Hello, I'm Dr.

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

This lesson is about voltage in parallel circuits and it comes from the Resistance and Parallel Circuits unit.

Today's outcome is I can describe the rule for voltages in a parallel circuit.

Let's begin.

These are today's keywords.

Firstly, we have voltmeter.

A voltmeter is a device connected in parallel with components to measure the voltage that goes across them.

Voltage is the next keyword, and voltage across a component describes how hard the current is pushed through it.

Potential difference is the next set of keywords, and these are a more formal term for voltage.

However, they can be used interchangeably, because they refer to the same electrical quantity.

Next, we have parallel circuits, and this is a circuit with junctions and separate loops.

This is a parallel circuit.

Finally, we have branch.

Each separate loop in a parallel circuit can be described as a branch.

If you'd like to look through these definitions, pause the video now, but do look out for the keywords in the video.

This lesson on voltage in parallel circuits has two parts.

The first part is about measuring voltage in a parallel circuit.

The second part is a rule for voltage in parallel circuits.

Let's start on measuring voltage in a parallel circuit.

A voltmeter shown here as a photo, and also as the circuit diagram symbol measures voltage in volts.

Potential difference is the scientific term for voltage.

Voltmeters are always placed across, or in parallel, with a component as shown over here.

So the component to be tested is placed in the circuit, and the voltmeter has two wires connected to either side of it to measure the voltage across it.

The voltmeter shown below is measuring the voltage across a lamp.

You can see that there's two wires connected either side of the lamp.

This voltmeter is measuring the voltage across the battery.

The two wires are connected to either side of the battery.

This voltmeter measures how hard the current is being pushed through the lamp by the battery.

When you're using a voltmeter in practise, you first connect the circuit and ensure it works.

You can then add the voltmeter in parallel to the component that you want to measure the voltage across.

Voltmeters should not be placed in series.

They stop the current from flowing.

So when you're building your circuits, do check your voltmeters and also a good way of doing this is to add them at the end of the circuit.

We've covered quite a bit of material.

Let's do a check for understanding.

What will happen to the two lamps numbered one and two in the circuit shown below? The choices are A, only lamp one will light.

B, only lamp two will light.

C, both lamps will light.

And D, lamp one will be dim and lamp two will be bright.

Pause the video now, study the circuit carefully, make a choice and come back to check how you've done.

Welcome back.

If you chose only lamp two will light, that's correct.

Lamp one won't light because the voltmeter is connected in series with it, and the voltmeter stops the current flowing.

You may have thought that lamp one might be dim while lamp two will be bright, but the voltmeter resistance is so high that lamp one should not light at all.

Well done if you got that right.

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

In this circuit, the voltmeter is now measuring how hard the battery is pushing current through the branch with the resistor on it.

Notice how the voltmeter has connections either side of the resistor.

Let's do a check for understanding.

Which of the circuits shown below correctly shows how to measure the voltage across lamp X? Pause the video now, study the circuits carefully, make your choice, and then come back to check your answer.

Welcome back.

If you chose answer A, that's correct.

This voltmeter is connected across lamp X, so it measures the voltage across lamp X.

In circuits B and C, the voltmeter is connected in series, or a sort of series/parallel connection.

If you find the circuits confusing, a good tip is to think, if I unplug the voltmeter, will the circuit still carry on working as it did before? It should.

The voltmeter should not affect the working of the circuit.

We've reached the end of the section on measuring voltage in a parallel circuit.

So now's the chance to practise what you've learned.

You're going to set up circuit A, which is shown in the diagram.

Circuit A has a battery, a bulb, and a resistor.

You're going to measure the voltage across each branch.

You'll record all your measurements, and then you're going to repeat for the remaining circuits, B, C, and D.

Circuit B has the same battery, a bulb, and a motor this time.

Circuit C has two more batteries, a resistor and the motor.

And finally, circuit D has three batteries, a buzzer and a resistor.

Pause the video now, carry out your experiment, and then come back to see what sort of results you're expecting to get.

Welcome back.

These are a sample set of results.

In circuit A, the voltage across the lamp is 1.

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

4 volts.

The voltages are the same.

In circuit B, the voltage across the lamp is 1.

5 volts, and across the motor is 1.

5 volts.

These are slightly different from circuit A, but in the same circuit, they are the same.

In circuit C, the voltage across the resistor with two batteries is 3.

1 volts, and across the motor it is also 3.

1 volts.

So the voltage is the same across those two components in circuit C.

However, since there are two batteries, the voltage is twice or roughly twice what it was before.

Finally, in circuit D, across the buzzer, the voltage is 4.

3 volts.

And across the resistor the voltage is 4.

3 volts.

Again, the voltage is the same across each branch, and also it's almost three times as much as the voltage from a single battery.

Well done if you've got similar sorts of readings.

We've now reached the second part of this lesson, a rule for voltage in parallel circuits.

We're going to look at voltage across parallel branches.

The voltage across the lamp was 1.

4 volts.

And the voltage across the resistor was also 1.

4 volts.

The voltage across the battery is also 1.

4 volts.

If we set up a circuit with three voltmeters like shown in the circuit below, all the voltmeters will read the same voltage.

The voltage across the battery is the same as the voltage across each of the two parallel branches.

Let's do a check for understanding.

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

0 volts.

The choices are A, the lamp has 1.

0 volts, and the resistor has 0.

0 volts.

B, the lamp has 1.

0 volts and the resistor has 1.

0 volts.

And finally C, the lamp has 2.

0 volts and the resistor also 2.

0 volts.

Pause the video now, check the circuit carefully, make a choice, and then come back to see how you've done.

Welcome back.

If you chose answer C, the lamp has 2.

0 volts and the resistor also has 2.

0 volts, that's correct.

Each branch is connected directly to the battery, so each branch will get 2.

0 volts as supplied by the battery.

If you chose answer B, you have to remember that the two components do not share the 2.

0 volts.

They don't get half each.

They get the full voltage.

And in answer A, the resistor will also get some voltage.

Well done if you got that right.

Let's look at how we connect a voltmeter and how it affects the reading in a parallel circuit.

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

Remember that the wires don't have a resistance that affects the way the circuit works.

This simulation shows that the voltage does not change as the probes slide along the wires.

Each time they move to test the voltage across a different component, the voltage remains at nine volts.

Remember, the wires don't affect the voltage.

Let's do a check for understanding.

What is the voltmeter measuring the voltage across in the circuit shown? Is it A, only the battery? B, only the resistor? C, only the lamp? Or D, any of the three components? Pause the video now, look carefully at the circuit, remembering what we've been talking about in terms of the wires and connections.

Make a choice, and then come back and check how you've done.

Welcome back.

If you chose answer D, that's correct.

The voltmeter is actually measuring the voltage across any, or all of the components.

Sliding the wires along to connect closer to the bulb, or the other side of the resistor, or even the battery, would give you the same voltage as we saw in the simulation before.

Remember that the wires do not affect the measurement.

Well done if you got that right.

Here's a circuit with three lamps.

These lamps in the parallel circuit are identical.

The voltage across the branch with one lamp is the same as the voltage across the branch with two lamps.

Remember, each branch gets the same voltage.

This is because it's connected directly to the battery.

We can see this in a picture.

Each branch forms a direct loop to the battery.

The single lamp branch has a direct loop to the battery, and the two lamp branch has a direct loop to the battery as well.

So the rule for voltage in a parallel circuit is the voltage across each branch of a parallel circuit is the same as the voltage of the battery.

The single bulb here gets a voltage of three volts, and the double bulb connection also gets a voltage of three volts across both bulbs.

Let's do a check for understanding.

We have here a 3.

0 volt battery, and in parallel are connected a six ohm bulb, and a three ohm resistor.

What is the voltage across the lamp in the circuit shown? Is it A, 3.

0 volts? B, 2.

0 volts? C, 1.

5 volts? Or D, 1.

0 volts? Pause the video, study the circuit carefully, make a choice, and then come back to check your answer.

Welcome back.

If you chose 3.

0 volts, that's correct.

The voltage across the battery will be the same voltage across the lamp.

Now let's check what the voltage across the resistor in the circuit might be.

Is it going to be 3.

0 volts, 2.

0 volts, 1.

5 volts, or 1.

0 volts? Again, examine the circuit carefully, make a choice, and then come back to check your answer.

Welcome back.

If you chose 3.

0 volts, that's correct.

It's going to be the same voltage as the voltage across the battery and also across the lamp.

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

So here we have a circuit where there's a bulb which has six ohms resistance, and the voltmeter is measuring the voltage across it.

The voltage will be 3.

0 volts, which is the same voltage supplied by the battery.

If we add another voltmeter, this time connected across the wires, the voltage across the lamp will still read 3.

0 volts, but the voltage across the wire will be 0.

0 volts.

The leads have zero resistance, so a voltage of zero across them is what you will find.

A component needs resistance to be able to give a voltage.

Here's a check for understanding.

What are the correct readings for voltmeters one, which is connected across the battery, two, which is connected across wire, and three, which is connected across a resistor in the circuit shown? Are they A, one is 3.

0 volts, two is 3.

0 volts and three is 3.

0 volts? Or B, one is 3.

0 volts, two is 0.

0 volts, and three is 3.

0 volts? Or is it C, one is 3.

0 volts, two is 1.

0 volts, and three is 2.

0 volts? Pause the video now, look carefully at the circuits, and where the wires are connected, and then choose your answer and then come back to see how you've done.

Welcome back.

If you chose answer B, that's correct.

The voltage across the battery is 3.

0 volts as shown.

The voltage across the wire is 0.

0 volts because the wire has no resistance, and then the voltage across the resistor, voltmeter three, will read 3.

0 volts.

Resistors do have resistance and it's the only component in the circuit, so it will have the full voltage.

Well done if you've got that right.

That was quite complicated.

We've reached the end of the section, so here's a task to practise what you've learned.

We have a set of statements here.

Use these statements to describe how to measure the voltage across a lamp.

Some of these statements are not needed.

Starting with the first one, which we've done for you, you're going to set up the circuit and make sure it's working.

The choices of the next statements are, you can connect a new wire between the voltmeter and one side of the lamp.

You can connect a new wire between the voltmeter and the other side of the lamp.

You can collect a voltmeter, which is labelled with a letter V.

Plug it into the voltmeter.

Collect a voltmeter, which is labelled with a letter V or a letter A.

Take out one of the wires from the lamp.

The wire from the plus side of the voltmeter should be closest to the plus side of the battery.

The wire from the plus side of the voltmeter should be furthest from the plus side of the battery.

Look through these statements carefully.

Pick the ones you need in the correct order and write out the steps for measuring the voltage across a lamp.

Pause the video now.

When you've completed the task, come back and see how you've done.

Welcome back.

This is the set of statements in order that you should have.

After you've set up the circuit and make sure it's working, you should first collect a voltmeter, which is labelled with a letter, capital V.

You'll then connect a new wire between the voltmeter and one side of the lamp.

After you've done that, you connect a new wire between the voltmeter and the other side of the lamp.

And you should ensure, lastly, that the wire from the plus side of the voltmeter should be closest to the plus side of the battery.

If you identified those statements correctly, and put them in the correct order, really well done.

You've now finished this lesson.

Let's summarise what we've learned.

A voltmeter is connected to each side of a battery or component and measures the voltage or potential difference in volts, which is given by capital V.

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 battery that have the same voltage.

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

Well done on completing this lesson.

I hope to see you again soon.