, which is a parallel circuit, voltage across parallel branches.

So here the voltage across the lamp was 1.

4 volts, and the voltage across the resistor was also 1.

4 volts.

So this links to the previous task that we just did.

The voltage within each branch was the same.

What we can also say is that the voltage across the cell is also 1.

4 volts.

The three voltmeters in the circuit below will all read the same voltage.

So this is the same circuit as the previous slide.

The voltage across the cell is going to be the same as the voltage across each of the two parallel branches.

Let's check our understanding of that.

So here we've got a circuit diagram.

It's a parallel circuit that consists of a cell and then a lamp, and a resistor.

What are the voltages across the lamp and the resistor in the circuit below? The cell has a voltage of 2.

0 volts.

So, is it A, the lamp has one volt, and the resistor has zero volts? B, the lamp has one volt, and the resistor has one volt, too? Or C, the lamp has two volts, and the resistor has two volts? Well done here if you went for C.

Remember, the rule is that the voltage in the branches is the same as the voltage of the cell.

So here, if the cell has a voltage of two, then the lamp will have a voltage of two, and also the resistor will have a voltage of two.

So great job if you've got that right.

A voltmeter that is connected anywhere here, where the red lines are, to anywhere here will give the same reading.

Another way of representing this is with this simulation here.

In the simulation below, the voltage does not change as the probes slide along the wires.

So the wires have no resistance, which means that as long as the voltmeter is across a component, the voltage will be the same.

The wires do not affect the voltage.

Let's check our understanding of that concept.

What is a voltmeter measuring the voltage across in the circuit shown? So you can see a circuit there that consists of a cell and then a resistor and a lamp, and it's a parallel circuit, and there's a voltmeter connected there.

And what is that voltmeter measuring the voltage across? A, only the cell; B, only the resistor; C, only the lamp; or D, any of the three components? This answer is D.

So, that voltmeter, as long as it is across a component, it will measure the voltage of any of those three components because the rule is the voltage of the cell is going to be equal to the voltage of the components in the first branch and the second branch.

And remember, the wires do not affect the measurement.

So well done if you manage to get that right.

Great job.

Let's look at this rule in a little bit more detail now.

Look at the circuit below, and if we said the lamps in the parallel circuit below are identical, the voltage across the branch with one lamp is going to be the same as the voltage across the branch with two lamps.

Each branch forms a direct loop to the cell.

So we can see here the single-lamp branch has a direct loop to the cell, and also the two-lamp branch has a direct loop to the cell, too.

The rule for voltage and parallel circuits is: The voltage across each branch of a parallel circuit is the same as the voltage across the cell or the battery.

So here in this circuit, the cell has a voltage of three.

That means that the voltage across the first branch will also be three, and the voltage across the second branch will also be three.

What this does mean, though, is that because the second branch consists of two lamps, the voltage across each lamp will be half of three, which is 1.

5, because they share the three volts within that branch.

Let's check our understanding of that idea.

So here we've got a circuit diagram, and the question's asking, what is the voltage across the lamp in the circuit below? And it tells you in that circuit that the voltage of the cell is three volts.

It's also told you that the lamp has a resistance of six ohms and that the resistor has a resistance of three ohms. But the question's asking, what is the voltage across the lamp in the circuit? Is it A, three volts; B, two volts; C, 1.

5 volts; or D, one volt? Now, hopefully, here we went for A.

It's three volts.

If the voltage of the cell is three volts, then the voltage across the first branch will also be three volts.

So in this case, the lamp has a voltage of three volts.

Now let's have a look at this question.

So same circuit here.

What is the voltage across the resistor in the circuit below? Remember, the cell has a voltage of three volts.

Is it A, three volts; B, two volts; C, one volt; or D, one volt? Now this is a very similar question to the last one.

We should have, again, A.

The voltage is the same in each branch, so therefore it's going to be three volts.

Well done if you manage to get that right.

A voltmeter needs to be connected across a component with resistance to measure a voltage.

Which we can see on this circuit diagram here we have a cell that has a voltage of three volts.

There is a lamp in there, and you can see that the voltmeter has been connected to either side of the lamp, so it's measuring the voltage across that lamp, and the lamp has a resistance.

In this case, the voltage across the lamp would be three volts because it's going to be the same as the voltage that comes from the cell.

If we look at the next circuit here, we've now got a cell and a lamp with a voltmeter around it, and then there is also a voltmeter that is just connected over wires.

There is no component within that voltmeter wires.

The voltage across the voltmeter where it's just connected to the wires is going to be zero.

The leads have zero resistance, so have a voltage of zero across them.

Let's see if we've understood that.

What are the correct readings for voltmeters one, two, and three in the circuit shown? So we have a look at that circuit.

You've got three voltmeters.

One is across the cell of three volts.

One is across the wires, which is two.

And number three voltmeter is across the resistor.

So remember, you are going to say what the correct readings are for each one of those voltmeters.

Is it A, one equals three volts, two equals three volts, and three equals three volts? B, one equals three volts, two equals zero volts, and three equals three volts? Or is it C, one equals three volts, two equals one volt, and three equals two volts? Have a go and select what you think is the correct answer.

We should have got here B.

B is correct because the voltmeter has a voltage of three, so one will be three.

And also, the voltage across the resistor is going to be the same, which is going to be three.

But the voltage across the wires is going to be zero, so number two is zero.

And that is because wires do not have a resistance.

Really, really great job if you manage to get that right.

Well done.

We're ready now to move on to the last task of the lesson, which is task B.

And you are going to use the statements to describe how to measure the voltage across a lamp.

Some of the statements are not going to be needed within this answer, so you need to select which statements are correct and also put them into the correct order.

So the first statement has been done for you.

You're going to set up the circuit and make sure that it is working.

So let's read through what each of the statements are.

"Connect a new wire between the voltmeter and one side of the lamp." "Connect a new wire between the voltmeter and the other side of the lamp." "Collect a voltmeter, which is labeled with a letter V." "Plug it into the voltmeter." "Collect a voltmeter, which is labeled with a letter V or letter A." "Take out one of the wires from the lamp." "The wire from the positive side of the voltmeter should be closest to the positive side of the cell." "The wire from the positive side of the voltmeter should be furthest from the positive side of the cell." I'm sure you can do a fabulous job of this.

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

Let's see how we did.

So the first statement's been done for you.

You're going to set up the circuit and make sure that it is working.

Then number two, collect a voltmeter, which is labeled with the letter V.

Doesn't have a letter A.

You must use the component with a letter V.

Then number three, connect a new wire between the voltmeter and one side of the lamp.

Number four, connect a new wire between the voltmeter and the other side of the lamp.

And then finally number five, the wire from the positive side of the voltmeter should be closest to the positive side of the cell.

Fantastic job if you manage to get those statements right and put them in the right order.

If you need to pause a video to correct your work or add in any extra detail, then please do.

But we're going to move on next to summarize what we've learned in today's lesson.

Well done, you've done a fabulous job today on voltage and parallel circuits.

So let's have a look at what we've learned.

We started off by saying a voltmeter is connected to each side of a cell battery or component and measures the voltage or potential difference in volts.

We then said the voltage is like the strength with which current is pushed around a circuit through the components.

The branches in a parallel circuit are like loops connected directly to the cell or battery that have the same voltage.

And a branch with high resistance will get the same voltage as a branch with low resistance.

Really great job with today's lesson.

I've really enjoyed teaching it.

I hope you have too, and I look forward to seeing you next time.