Hello and welcome to this lesson called "Heating by the Sun." This is from the unit called "Our solar system and beyond." Now, in this lesson, we are going to do an experiment to look at how the Sun heats the Earth.

So how can we do an experiment with the Sun and the Earth? Well, the answer is we're going to do what scientists do a lot of the time, which is to build a model of the things they really want to test, and then test that model, doing an experiment on the model.

And if the model is a good representation of the Sun and the Earth, the features of the Sun and the features of the Earth, then the results of the experiment on the model, hopefully, will also apply to the Sun and the Earth themselves.

So let's have a look at how we're gonna do that this lesson.

The outcome of this lesson is hopefully by the end of the lesson you'll be able to explain why the heating effect of the Sun is greater at midday than at sunrise.

Here are some keywords, we're gonna be looking at this lesson, heating, ray, angle, and thermometer.

Each word will be explained as it comes up in the lesson.

This lesson is split into two parts.

The first section of the lesson is going to introduce the practical, where we're gonna investigate heating of a surface at an angle.

And in the second part of the lesson, we'll look at how the results from that experiment can hopefully be applied to Earth.

So let's get going with the first section.

So we know that objects that give out light can cause heating effect.

Now how do objects like the Sun that give out light, how do they cause that heating effect? Well, the answer is they're giving out light and they're also giving out other kinds of radiation, which then get absorbed by surfaces.

And when surfaces absorb light or when they absorb other invisible kinds of radiation are things like infrared, ultraviolet, that those kind of rays which aren't part of the visible light spectrum.

When objects absorb that kind of radiation, they get warmer.

So how can we represent that? So in diagrams, all of the kinds of light given out by an object can be represented by drawing rays, which are straight lines with an arrow, which show the direction the light travels.

And in the lesson, I'll just say light, but when I say light in this lesson, I really mean all of the different kinds of light given out by glowing objects like the Sun, so includes the invisible kinds of light like infrared and ultra and that kind of thing.

But I'll just say light from now on and you'll know that when I say lights during the session, I mean all of the different kinds of light, which are all represented by the rays that we draw going out from the glowing object.

And we've said that if rays hit the surface of another object, they might be absorbed, making the object hotter.

So now, we've looked at what rays are, let's look at the Sun in more detail.

So during each day the Sun appears to move across the sky in a curve from east to west.

So you have to imagine looking south, so standing on the sheet of paper with the compass points on it.

So imagine standing on that piece of paper and looking south, then it at sunrise, you would always see the sunrise to your left if you're looking south, because that's the east and that's where the Sun appears every morning.

And then the sunrise is higher in the sky until the Sun gets to midday.

And at midday, the Sun is south, due south in the sky, it's neither east nor west from the UK.

And then the Sun continues its journey across the sky and the Sun continues to move from east to west and from high up to low down, so the Sun sets in the west, moving in that arc or arch across the sky.

Now it's important to say the Sun does not actually move higher above Earth surface and then lower back towards Earth surface for sunset.

That doesn't happen.

The Sun stays the same distance from Earth.

So why does it look like it moves across the sky? Well, of course, it's because Earth is spinning.

So we are standing on a spinning sphere, looking at an object which is not moving.

So as we spin on our sphere, the Earth, we're effectively spinning from west to east in the picture.

So as we spin from west to east in the picture, it looks to us as if the Sun is moving from east to west, when in fact it's us spinning.

Let's look at a model of that.

So the Sun appears to move in this curve or arch or arc, A-R-C, arc across the sky because Earth is a sphere that's spinning.

So we can show that by looking at a globe.

So in this picture, you can see the top of a globe where I've stuck a stick on the UK, pointing straight up off the surface of the globe into the sky from the UK.

And in this picture, this is representing sunrise because you can see in that picture how look at the direction of Earth's spin and look at where the UK is under that blue tank matchstick.

The UK is just moving out of darkness out of night into daytime.

So that's sunrise.

So you can see that rays from the Sun are at a large angle.

The Sun is at a large angle to straight up.

Remember the matchstick shows the direction of straight up and at sunrise, rays from the Sun are at a large angle to straight up.

So effectively, if you are looking straight up in the sky, the Sun is very low down in the sky.

It's a large angle compared to straight up.

So that's why the Sun appears low down in the sky at sunrise.

But because Earth spins, at midday, the Sun is then at a smaller angle to straight up, so appears higher in the sky.

So this is midday now because Earth has spun and the UK has moved round on Earth on the spinning Earth.

So the UK has moved from the position in the first picture to the position in the second picture where the matchstick is now.

And look at the angle of rays from the Sun to straight up.

Now it's a smaller angle, okay? So if the Sun appears at a smaller angle to straight up, then it's higher in the sky.

So the Sun has gone from lower in the sky at sunrise in the first picture to higher in the sky, closer to being straight up in the sky from the UK at midday.

So in those pictures, Alex is looking straight up into the sky along the matchstick.

So you can see in the first picture, he has to tilt his head through a large angle to look at the Sun.

So the Sun's low far from being straight from the sky.

And in the second picture, Alex has to tilt his head through a smaller angle to look at the Sun because, so the Sun is appearing higher in the sky at that point.

And that's all because of Earth is a sphere that we're on which is spinning.

Just a reminder to never actually look directly at the Sun.

This is just a model to help us understand why the Sun moves across the sky in an arc.

So from a large angle to straight up so low down in the sky to a small angle to straight up, so higher up in the sky at midday.

Let's do a quick check on this now, why does the Sun's height in the sky vary over a day? Is it because the Sun orbits Earth? Is it because Earth orbits the Sun? Is it because Earth spins? Or is it because the Sun moves up and down whilst Earth spins? Which is the correct option a, b, c, or d? Choose now, five seconds.

Well done if you chose option c.

Okay, the Sun definitely does not orbit Earth.

So a is completely wrong, okay? b is true in the sense that Earth does orbit the Sun, but that's not what causes the path of the Sun across the sky that's caused by the fact that Earth is spinning.

So it looks like from us on the spinning Earth, it looks like the Sun moves across the sky in an arc.

And d is certainly not true as well.

The Sun is stationary, it does not up and down towards the Earth spins.

So the answer was c.

Well done if you've got that.

Okay, so it also feels hotter at midday than at sunrise or sunset.

Oh, and here's Alex again.

What's Alex saying? Alex is saying, "Maybe it feels hotter at midday than at sunrise or sunset.

That's maybe that's also because the UK is at a different angle to the Sun at different times of day.

Help me test out this idea." All right then Alex, let's give it a go.

So what we're gonna do this lesson is we're gonna use a piece of card as a model for the UK.

There's the piece of card and it's attached to a thermometer.

So the piece, there's a thermometer attached to the back of the card that can help us find out if the angle to the lamp, which is gonna represent the Sun, does that change the heating effect? So I'll show you in more detail what this is gonna look like now.

So when the card is side-on to the lamp, that's gonna model sunrise, like the UK being kind of side-on to the Sun's rays.

And then as Earth spins, we could model that by turning the card, by spinning the card.

So let's spin the card.

So it's 45 degrees to the lamp that might model mid-morning, for example, 9:00 AM, perhaps when the UK is in this position compared to the Sun's rays 'cause the Earth has spun a little bit.

And then when the card is face-on to the lamp, that would model midday.

So when the UK is closest to being face-on to the lamp.

So what we want to test is, in these three different situations, which one is gonna heat up the card the most and can that explain why midday? So the right-hand image is hotter than sunrise, the left-hand image.

Let's look at Alex's prediction here.

Alex thinks that the closer the card gets to face-on, the hotter the card will get in two minutes.

Just take a moment to consider do you agree with that prediction? Do you think it's correct? And just take a moment to think about why you think that prediction could be correct.

Pause the video now and just have a think and we will return to that later.

Right, so before we get going with the practical, let's discuss the three main variables.

The independent variable, which is the variable you change or select values for.

The dependent variable, that's the variable that you measure or observe to get your results or to see if it changes or to see how it changes.

And then control variables, variables that must remain the same throughout investigation.

So the results and the conclusion are valid.

So what are we gonna do? What's Alex gonna do? Alex is gonna investigate how the heating angle affects the card's temperature after two minutes.

So which one of those is the independent variable for this investigation? So the thing that the experimenter changes or sets.

Five seconds to decide.

Okay, well done if you said the independent variable is the angle the card is held to the lamp because that's what we are gonna change or set as the experimenter.

Okay, new question now, but the same options to choose from.

Which of these this time is the dependent variable for this investigation or the dependent variable for this investigation where Alex is gonna investigate how the heating angle affects the card's temperature after two minutes, what's the dependent variable that Alex is going to observe or monitor to see how it changes or if it changes? Five seconds to decide.

Well done if you said, a, the temperature after two minutes is the thing that Alex is going to observe and monitor to see if it changes or how it changes when he changes the angle of the card to the lamp.

So that's the dependent variable.

Okay, final question about variables.

Same options, but this time, which are control variables that have to be kept the same, so they don't affect the results and it's a fair test and conclusions will be valid, which are control variables that have to be kept the same in every test because they're not what we're testing in this experiment.

Five seconds to decide, off you go.

Okay, so it's the only letters we've not said yet.

It's the power of the lamp, the distance the card is held from the lamp and the starting temperature of the card.

All of those have got to be kept the same.

So they don't make the temperature warmer or colder after two minutes because we're not investigating the power of the lamp or the distance the card is held from the lamp or the starting temperature of the card and how they affect the temperature after two minutes.

We're only investigating how the angle the card is held to the lamp affects the temperature after two minutes.

Therefore, we've got to keep b, c, and e the same, otherwise 'cause if we didn't, they might affect the results and then we wouldn't know if the temperature was warmer after two minutes because of a different angle or because of a different distance or a different power of the lamp or a different starting temperature.

So we've got to keep those things the same.

So we know that any different temperatures after two minutes can only have been caused by a different angle to the lamp.

And that's why we have to keep control variables the same in experiments.

So we know that any differences in results can only have been caused by the one thing we changed our independent variable.

Okay, if you are in a classroom, it's time to do the experiment now.

So here's step one.

Set up the equipment with the thermometer bulb touching the card.

That bit might be done for you.

The attaching the thermometer bulb to the card.

Turn the card to the angle you want.

Use a ruler to check the distance between the lamp and the thermometer.

You'll teacher will tell you a suitable distance to use for your equipment and that should always be the same in every test that you do.

Finally, in step one, it's really important that you turn on the lamp away from the card to let the lamp warm up to full power because if you do that during a test, if you first turn on the lamp, then the first minute the lamp's on it might not be at full power and therefore, it won't be a fair test if the lamp is at full power for the full two minutes for the other tests.

So you've got to make sure that the lamp is going to be at full power for the full two minutes.

So turn on the lamp for maybe 10, 20, 30 seconds away from the card first to give the lamp the time it needs to get to full power.

Then you can do step two, which is recording the starting temperature of the card and then moving the lamp into position and timing two minutes with the lamp on full power for the full two minutes.

Then at the end of the two minutes, you can record the final temperature of the card for that angle.

And then step four is to repeat that process for the other two angles, ensuring the same starting temperature of the card.

So you might need to allow the card some time to cool down between each test.

It might help to mark the angles below the apparatus, so you can easily adjust it to the correct angle.

Okay, so if you are in a classroom situation or you have the equipment available, then you can do that practical now.

So you could pause the video and I'll see you again afterwards.

If you don't have access to equipment, then there'll be some example thermometer readings on the next slide, which you can read the thermometers before looking at the example results table, which I'll provide in the following slide after that.

So you can check your thermometer readings.

Okay, pause video now off you go with that experiment task.

If you didn't have the equipment available or if you need some sample results, you can have a go at reading these three thermometers to collect sample results in this experiment.

So pause video now if you need to do that.

Right, I'm gonna give you some feedback on the experiment task now.

I'm gonna show you a set of sample results based on these thermometer readings, you can see on the screen now.

So the thermometer readings are when the card was at nought degrees, which is side-on to the lamp or nought degrees to the raise.

The thermometer reading after two minutes was 19 degrees C.

For the 45 degree angle, the thermometer reading after two minutes looks like it was 27 degrees C.

And when the card is face-on to the lamp or 90 degrees to the raise, looks like the thermometer reading after two minutes was 37 degrees C, it's closer to 37 than 36.

We're gonna go with 37 that final thermometer reading.

So that would mean that a sample results table from the experiment would look like this.

So well done for your efforts with that experiment and hopefully your data follows the same kind of pattern as this sample data does.

So that brings us to the final section of the lesson where we can now look at applying those results to Earth.

So we need to draw a conclusion first, a conclusion to an experiment states what you found out in an experiment and how you know this.

So conclusion will often start with I found out that then write what you found out.

So here are Alex's results.

They're the sample results I showed you just now.

Fill in the blanks to complete his conclusion.

The greater the angle to the card to the rays, the what? The temperature after two minutes.

This means that the more face-on the card is to the lamp, the what? The heating effect.

So have a go at filling in those blanks now.

Pause the video if you need to.

Five seconds off you go.

Okay, I think it should have been fairly clear that the greater the angle to the card to the rays, so the greater the angle, the higher the temperature after two minutes or any word that means higher is fine.

So this means that the more face-on the card is to the lamp, the greater the heating effects or any word that means greater, bigger, stronger.

Well done if got those right.

So let's now explain that conclusion.

So when there was a small angle between the card and the rays, then the card absorbed less light, giving a smaller heating effect.

So the card didn't heat up as much when it was close to being side-on.

So if you imagine looking at the front of that card, there's only two rays hitting it.

Whereas at an angle of 45 degrees, the card actually absorbs more rays of light.

So the heating effect is greater.

You can also see how the rays are, there's more rays in the same area.

So the rays hitting the surface are actually less spread out than they were compared to when the angle was smaller in the first diagram.

And then what about when the cards face-on? Well, when the card's face-on, there's an angle of 90 degrees between the surface and the rays.

It's being heated kind of from directly overhead if you like, or from face-on.

So this is when the card absorbs the most light or most rays of light that it can because it's face-on to the rays.

So the heating effect is the greatest.

And you can see that actually because there are more rays spread over the same area, the rays are actually less spread out over the same area than they were when the card was at an angle.

So when a card is heated from face-on the same area absorbs more rays of light or the rays of light are less spread out over the surface because of the change in angle.

And that's why you get a greater heating effect when something is face-on, compared to with something it's at a different angle.

Okay, let's check what we've just gone through.

Use the words more or less to fill each gap.

The closer a surface is to being face-on, is it more or less light that's gonna be absorbed by the surface when it's closer to being face-on? Is it more or less spread out the light will be across the surface? And is it more or less the temperature of the surface will rise? Pause the video now to make sure, you've got those the right way around for each blank.

Off you go.

Okay, let's check how you got on.

Well, if the surface is closer to being face-on, it's gonna absorb more light, okay? Because it's presenting a bigger area.

So more light rays hit it.

That means on the surface the less spread out the light will be, okay? Remember those diagrams from the previous slide if you're not sure why that is.

And then the final one, the closer a surface is to being face-on the more the temperature will rise.

Very well done if you've got all three of those.

So let's now apply the results to Earth and midday and sunrise.

So at midday, the UK is closer to being face-on to the Sun's rays.

That means, we know at midday, the Sun is higher in the sky.

So a patch of ground is basically lit from directly overhead.

The patch of ground is kind of face-on to where the Sun is in the sky.

So that's the view from Earth.

But the view from space, we can see that it's because the Earth has spun to bring the UK closer to being face-on to the Sun's rays.

So what this means is, the Sun's light is less spread out over the ground, so the heating effect is greater at midday.

And remember that the Sun is no closer or further from Earth at this point.

Whereas as sunrise, the UK is closer to being side-on to the Sun's rays.

We know that the Sun that makes the Sun look lower in the sky at that time of day.

So the Sun's rays, when they hit the same size patch of ground, the Sun's rays end up more spread out over the ground.

Even though, they're the same kind of distance apart, it's when they hit the ground, they're more spread out over the same area, the same size area, which gives you a lower heating effect at that time of day at sunrise.

And just to reiterate again, if the Sun does not get any closer or further from Earth.

Okay, let's do a check on what we just said then.

Which two statements explain why the Sun heats places on Earth more strongly at midday? So we're looking for two statements that help explain why it's warmer at midday? a, at midday, Earth is closer to the Sun, b, at midday, the temperature of the Sun is hottest, c, at midday, the Sun is high in the sky, d, at midday, the Sun's rays are less spread out over the ground.

Which two help explain why the Sun heats places more at midday than a sunrise? Pause the video now to choose two of these, which are correct.

Okay, let's see how you got on.

Well, a and b are incorrect.

Earth does not move any closer to the Sun.

Earth stays the same distance from the Sun and the Sun stays the same distance from Earth.

And likewise, the temperature of the Sun doesn't change either.

So it's c and d that are correct.

At midday, the Sun appears highest in the sky because of the rotation of the Earth and that causes a midday, the Sun's rays to be less spread out over the ground, causing a greater heating effect if there's more rays in the same area because they're less spread out over the ground when they hit the ground.

Well done if you've got both of those.

Okay, let's do a final task then based on what we've learned this lesson.

So this task, you have to apply what you've learned this lesson to a slightly different situation, but still to do with the Earth and the Sun.

So this task is about Earth's polar regions, the Arctic, the North Pole, and Antarctica where the South Pole is.

So in this task what I wanted to do is write an explanation of why the temperature in the polar regions is always lower than the temperature in countries close to the equator.

And I'm sure you know which picture is which.

So you should use ideas in this lesson about angles and the Sun's rays.

And you could include a diagram of the Sun's rays hitting Earth.

So have a good think about this, perhaps discuss with people around you and then put pen to paper and try and write that explanation using the ideas, we've learned this lesson about the heating effect when a part of the Earth is face-on to the Sun and the heating effect when apart of the Earth is at a different angle to the Sun.

Pause the video now have a good go at that task.

Well done for your effort with that task because applying scientific knowledge to a new situation can be tricky.

So I'll give you some feedback now.

So here's an example diagram that you might have drawn that might have helped you explain.

Now, the Sun's rays when they hit Earth, they're all effectively coming straight from the Sun and about the same distance apart.

So why does that cause temperature in the polar regions to be always lower than the temperature close to the equator? So here's an example explanation.

Don't worry if yours isn't identical to this, as long as it covers similar ideas, that's brilliant.

So near Earth's poles, Earth's surface is at a smaller angle to the Sun's rays.

So the Sun's rays are gonna hit the surface at this small angle and they're gonna be more spread out over the ground.

Look at those top rays of light and they're further apart from each other on Earth's surface than rays of light that hit near the equator, which are closer together.

And what that means is, there's a smaller heating effect at the poles leading to lower temperatures there.

However, near the equator, Earth's surface is much closer to being face-on to the Sun.

So the Sun's ray hit the surface, hit Earth's surface at the equator at a much larger angle, close to 90 degrees between the surface of Earth and the Sun's rays closer to 90 degrees.

Therefore, the Sun's rays are less spread out over the ground.

So there's a greater heating effect closer to the equator.

So very well done if your answers were along those lines.

You should maybe take a moment now to pause the video at this point and make any adjustments or improvements to your explanation, so you've taken the most you can from this feedback.

So that's the end of this lesson on "Heating by the Sun." Here's a summary.

Heating a surface from directly overhead or face-on is more effective than heating the surface at a different angle.

And have a look at the diagrams to remind yourself why.

When a surface is face-on in the first diagram, rays of light are less spread out when they're hitting that surface face-on.

Whereas in the second diagram, at a different angle, the rays of light, even if they're kind of the same distance apart, when they hit the surface, they're more spread out over that surface, 'cause in the first diagram, you've got the six rays of light, all six rays of light hit the surface.

So when they hit the surface, they're closer together.

Whereas in the second diagram, out of those six rays of light, only four of them are now hitting the surface.

So when they hit the surface, they're a bit more spread out over the surface, they cause a lower heating effect.

The Sun appears to to move in an arc across the sky each day because of Earth's spin.

And Earth's spin changes the angle at which the Sun's rays reach a place on Earth and how strongly that place is heated at different times of day.