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Hello, my name's Dr.

George.

This lesson is called "Measuring the resistance of a filament lamp" and is part of the unit, "Circuit Components".

You're going to do an investigation as part of this lesson.

The outcome for the lesson is I can find the resistance of a filament lamp by measuring the current and p.

d.

for a suitable range of p.

d.

I'll be using these key words in the lesson.

I'll introduce them as they come up, but this slide is here in case you want to come back anytime and remind yourself of the meanings.

The lesson has two parts.

They're called investigating the resistance of a filament lamp and checking for errors in electrical investigations.

A filament lamp contains a thin coil of wire.

And when an electrical current flows through the wire, it gets hot and glows brightly.

I'll show you a circuit you can use to investigate a filament lamp.

You'll want to measure the p.

d.

across it and the current through it so you can find its resistance.

So connect the lamp to a DC supply that's direct current, current that flows in one direction only.

And in series with the lamp, you'll need a variable resistor.

You can use that to vary the p.

d.

across the lamp.

The p.

d.

across the supply is shared between the resistor and the lamp.

It's shared in the ratio of their resistances.

So as you change the resistance of the variable resistor, you'll change the p.

d.

across the lamp.

You'll need an ammeter in series to measure the current, and a voltmeter in parallel with the lamp to measure the p.

d.

across it.

And you can use those to calculate resistance of the lamp over a wide range.

So we're going to find out whether the lamp has the same resistance in any conditions or whether it's resistant to changes as the p.

d.

and the current change, and which of the circuits below is the correct one to measure p.

d.

and current when investigating a filament lamp.

And with short questions, I'll wait five seconds.

But if you need longer than that, press pause while you're thinking and press play when you have your answer ready.

The correct answer is circuit a.

In circuit b, the ammeter and voltmeter are connected the wrong way around, and in c, the voltmeter is in series with a lamp when it should be in parallel with it.

So here's the circuit that you can use and the p.

d.

across the lamp and its brightness will be changed by adjusting the variable resistor.

You can also disconnect the power supply and then you'll have a p.

d.

of 0 volts.

You can probably guess what the current's going to be when you do that.

Now to get the most accurate results, it's helpful to do a quick check before you start the experiment for what are the largest and smallest values of the variables that you can use.

The difference between these is called the range.

So what you might do is try out different values of the variable resistance and see what potential differences that gives you across the lamp and what range of currents as well.

You can choose the most accurate ammeter and voltmeter to measure the range of values that you're going to get.

Let's think about that.

Which of the ammeters here is most appropriate for measuring currents in the range 0 to 2 amps to three significant figures? The correct answer is b.

B and c look very similar.

They're both measuring amps, but the decimal point is in a different place.

So c can measure larger currents perhaps up to 99.

9 amps.

But if you are measuring smaller currents in the range of nought to 2 amps, you are only going to get two significant figures, things like 1.

7.

Whereas in b, you can get two decimal places and that will give you three significant figures.

Ammeter a is measuring in milliamps, so it can't go higher than 9.

99000th of an amp.

That's not going to cover the range 0 to 2 amps.

Your results need to include enough measurements spread across the whole range that's available so that you can make a valid conclusion.

A conclusion that's valid is answering the question that you are asking with your investigation.

It isn't easy to adjust a variable resistor to get exact round values of p.

d.

across the lamp, like 2.

00 volts, but you don't need to.

It's much better to just take values that are roughly evenly spread across the range and you'll find that's a lot quicker.

For example, here's a set of p.

d.

s that could be used.

Don't try to copy these.

You see that they're not exact round numbers, but they are quite evenly spread within the available range of nought to 12 volts.

And if you take a lot of measurements and it doesn't take very long, you don't need to take any repeats.

We take repeat measurements and average to get more accurate results so that we hope our graph will show the true relationship between the variables.

But instead of having a smaller number of points that are very accurate, we can have a larger number of points and each one may not be as accurate, but it should be enough for us to be able to see where the best fit line should go.

With so many results, it should be very obvious if there are any errors when you plot your graph.

If there have been any areas in a measurement, they won't sit on the best fit line.

During this experiment, you're going to swap the positive and negative connections on the power pack that lets you make the current flow in the opposite direction so that you can check whether the lamp behaves the same way when current flows the other way.

Start off with the ammeter and voltmeter connected so that they give positive readings.

Take your set of readings over a range of p.

d.

s and then swap the connections to the power pack and take another set of measurements.

Like this.

So why are many readings taken in electrical investigations rather than repeating them? Press pause while you read the options and choose your answer.

And as I said earlier, because it's often easy and quick to take electrical readings, it's quite easy to just move on to the next p.

d.

, take a reading and so on.

In some types of experiment, it takes longer to set up the next value of your independent variable, and so it's more worthwhile to take repeat readings for that value.

So I'd like you to go ahead and do this experiment.

You're going to investigate the p.

d.

across and current through a filament lamp as its brightness varies.

You'll need to create a table like this before you start taking measurements.

And you're not only going to record p.

d.

and current and calculate resistance, I'd like you to record the brightness of the lamp as well.

Describe how bright it is, describe its colour, and then when you've got your results, plot a graph of current against potential difference and then draw a best fit line through the points and look at the way the points are arranged and try to see if they suggest a straight line relationship or a relationship that is represented by a curved line.

Press pause when you do the investigation and press play when you have your set of results ready.

I hope that went well.

I'll show you a set of sample results.

And if you didn't for any reason manage to get a set of your own results, you could use these to plot a graph.

We can probably notice some things in this table.

We can see current going up when the p.

d.

gets larger, but it's hard to see what the exact relationship is, and that's part of why we draw a graph.

And the graph of these results looks like this.

I'll talk more about this graph in the next part of the lesson.

So let's go on to checking for errors in electrical investigations.

Here are those data points again, and the arrangement of these points isn't really suggesting a straight line.

It seems to fit a curve and we can draw a smooth best fit curve.

We don't force it to go through every point, but it does go close to most of these points.

But there is one point here that is very different.

It doesn't fit the pattern of the others and it seems to be an anomaly.

It's very unlikely that the real relationship between current and p.

d.

here has this bump in it.

It's much more likely that some sort of error happened when that measurement was taken.

Notice as well that the points on this graph cover the full range of p.

d.

that's available -12 to +12 volts and are reasonably evenly spaced doesn't have to be perfect.

Which of the following statements about the data shown on this graph is correct? The data points are not evenly spaced.

We don't know if the range is large enough.

There isn't actually a scale here and we don't know if there are too many anomalous points.

It's hard to tell when we have this big gap in the graph.

The anomalous point on this graph could have been caused by measurement errors.

It could be that for some reason, the current was smaller than expected or smaller than it should have been, or the p.

d.

was larger than expected.

So when you get an anomalous point on a graph, you can think about that, what type of error might have caused that point to end up there? The cause of the anomaly could be a combination of these things and other possible errors.

What could have caused the anomalous result below? Press pause when you're thinking about this.

It could have been caused by a higher than expected current reading.

You can see that at the p.

d.

of this point, the curve suggests that the current should be lower or it could have been caused by for some reason, a lower than expected p.

d.

reading.

And what did we learn about a filament lamp from this investigation? As the p.

d.

across the lamp increases, the lamp's brightness increases.

We can see that from the descriptions.

And also, the current increases, which you would expect when you increase the p.

d.

across a component.

And we find that the resistance of the lamp changes.

The resistance increases as the p.

d.

increases.

So you can't simply state the resistance of a lamp like this.

It has a varying resistance.

And we find the same sort of thing when the p.

d.

direction is reversed.

As the p.

d.

gets larger, the current also gets larger just in the opposite direction from before.

Let's think about resistance.

Where is the resistance of the filament lowest on this graph? And we know from the table that the resistance is lowest where the current and p.

d.

are lowest.

Which of the following statements is correct when the p.

d.

across the filament lamp has increased from nought to 12 volts? Press pause while you read the options.

And the only correct statement here is the brighter a filament lamp, the greater its resistance.

And you should be able to see that from looking at your table.

You can calculate resistance from the graph as well.

For instance, you can find the resistance at a p.

d.

of 2.

0 volts by drawing a vertical line from 2 on the x axis up to the best fit line, and then where it meets the best fit line, draw a horizontal line across to the y axis and that shows you the current at 2 volts is 0.

7 amps.

And we just draw these lines to help guide the eye.

We're just trying to see what is the current where we're at 2 volts on this best fit line.

Let's calculate the resistance now from that p.

d.

and current.

Resistance is p.

d.

divided by current.

And we get 2.

857, et cetera ohms. The calculator shows many digits here, but it's not reasonable to think that we know the resistance to the nearest thousandth of an ohm from the best fit line where we've only read to one or two significant figures.

So we could give two significant figures in our answer.

2.

9 ohms. Now your turn.

What is the resistance at a p.

d.

of 6.

0 volts? Press pause while you're working this out.

Press play when you're ready to check your answer.

The correct answer is 3.

8 ohms, and here's why.

If we read from 6 volts on the x axis up to the best fit line and then across we get 1.

6 amp current, and then resistance is p.

d.

divided by current and to two significant figures, we get 3.

8 ohms. Well done if you got that.

And I have some longer questions for you.

Press pause while you're reading and writing down your answers And press play when you're finished.

And let's look at example answers.

So describe what happens to the resistance of a filament lamp as the p.

d.

has increased from 0.

As the p.

d.

across the filament lamp has increased, the resistance also increases.

There's really not much more you can say about that.

Then describe how the resistance of the lamp is linked to the brightness and temperature of the lamp.

The resistance of the lamp increases as the brightness and temperature of the filament lamp increases.

And finally, describe what issues there are with the data shown on the graph and explain how to improve it.

Well, there's an anomalous point at the bottom right of the graph.

More measurements could be taken to check it.

And the readings are not evenly spaced out.

There's a gap in the middle.

The experimenter should have taken measurements at regular intervals of p.

d.

to avoid this gap.

So well done if you got the answers to those questions right.

And now we've reached the end of the lesson, so I'll give you a summary.

Increasing the p.

d.

across a filament lamp increases the current through it, and the lamp gets brighter and hotter.

The lamp behaves in the same way if the p.

d.

is reversed and current is pushed the other way through the lamp.

Using pairs of readings from a graph of current against p.

d.

allows resistance to be calculated using R equals V divided by I.

Resistance is p.

d.

divided by current.

The resistance of a filament lamp gets bigger, the brighter it is.

So well done for working through this lesson.

I hope your investigation went well and you were able to draw a graph, and I hope to see you again in a future lesson.

Bye for now.