Hello, my name's Dr.

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

Today's lesson is called Electrical Resistance and it comes from the resistance and parallel circuits unit.

Our outcome today is I can compare the electrical resistance of different components using measurements of current.

Let's begin, starting with our keywords today.

The first one we have is resistance.

This is a measure of how hard it is for current to flow through a component.

The next keyword is ohm and the symbol is the Greek letter Omega.

Ohm is the unit for resistance.

Next we have ammeter, and ammeter measures the current that flows through a circuit or a component.

And the keyword that follows that is amp or ampere, and the short for that is capital A.

Current is measured in amps.

Our last set of keywords are control variable.

When we do an investigation, we need to only test one thing at a time.

The other variables, things need to be kept the same or constant.

This allows for a fair test.

Here are the keywords defined.

If you'd like, pause the video so you can read them, but look out for them in the rest of the lesson.

This lesson on electrical resistance has two parts.

The first part is about current through different components.

The second part is about electrical resistance itself.

We'll first start with current and different components.

In this circuit shown, we can see that the bulb brightness and the current depend on the voltage of the cell.

This is an animation of the circuit and it shows how increasing the voltage of the cell also increases the current and it increases the brightness of the bulb.

Here is a check for understanding.

Which of the following three things will happen.

If another cell is added in series and correctly aligned to the circuit shown, The choices are A, the current will increase.

B, the voltage will increase.

C, the number of charges in the wires will increase.

And D, the bulb will get brighter.

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

Welcome back.

If you chose, the current will increase.

That's correct.

If you also chose, the voltage will increase.

That's also correct.

And finally, if you chose, the bulb will get brighter.

That is also correct.

The only one that doesn't happen is the numbers of the charges in the wires will increase.

The numbers of charges in the wires stays the same.

It's just the way they flow through the wires.

The circuits that'll appear below will show how cells can light a bulb.

One bulb is lit brightly by one cell.

If you have two cells, they'll light the bulb more brightly.

On the other hand, if you have two cells that point against each other in opposite directions, they cancel out and the bulb does not light at all.

Something to note in real life is that the cells don't often have exactly the same voltage, so some current will flow.

The bulb might light very dimly.

In this animation, the cells are exactly the same voltage, so they fully cancel each other out.

The electrons are not shown flowing.

We'll now look at increasing the number of bulbs which will make them dimmer.

One bulb will still be brightly lit by one cell.

If we have one cell lighting two bulbs, the two bulbs are not as bright as before, and if we have three bulbs with the one cell, they're even less bright.

Increasing the number of bulbs makes it harder for the current to flow.

Here is a check to see how you've understood the last bit of teaching.

Which of the following statements are correct when identical bulbs are added in series to a circuit? The choices are, A, adding more bulbs will make them brighter.

B, adding more bulbs will decrease the current.

C, adding more bulbs will increase the voltage.

And D, adding more bulbs will decrease their brightness.

Pause the video now.

Make your choices and then come back to check your answers.

Welcome back, if you chose answer B, adding more bulbs will decrease the current, that's correct.

If you also chose D, adding more bulbs will decrease their brightness, that's also correct.

Answer A was incorrect because adding more bulbs did not make them brighter and answer C was not correct.

Adding more bulbs didn't increase the voltage.

It just decreased the current and the brightness.

Well done if you got those right.

In this circuit shown the size of the current depends on the following things.

The voltage of the battery, which in this case is 1.

5 volts.

The resistance of the components in the circuit.

Resistance is a measure of how hard it is for the current to flow through them.

Let's check your understanding.

Which of the circuits shown below has the highest resistance? Is it A the circuit with one bulb and one battery? Or B, two bulbs and one battery? Or C, three bulbs and one battery? Pause the video now make your choice and then come back to check your answer.

Welcome back.

If you chose answer C, that's correct.

The three bulbs increase the resistance by three times.

Each bulb adds to the resistance so that circuit had the highest resistance.

Well done if you got that right.

These two circuits shown below are identical except for the bulbs.

So the battery and the ammeter and the wires are all identical.

In the first circuit he bulb in this circuit has a higher resistance and the current on the ammeter reads 0.

20 amps.

In the second circuit the bulb in this circuit has a lower resistance and the current on the ammeter is 0.

90 amps.

Let's do a quick recap for understanding.

Which of the circuits shown has the lowest resistance.

The cells are identical.

The first circuit has 0.

10 amps.

The second circuit has 0.

14 amps.

And the last circuit has 0.

22 amps.

Pause the video now, make your choice and then come back to check your understanding.

Welcome back.

If you chose answer C, that's correct.

The circuit with the highest current will have the lowest resistance.

The other two circuits have a lower current, so they will have a higher resistance.

Well done if you got that right.

Let's try another check for understanding.

We've got two circuits, one and two.

In circuit one there's a 1.

5 volt cell, which is in a circuit with one bulb and ammeter that reads 0.

8 amps.

The second circuit has two cells to make a battery of three volts and there's a resistor with ammeter connected that reads 0.

8 amps.

Is this statement true or false? Circuit one has a lower resistance than circuit two.

Pause the video now study the two circuits and make your choice true or false, and then come back to check how you're doing.

Welcome back, if you chose true, that's correct.

Circuit one does have a lower resistance than circuit two.

Now's the time to justify your answer.

The choices are, circuit one has a higher voltage across the bulb and circuit two needs a higher voltage to give the same current.

Which of these is a good justification for the answer true? Pause the video, now read through those statements, look at the circuits, make your choice and then come back to see how you've done.

Welcome back.

If you chose answer B.

Circuit two needs a higher voltage to give the same current.

That's correct.

If you need a higher voltage to give the same current in a different circuit, that circuit will have a higher resistance, so circuit two has a higher resistance than circuit one, so the first statement circuit one has a lower resistance is true.

Well done if you got that right.

That's quite a complex and difficult question.

Now's the chance to do a practise task, a practical to see how you've got on.

You're going to set up the circuit as shown below and place each of some different components into the space shown marked unknown component.

Make sure it's connected up into the circuit.

Then you'll measure the current through it.

Record the current for each of these components in a table and then rank them in order of resistance.

After you've completed your table, predict what will happen to the order of resistances if another cell is added in series and correctly aligned to the circuit.

When you've done that, carry out the test again, this time with two cells and record the results in another table.

Compare the two tables and then comment on your prediction from two and explain if this was a fair test.

Pause the video now, go ahead and do the practical and when you're finished, come back and we'll see how you got on.

Welcome back, here are a set of results that you can compare yours with.

The results are for a 1.

5 volt bulb, a 6.

5 volt bulb, a pencil that's been sharpened at both ends so that the graphite lead can be tested.

And then two resistors of different values.

Their currents were as follows, the 1.

5 volt bulb had a current of 0.

187.

The 6.

5 volt bulb had a current of 0.

043, so significantly less.

The pencil led had a current of 0.

056.

The first resistor had a current of 0.

088.

Second resistor 0.

033.

In terms of the order of the resistance, resistor two was the highest resistor.

Next was the 6.

5 volt bulb with the second highest resistance.

Then the pencil lead with the third highest resistance.

Resistor one was the fourth highest resistance.

And then the 1.

5 volt bulb had the lowest resistance In part two, the prediction is the order of resistances will stay the same as the components have not changed.

The resistance of the circuit depends on the components, not the voltage, so increasing the voltage will increase the current through the circuit, but the resistance will stay the same for each component.

With two cells and the components in the same order.

The 1.

5 volt bulb had 0.

391.

The 6.

5 volt bulb had 0.

088.

The pencil lead at 0.

117.

Resistor one had 0.

185.

Resistor two had 0.

066.

The order remained the same.

Looking down the column 5, 2, 3, 4, 1 was the same as in part one.

So the order of resistances did stay the same.

Commenting on the prediction, the prediction was correct.

The order of resistances stayed the same.

This is because the components to be tested were the same each time.

The resistances of the different components.

The bulbs, pencil and resistors were the independent variables in each test.

The other variables were the rest of the components in the circuit, these were kept the same as they were the control variables.

If you got similar results, really well done.

The main point was to find that the resistances stayed in the same order.

We've now reached the second part of the lesson.

Electrical resistance.

Resistance is a measure of how hard it is for current to flow.

This resistor has a low resistance so it's easier for current to flow through it.

This next resistor has a high resistance, so it's harder for the current to flow through it.

The coloured bands show that the resistors are different.

Let's do a check for understanding.

Which of the resistors below is the most difficult for current to flow through.

Resistor A has a high resistance.

Resistor B has a medium resistance.

And resistor C has a low resistance.

Pause the video now make your choice and then come back to check your answer.

Welcome back, if you chose answer A, the resistor with the high resistance.

That's correct, that's the one which is most difficult for current to flow through.

The medium and low resistance resistors will be easier for current to flow through.

Resistance is measured in ohms. The stripes on a resistor are like a barcode that tells you the value of its resistance.

You can look these up on the internet to see what they are.

The first resistor is a 16 ohm resistor.

The colours on the second resistor tell you that it's a 42 ohm resistor.

It's easier for current to flow through the 16 ohm resistor than it is for the 42 ohm resistor, so a higher current will flow through it.

Here's a check for understanding.

Which of the resistors below is the easiest for a current to flow through? The same symbol is used for all of them, but the resistances are shown.

Resistor A has a resistance of a 100 ohms. Resistor B has a resistance of 101 ohms. And resistor C has a resistance of 102 ohms. Pause the video now, make your choice and then come back when you're ready to check your answer.

Welcome back.

If you chose answer A, that's correct.

This resistor has the lowest resistance, so it's going to be the easiest one for current to flow through.

In a circuit a resistor opposes the flow of current as shown by the green arrows.

If you add an identical resistor in series with the first one, it doubles the resistance, so that means if the voltage is the same because we haven't changed the cell, the current will be halved and the arrows are smaller.

The second resistor can be placed anywhere in a series circuit and it will still have the same effect on current.

It can be placed just after the first resistor.

It can be placed by the battery or cell.

It can be placed on the other side of the cell or it can be placed on the vertical sides of the circuit.

The resistors have an identical effect on the current wherever they're placed in the circuit as long as it's a series circuit.

Here's a check for understanding.

There are two circuits, circuit A and circuit B.

Is the statement true or false? Circuit A has a greater resistance than circuit B.

Pause the video now carefully study the two circuits.

Choose either true or false for the statement and then come back to check your answer.

Welcome back.

For the statement, circuit A has greater resistance than circuit B, the answer is false.

And these are two possible justifications for the answer.

A, the resistors are closer to the cell or B, the position of the resistors does not matter in a series circuit.

Pause the video now look at the two statements and choose the one that justifies the false answer.

Welcome back.

If you chose the position of the resistors does not matter in a series circuit, that's right.

That's the justification for the answer being false.

If you got that right, really well done.

Quite a difficult question.

in the simulation below the hands act like resistors.

If this hand on the top left grips tighter, the loop slows down everywhere.

If another hand gripped here at the bottom, the loop would slow down again everywhere.

The loop has to pass through all of the hands at the same time.

The position of resistors or hands does not change how they affect the current, the speed of the loop.

In this simulation, when the switch is closed, the current begins to flow through all the bulbs at the same time.

Each bulb opposes the flow of the current.

When the switch is closed.

Each time the switch is closed the current moves immediately.

The current has to be passed through all the bulbs at the same time.

Let's check your understanding.

In which order will the bulbs light up when the switch and the circuit shown is closed? We've got bulbs one, two, and three.

Two of the bulbs are either side of the cell and the third bulb is at the bottom of the circuit.

The choices are A, one lights up first, then three and then two.

B, one, two and three light up at the same time.

C, two lights up first, then three and then two.

And finally D one and two light together, and then finally three.

Pause the video now make your choice and then come back to check your answer.

Welcome back.

If you chose answer B, that's correct.

All the bulbs will light up at exactly the same time.

The current will flow through all the bulbs the moment the switch is turned on, bulbs closer to the cell or nearer the positive or negative end of it so not get current sooner.

All the bulbs get the current at the same time.

If you got that right, well done.

We've come to the end of the section.

Here's a practise task for you to see how you're getting on.

Laura and Jacob are trying to light some decorative Christmas lights as brightly as possible.

The circuit containing identical bulbs is shown below.

You can see it has one cell and then in a series circuit there are five bulbs with a switch.

Laura says, "The bulbs should be as near to the cell as possible so they light up quickly and brightly." Jacob says, "The bulbs can be placed anywhere in the circuit, it won't affect when they light up or how brightly." State whether each statement is correct or incorrect, and then explain why you've said this.

Pause the video now, read through the statements and study the circuit and then write out your answers and when you're ready, come back to check how you've got on Welcome back, look at your answers and see if you've written anything similar.

Laura's statement is incorrect.

The bulbs do not need to be near to the cell as a position in the circuit does not affect their brightness.

Jacob's statement is correct.

The position that the bulbs are in does not affect their brightness.

The bulbs will also be equally bright in a series circuit as they are identical and therefore have identical resistances.

All the lights oppose the current when the circuit is switched on.

It has to be pushed through all of the lights at the same time.

If you managed to get some of those right, well done.

We've reached the end of the lesson.

Let's summarise what we've learned.

Resistance is a measure of how hard it is for current to flow through a component in a circuit.

It is measured in ohms. Current can be measured with an ammeter and can be used to judge the amount of resistance in a circuit.

Increasing the voltage across a component will increase the current flowing through it, but the resistance will stay the same.

Adding resistors anywhere in a series circuit will affect the current everywhere.

You've done well to complete this lesson.

I hope to see you again soon.