Hello, I'm Dr.

De Mello and I'll be teaching you today's lesson.

This lesson is called Measuring Current in a Parallel Circuit and it comes from the unit on Resistance and Parallel Circuits.

Today's outcome is I can describe how to measure current through each branch of a parallel circuit.

Let's look at today's keywords.

The first one is ammeter.

This is a device put into circuits in series and it measures the current flowing through that branch that it's placed in.

The next one is amps and these are the units that current is measured in.

It's short for amperes and represented by the symbol capital A.

Our next key word is branch and this is a separate loop in a parallel circuit.

A parallel circuit will have separate loops which are branches.

Finally, we have parallel circuit and this is a circuit with more than one complete loop from one end of a battery round to the other end.

This lesson about measuring current in a parallel circuit has two parts.

The first part is measuring current in a parallel circuit and the second part looks at the rule for current in parallel circuits.

Let's begin with current in a parallel circuit.

An ammeter measures the current that flows in a wire in amps.

We've got the symbol for an ammeter here and we've also got a photo of an ammeter that can be used in labs.

The current in a wire is the amount of electric charge that moves through a branch of wire each second.

In this animation, you can see the charge as electrons moving through the wire.

The current is how much of this charge moves through.

This animation shows how current flows in a parallel circuit.

A large current flows in the battery branch and you can see this because the charges are moving quite quickly through the battery.

Smaller current flow in the other branches and you can see that the charges are moving more slowly, meaning there is a lower current.

Let's check your understanding with a true or false statement.

The statement is the current in all parts of a parallel circuit is the same.

Is that true or false? Pause the video now, choose either true or false, and then come back to see how you've done.

Welcome back.

If you chose false, that's correct.

Now's the time to justify your answer.

The choices are a, at a junction, the current splits and so is reduced.

Or B, each branch has the same resistance and so the same current.

Pause the video now, pick the justification you think is correct, and then come back to see how you've done.

Welcome back.

If you chose at a junction, the current splits and so is reduced, that's correct.

A large current will split into smaller currents.

Justification b, each branch has the same resistance and so the same current is technically correct but it's not the correct justification for the false answer.

Well done if you got that right.

When measuring current in a circuit, the current in each branch is measured differently.

We're gonna look at how that's done.

So to measure the current through the cell, we would break the circuit, so we'd unplug the leads making a gap, and then we'd connect the ammeter in that gap by plugging it into the lead ends that we've just opened up.

So summarising that, to measure the current through the cell, you break the circuit near the cell and then connect the ammeter.

The ammeter must be connected directly to the cell.

You can see this purple highlighted line shows that the ammeter is directly connected to the cell.

The ammeter can be placed in different positions.

It can be placed lower down over here, and again, it's directly connected to the cell and you can see this with a purple line.

It could be placed over here on the other side of the cell.

Again, there's a purple line directly connected to the cell.

Or it could be placed a bit closer.

Again, the purple line shows it's directly connected to the cell.

The ammeter can be connected anywhere along the top branch before the junctions.

You can see along the highlight of purple wires there's no junctions.

It's a single wire, so the current cannot be split, so you'll measure the full current.

As soon as you pass the junctions just below the purple highlights, the current will split and you won't be measuring the current through the cell.

Let's do a check for understanding.

Which of the following circuits below show an ammeter in the correct position to measure the current through the cell.

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.

The ammeter is just beside the cell and it's directly connected to it, so it will be measuring the current through the cell.

If you also chose answer c, that's also correct.

This time the ammeter is on the other side but it is directly connected to the cell.

Answer b is not correct because the ammeter is below the junction.

There's a junction in between the ammeter and the cell so the current will split off.

Well done if you got that right.

Here is a circuit with a resistor in one branch and a lamp in the other.

To find the current through the resistor, the ammeter should be connected in this middle branch.

You can see the middle branch highlighted with the purple lines.

The ammeter can be placed here or it could be placed over here anywhere along that branch that was highlighted with the purple lines.

In either of these two positions, the ammeter would measure just the current going through the resistor.

Here is a check for understanding.

Which of the following circuits show an ammeter in the correct position to measure the current through the resistor? Pause the video now, study the circuits carefully, and then come back to see how you've done.

Welcome back.

If you chose answer b, that's correct.

Here the ammeter is placed just beside the resistor in the resistor's branch so it measures the current that flows through the resistor.

There's a direct connection between the ammeter and the resistor.

In circuit A, the ammeter is measuring the current after it has passed both through the resistor and the lamp.

Those two current add up, so the ammeter is measuring the total current through the resistor and the lamp.

A is not correct.

And in answers c, the ammeter is not correctly connected.

It needs to be connected in series with the resistor, not in parallel as it is over here.

Well done if you got that right.

In the same circuit as before, if you wanted to find the current through only the lamp, the ammeter should be connected in the lower branch, this branch that we've highlighted purple.

The ammeter could be placed in any of these positions.

Over here just after the junction, over here just beside the lamp, over here just after the lamp, over here just before that junction.

In any of these positions, the ammeter would be in that purple highlighted branch.

Let's do a check for understanding.

Which of the following circuits show an ammeter in the correct position to measure the current through the lamp? Pause the video now, look carefully through the three circuits.

Choose the one or ones you think is correct and then come back to check your answer.

Welcome back.

If you chose answer a, that's correct.

The ammeter over here is in the bottom branch.

It's got a direct connection to the lamp, so it's measuring the current that is passed through the lamp.

Answer b, the ammeter is connected in parallel.

Ammeters should not be connected in parallel, they should be connected in series, so answer b is not correct.

In answer c, the ammeter has three connections.

Ammeters only have two connections, so answer c is not correct as well.

Well done if you got that right.

We've reached the end of this section.

Now is the chance for you to practise what you've learned.

You're going to set up circuit A shown below.

Circuit A has one cell and it's got two resistors in parallel, a five ohm resistor and a 10 ohm resistor.

You're going to measure the current through the cell.

You're then going to measure the current through each branch with the resistors.

You record all your measurements and then you're going to repeat steps one, two, and three for circuits B and C.

Circuit B has a five ohm resistor and a lamp and circuit C has a 10 ohm resistor and a lamp.

Pause the video now.

Go ahead and set up the circuits and measure the currents, and then come back to see how you've done.

Welcome back.

Here's a set of sample results for the current in the different branches.

So for circuit A, in the cell the current was 0.

45 amps.

In the five ohm resistor, the current was 0.

30 amps.

In the 10 ohm resistor, the current was 0.

15 amps.

In circuit B, the current through the cell was 0.

50 amps, through the five ohm resistor, it was 0.

30 amps, and through the lamp it was 0.

20 amps.

Finally, in circuit C, the current through the cell was 0.

35 amps, through the 10 ohm resistor 0.

15 amps, and finally through the lamp 0.

20 amps.

In all the circuits, even though the same cell was used, slightly different currents flowed through the cell.

We'll examine these results more carefully in the next section.

This next section is the rule for current in parallel circuits.

In circuit A, the current in the branches were in the cell branch, 0.

45 amps.

In the five ohm branch, it was 0.

30 amps and in the 10 ohm branch it was 0.

15 amps.

0.

30 plus 0.

15 equals 0.

45.

The current in the cell branch is equal to the sum of the current in the other branches.

Looking at the branches, the five ohm branch has 0.

30 amps.

The 10 ohm branch has 0.

15 amps and these two come together at the junction on the right and they join up to make 0.

45 amps, which runs through the cell.

In circuit B, the currents in the branches were the cell had 0.

50 amps.

The five ohm resistor had 0.

30 amps and the lamp had 0.

20 amps.

0.

30 plus 0.

20 equals 0.

50 amps.

Again, the current in the cell branch is equal to the sum of the current in the other branches.

Finally, in circuit C, the current in the branches were 0.

35 amps for the cell, 0.

15 amps for the 10 ohm resistor, and 0.

20 amps for the lamp.

0.

15 plus 0.

20 equals 0.

35 amps.

Again, the current in the cell branch is equal to the sum of the current in the other branches.

Let's try a check for understanding.

What is the current through the cell in the circuit shown below.

Pause the video, look carefully at the circuits noting the resistors, lamps, and the currents flowing through them.

Make a choice and then come back to check your answer.

Welcome back.

If you chose 0.

5 amps, answer d, that's correct.

0.

2 amps plus 0.

3 amps gives you 0.

5 amps.

The current in the branches adds up to 0.

5 amps.

Well done if you got that right.

The rule for current at junctions is that the total current flowing into the junction equals the total current flowing out of the junction.

The current going in shown by this arrow equals the current going out shown by the two smaller arrows.

The smaller arrows add up in size to the bigger arrow.

If you look at the animation, you can see the current in the single wire is shown by the charges moving quite fast.

In the two separate wires, the current splits up.

Some current goes through the top wire and some current goes through the bottom wire.

The movement of the charges is slower in these wires, so the current is lower.

However, the same number of charges are coming in at the left as going out at the right in total.

So the total current flowing into and out of a junction is the same.

If 0.

45 amps flows into the junction, 0.

3 amps and 0.

15 amps will flow out.

0.

3 plus 0.

15 equals 0.

45 amps flowing out of the junction.

The current flowing in equals the current flowing out.

On the other side of the circuit, 0.

30 amps and 0.

15 amps added together give you 0.

45 amps that's flowing into the junction.

0.

45 amps flows out of the junction.

The current before equals the current after the junction.

Here's a check for understanding.

What is the current through the lamp in the circuit shown? Pause the video, study the circuit carefully, make your choice of answer, and then come back to see how you've done.

Welcome back.

If you chose answer a, 0.

2 amps, that's correct.

If you take away 0.

4 and 0.

2 from 0.

8, you're left with 0.

2 amps to flow through the lamp.

Well done if you got that right.

The current in the branch with the lower resistance is higher because resistance opposes the flow of current.

Five ohms is the lower resistance and 0.

3 amps flows through the five ohm resistor.

That's the higher current.

Let's check your understanding of that last point.

Which of the following gives the correct current through the resistor and lamp in the circuit shown below? There's 0.

6 amps coming out of the cell and then that splits up between the 10 ohm resistor and the five ohm lamp.

The choices are a, the resistor has 0.

6 amps and the lamp has 0.

6 amps.

B, the resistor has 0.

6 amps and the lamp has 0.

4 amps.

C, The resistor has 0.

3 amps and the lamp has 0.

3 amps.

Or d, the resistor has 0.

2 amps and the lamp has 0.

4 amps.

Pause the video now, make your choice, and then come back to see how you've done.

Welcome back.

If you chose answer d, the resistor has 0.

2 amps and the lamp has 0.

4 amps, that's correct.

The resistor has a higher resistance than the lamp, so it'll have a lower current.

In fact, the resistor is twice the resistance of the lamp, so it has half the current that flows through the lamp.

Well done if you've got that right.

Let's analyse the results from the task in more detail.

With the same cell, the current through identical components, those with the same resistance, is the same.

The current in the five ohm resistor in circuits A and B was the same, 0.

30 amps.

The current through the 10 ohm resistor in circuits A and C was 0.

15 amps.

And finally in circuit B, the current through the lamp was 0.

20 amps and in circuit C, the current through the lamp was 0.

20 amps.

It's the same.

Since the cell is pushing the current with the same voltage, the current through the same resistances will be the same.

Let's try a check for understanding.

Is this statement true or false? The current through the lamp in the circuit shown below will be 0.

3 amps if it has a resistance of seven ohms. Is this true or false? Pause the video, look at the circuit carefully, make a choice, and then come back to check your answer.

Welcome back.

If you chose true, that's correct.

Now is the time to justify your answer.

Your choices are a, fewer components on a branch will cause the current to change or b, the voltage pushing the current through each branch is the same.

Pause the video now, look at the justifications, and choose the correct one, and then come back to see how you've done.

Welcome back.

If you chose b, the voltage pushing the current through each branch is the same, that's correct.

If the resistance and the voltage pushing through the resistance is the same, the current that flows will be the same.

Justification a isn't correct because fewer components on a branch may not cause the current to change if they have the same resistance.

It's the resistance, not the number of components that's important.

We've come to the end of this section.

Here's a task to practise what you've learned.

Laura and Lucas have built the circuit shown below and they're predicting the size of the current through the lamp and the resistor.

The lamp has a resistance of four ohms and the resistor has a resistance of two ohms. Laura says, "A larger current flows through the resistor than the lamp because it has a smaller resistance." Lucas says, "A larger current flows through the lamp than the resistor because it's closer to the cell." Identify whether each pupil is correct or incorrect and explain why.

Pause the video now.

Study the circuit carefully and the statements Laura and Lucas have made.

Write out your answers and then come back to see how you've done.

Welcome back.

Laura is correct and Lucas is incorrect.

The voltage across both branches is the same.

There will be a bigger current through the resistor since it has a lower resistance than the lamp.

The distance of the components from the cell does not affect the current as the wires do not have resistance and they don't resist the flow of current.

Well done if you wrote down similar statements.

We've reached the end of this lesson.

Let's summarise what we've learnt.

In a parallel circuit, current flows in all the branches.

The current splits at the junctions, and the total current going into a junction is the same as the total current coming out of that junction.

When measuring the current in a loop, an ammeter must be placed in between the junctions at each end of the loop.

Each loop will have the same voltage across it, so the current in the loop will depend on the resistance of the loop.

A high resistance will give a lower current.

You've done really well to complete this lesson.

I hope to see you again soon.