# Lesson video

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- Hello, and welcome to this lesson on power and energy.

My name is Mrs. Robinson, and I will be taking you through this lesson today.

You will need a calculator, so if you do not have one, please pause the video and go and get one now.

So in today's lesson, we'll be looking at power and energy, and our outcome is to learn what the relationship is between power and energy.

The keywords today that we'll be using are rate, which is a measure of how quickly something happens, and power, which is the rate at which energy is transferred.

So with the power keyword, we will go into a little bit more detail with that one.

This is your lesson outline for today.

So we're gonna be looking at power and energy and the relationship between them, the power equation, converting units, and rearranging the power equation.

So we're gonna start off with power and energy.

So power is the rate at which energy is transferred, okay.

So we're gonna look at some examples, and I want you to tell me which you think is the most powerful.

So would walking, or jogging, or sprinting be the most powerful? You can pause the video here and have a little think about it, and then start playing the video again once you have an answer.

So out of those, I hope you had sprinting in mind for that one.

So we would say sprinting is the most powerful.

Hopefully you've thought about maybe why that is, and were thinking about how much energy we're maybe using in that kind of period of time, so how quickly we are using energy.

So if we're sprinting, we're using a lot more energy in order to sprint.

So here, we're gonna look at a different example, and I'd like you to also think about the reason why.

So we've got two kettles.

The one on the left has a volume of a thousand millilitres of water, and it takes 50 seconds for it to boil the water.

The one on the right has a volume of a thousand millilitres, so exact same volume of water, but it takes 90 seconds for the water to boil.

So which one would you say is more powerful and why? So pause the video again here, have a little think about which one you think is more powerful and what your reasons are for that.

When you're ready to play the video again, then you can go ahead with that.

Okay, so hopefully we've got an answer.

We've picked which one we think is more powerful, and we've come up with reasons why as well.

So, hopefully, you have gone for the kettle on the left.

It's the one that takes 50 seconds for it to boil.

And hopefully we've kind of thought about a reason why as being how quickly it's been able to boil that water.

So it's a much shorter time of 50 seconds to be able to boil that water, as opposed to the 90 seconds.

So it'll use the same amount of energy, okay, to heat up both volumes of water, to boil those, but the kettle on the left is taking a much quicker time, so we'd say that's a higher rate.

So now, you can have a little go at this question yourself.

So which kettle is the most powerful? Is it kettle A, with a time to boil of 60 seconds? Is it kettle B, with a time to boil of 150 seconds? Or is it kettle C, with a time to boil of 80 seconds? So pause the video here, and then play it again once you're ready to move ahead.

Okay, so, hopefully you went for answer A.

So the time to boil was 60 seconds, which was less than the other kettles for the same volume of water.

So the one which is most powerful would be kettle A.

Next question, so which one is most powerful? Is it the car where it takes 20 seconds for it to reach 30 miles per hour, that's car A? Or is it the car that takes 10 seconds to reach 30 miles per hour, which is car B? Or would it be car C, which takes six seconds to reach 30 miles per hour? Again, pause the video while you get your answer, and then play again when you are ready.

Okay, hopefully, you went for answer C.

So car C would be the most powerful, because it's taking the least amount of time in order to get to your 30 miles per hour.

So now have a go at task one on your worksheet, and I hope you do well.

So pause the video here, and then resume when you are ready to move on.

Okay, so hopefully you've had a good go at that.

We'll go through the answers just now.

So, power is the rate at which energy is transferred, and for the most powerful hair dryer, that would be B.

It would take seven minutes to dry your hair, that's less time than the other hair dryers, that one is most powerful.

So hopefully, you've got those correct.

Okay, so the next section, we'll be looking at the power equation.

So power can be calculated using this equation, so power is equal to energy divided by time.

Power is measured in watts, energy is measured in joules, and time is measured in seconds.

Okay, let's do a little quick check from that.

So what is the correct equation for power? Is it A, power equals energy subtract time? Is it B, power equals energy divided by time? Or is it C, power equals energy multiplied by time? Pause the video here, and then when you're ready, continue on.

Okay, so I hope you got that right, it should have been power equals energy divided by time.

Okay, so I hope you got this one correct, so it would be power is energy divided by time.

Next question, which is the correct unit for power? So what is power measured in? Is it A, joules? B, seconds? Or C, watts? So again, pause the video here, and then resume when you are ready.

Okay, again, I hope you got this one correct.

It would be C, watts.

So well done, guys, if you managed to get those two questions correct.

So now, I'm gonna go through some example questions with you of how we would calculate power.

So first questions that I do, so I'm gonna go to this one, so just watch carefully.

So the question is to calculate the power of a toaster that transfers 20,000 joules of energy in 20 seconds.

So you've just got to remember your equation is power equals energy divided by time.

We've got two bits of information that are in that question.

We've got the 20,000 joules of energy, so that will be your energy that we'll substitute in.

And we've also got the time of 20 seconds.

So what I'll do next is I would write my next line, saying power is equal to 20,000 divided by 20.

Then you'd use your calculator to do that sum and then write down your final answer, which would be a thousand.

And then make sure you've got your units, which is watts.

You can either write it as watts in words, or you can write a capital W.

So let's do the We Do.

So you've got to calculate the power of a kettle that transfers 5,000 joules of energy in 10 seconds.

Hopefully you are already thinking what your first thing is that you need to do and you thought this, you'd be absolutely correct.

You would first write power equals energy divided by time.

So you would write your power equation.

And then hopefully you're already thinking what you need to do next.

You've got those two numbers in there, 5,000 joules of energy and 10 seconds.

So just be careful with which way around that we'll put this into the equation.

So you got this right, if you're thinking already, fantastic, it would be 5,000 divided by 10.

And then you're absolutely right, you put this into your calculator to get your final answer and then just don't forget your units.

So you would get 500 watts for this question.

So well done if you were thinking that along with me.

Okay, now you can have a little go at task two on the power equation.

Pause the video here and then play again once you're ready.

Okay, now that you've had a little go at those, we'll go through the answers.

So for A, you should have got a hundred watts, so that'll be making sure you do a thousand divided by 10.

For B, you should have got 1,800 watts.

So you've done 216,000 divided by 120.

And then for C, you've got two separate calculations there and then a little statement.

So your fan, you would have to do 17,160 divided by 520, which would've got you 33 watts.

And for the hair dryer, you would've done 27,000 divided by 300.

And that would've got you the 90 watts.

And then you would just say that the hair dryer uses most power.

So well done if you've managed to get those.

Okay, so the next section we're gonna look at is how to convert units.

So we're gonna look at first converting between kilowatts and watts.

So you might notice there as well, I've put in brackets, the shorter version that we can use is instead of writing kilowatts in full, we can write kW.

We've got make sure that K is a lowercase K and the W is a capital W.

And it's the same for watts, same idea.

We have a capital W for watts.

So one kilowatt is the same as 1000 watts.

So we can also write that as one kW is equal to a thousand W.

So that kW means kilowatts and the W means watts.

So how we do these calculations would be to multiply the kilowatts by a thousand to then convert it into watts.

So the example there is multiplying one by a thousand, which gets me 1000 watts.

So another example would be if we had three kilowatts, then we would take the three kilowatts and we would multiply that by a thousand to then get 3000.

So let's have a look at some more examples.

So if I like to convert five kilowatts into watts, and I've just got to remember one kilowatt is a thousand watts.

So five kilowatts would be five multiplied by a thousand.

So that would give you 5,000 watts.

So let's look at the next example.

So if I had to convert seven kilowatts into watts, then you've just got to remember that one kilowatt is a thousand watts.

And then, hopefully you've got this.

It would be seven multiplied by a thousand to get 7,000 watts and that's you done the conversion for kilowatts into watts.

So let's just do a couple of questions just to check that you've got that.

So what is six kilowatts in watts? Is it 6,000 watts, 600 watts, or 6 million watts? Pause the video here and then play again when you are ready.

Okay, so hopefully you got the right answer there and you got an answer of 6,000 watts.

Well done if you've got that.

Let's try another one.

So five kilowatts would convert to how many watts would that be? Would that be 500, five, or 5,000? Again, pause the video here and then play again when you are ready.

Okay, so hopefully you got the right answer of 5,000 watts for that question.

So well done if you've got that.

If you didn't get that, don't worry, maybe just go back a little bit and recap.

So we're going on to the next little bit.

It's still looking at between watts and kilowatts, okay.

So we're still doing those conversions, but it's going the other way this time.

So instead of going from kilowatts to watts, which is what we did before by multiply by a thousand, we're gonna go from watts to kilowatts.

So this time we've gotta think, well, 1000 watts.

So for every thousand watts I have, that's equal to one kilowatt.

So to do that we need to divide by a thousand this time, okay.

So if we had 4,000 watts, so we don't need to multiply this time, we've got 4,000 watts, we are dividing this time.

So that would give us the four kilowatts.

So 4,000 divided by 1000 to give us four kilowatts.

So now we're gonna look at some examples.

So I'm gonna convert 3000 watts into kilowatts.

So I just need to remember that a thousand watts is equal to one kilowatt.

So if I have 3000 watts, I'm gonna divide that by a thousand.

And then that will give me my three kilowatts.

Now let's look at the next example together.

So if I have 6,000 watts, I need to convert that into kilowatts.

Again, just remember 1000 watts equals to one kilowatt.

So if I have 6,000 watts, then I would do 6,000 divided by a thousand.

And then I should get six kilowatts for my example.

So now we're gonna do some little checks just to check that you have that.

So first question, 4,000 watts converts to how many kilowatts? Is it 40, 4, or 400? So pause the video here and then play again when you are ready.

Okay, so hopefully you've got that, that should have been your four kilowatts.

So let's go to the next question.

So if you have 12,000 watts, this converts to how many kilowatts? Is it 12, 120 or 1.

2? So pause the video here and play again when you are ready.

Okay, so hopefully you've got an answer of 12 kilowatts there.

So hopefully you've got the hang of this now.

So we're gonna have a little go at now converting between minutes and seconds.

So this is something that maybe you know around, but this is why we are going through, it's a little bit of practise.

So we're ready for the next section.

So just remember that one minute has 60 seconds in it.

So we might write it as one min equals 60s for seconds.

So if I was to convert from minutes to seconds, I would multiply it by 60, okay.

So let's look at an example.

I would take the three minutes.

And then I would multiply it by 60, which then would give me 180 seconds.

Okay, so let's have a look at some examples.

So if I had to convert six minutes into seconds, I've got to remember that one minute is equal to 60 seconds.

So I would do six multiplied by 60, which would then give me 360 seconds, which I can also write is 360s for seconds.

The next example I've gotta convert nine minutes into seconds.

So just think about well how many seconds is in one minute? Okay, so one minute is 60 seconds, but now I need to deal with nine minutes.

So what am I gonna have to do? I'm gonna have to do nine multiplied by 60, which it'll give me 540 seconds or you can write it as 540s for second.

Well done if you were get thinking ahead with that one.

Can be a little bit tricky with these but well done.

So now I'm gonna do a little check.

So if you had two minutes, that would be equal to, how many seconds would that be? 200 seconds, 120 seconds or 1,200 seconds? So pause the video here and then play it again when you are ready to move on.

Okay, so hopefully you have got 120 seconds for that one.

So it'll just be two times by 60 to give you 120.

So well done if you've got that one.

Right, so the next bit to do is that we are going to convert from seconds into minutes this time.

So a little bit different than what we did before.

Before we've multiply by 60 to get from minutes to seconds, but we're gonna go the other way this time.

So just remember in a minute there is 60 seconds, okay? But we're going the other way.

So 60 seconds, to get that into minutes, I would divide by 60 this time, okay? So an example would be if we had 360 seconds, then we would divide by 60 to then get my six minutes.

So let's have a look at some examples.

So if I had to convert 540 seconds to minutes, just remember you've got 60 seconds in a minute.

So 540 seconds, you would do 540 divided by 60, which then would be nine minutes.

So the next example would be to convert 420 seconds into minutes.

So just remember, first bit, every minute you have, you've got 60 seconds.

So if I had 420 seconds, that's right, you've got to divide it by 60.

Put that in calculator, then you should get seven minutes.

So now we'll do just a little quick check on that.

So if you had 120 seconds, that would be equal to how many minutes? Would it be 12 minutes, one minute or two minutes? So pause the video here and then play again when you are ready.

Okay, so hopefully you got two minutes for this one.

So you did 120 divided by 60.

So well done if you've got that.

Next example, if you have 240 seconds, is that equal to how many minutes? Is that four, 24 or 40? See and pause the video here, have a look with that question and play again when you are ready.

Okay, so hopefully you've had a go at that and you've got an answer of four minutes.

So if you did 240 divided by 60, that would give you the four minutes.

Okay, now you're gonna have a little go at task three, converting units.

Pause the video here and then play again when you are ready.

Okay, so let's go through the answers for this.

So for part A, you had to convert the following from kilowatts to watts.

So in order to do that, then multiply by a thousand.

In part three, it was 3.

6 kilowatts, it'd be 3,600 watt.

Don't forget about that decimal point there, it's important.

For part B, you're converting from watts to kilowatts.

So this time you have to divide by a thousand.

And again, just be careful for those decimal points.

So part two should be 1,200 divide by a thousand.

So you get 1.

2 kilowatts.

And then same again in part three, you had 4,120, so that'll be 4.

12 kilowatts.

So just be careful with those decimal points.

For part C, you had to convert from seconds into minutes.

So in order to do this, you would have to divide by 60.

You would've got these answers here.

Just be careful for part three.

It's 330 divided by 60, it'll be five points.

And then for part D, you had to convert from minutes to seconds.

So this time you were multiplying by 60.

Just watch out for your numbers there, especially in part two, don't forget that zero at the end.

So it'll be 72 multiplied by 60, which will give you 4,320.

Well done if you had a good go at those and you got all of those correct.

So now we're gonna go through rearranging the power equation.

So we're gonna use the equation that we had at the start, power equals energy divided by time and we're gonna rearrange those for energy and time.

First one we are gonna look at is rearranging for energy.

So remember the power equation was power equals energy divided by time.

So what we'll do is we're gonna focus on energy for this.

So we want it in a form where it's energy is equal to.

So to do this because we've got energy divided by time, we need to do the opposite of that divide by time in order to get energy by itself.

So we are gonna multiply that right hand side by time and we're gonna do the same on the left hand side.

So whatever you do to one side, you have got to do to the other.

And then because the energy divided by time we're also multiplied by time, then that would cancel out.

So we're just gonna be left with energy on the right hand side, and on the left hand side, you can have power multiplied by time.

And if we like, we can switch that around so it's in a little bit of a nicer format for us to use.

So I'm gonna move energy to the left hand side, power multiplied by time onto the right hand side.

So my equation now is gonna be energy equals power multiplied by time.

So now we're gonna go through some examples of how to use that energy equation.

So first part is calculate the energy transferred by a 2000 watt heater in 10 seconds.

So first we're going to write down the equation that we are going to use.

So it's energy is equal to power multiplied by time and then we're gonna substitute in the values that we know.

So we know that power is 2000 watts and the time is 10 seconds.

2000 multiplied by 10, which then would give us 20,000 joules of energy.

So just remember the units for energy is joules.

For the We Do, let's have a look together then.

So calculate the energy transferred by a 41,000 watt motorbike in five seconds.

So again, write down the equation, don't forget to do that important first step.

And then hopefully you've guessed it next that you are going to substitute in your numbers.

So energy is going to equal your power, which is 41,000 watts, multiply your time of five seconds.

And then do that calculation on your calculator.

You should get an answer of 205,000 joules.

Don't forget units as well.

Let's do a quick check with using that new energy equation.

So the energy of a 60 watt bulb on for 72 seconds is what? Would it be 1.

2 joules, 132 joules, or 4,320 joules? Pause the video here and then play again when you are ready.

Okay.

So hopefully you've got an answer there of 4,320 joules.

So you needed to multiply those two numbers together, 60 times by 72 to get that.

So now we're gonna look at rearranging the power equation, but in terms of time.

So it's a little bit trickier, this one, a little bit different, but I'm sure that you can manage with this.

So first, we'll have the equation power equals energy divided by time.

Wanting to focus on time.

That is how we want the equation to end up.

We want time equals, okay? So first thing we're gonna do is we're gonna multiply time by both sides so that time isn't a divide to make things a little bit easier for ourselves.

If I do it on the right hand side, I've got to do it on the left hand side as well.

So similar thing that we did before.

We get power multiplied by time is equal to energy.

Okay? Now we're still not quite there yet.

We've got another step to do because I want time equals to.

So because on the left hand side of that equation, I've got power multiplied by time, I just want time there, I don't want the power there anymore.

I'm gonna divide by power and I'm gonna do that on both sides.

So on the left hand side, I'm just gonna be left with time, but on the right hand side, it's gonna be energy divided by power.

So now we're gonna have a little go at some questions.

So the first one, I'm gonna go through it.

So just make sure you're paying attention and watching through.

The next one we will do together.

So how long does it take for 2000 watt kettle to boil water with 60,000 joules of energy? So first thing, we're gonna write down the equation.

So time is equal to energy divided by power.

And then we're gonna substitute our numbers in.

So energy is the one that's got the J afterwards, it's got the joules.

It's 60,000, and power, it's got the W, which is 2,000.

So we've got 60,000 divided by 2,000.

Do that on your calculator, we should get a time of 30.

And your time is always gonna be in seconds when you've got your units of power as watts and your units for energy as joules.

So the next one we're gonna look at together.

So how long has a 800 watt microwave been on for if it uses 17,600 joules of energy? So straightaway, hopefully you are thinking, right, I need to write down what the equation is for time.

So it's time is equal to energy divided by power.

Then what next? We need to substitute our numbers in.

So just make sure you have them the correct way.

So energy says we've got uses 17,600 joules of energy, that is our value for energy, and for power, it's at 800 watts.

So I'm gonna have 17,600 divided by 800.

And then put it on your calculator, you should get a time of 22 seconds.

So now you're gonna have a little go at some questions yourself.

So how long was a 6,000 watt oven left on for using 240,000 joules of energy? Was it 400 seconds, 40 seconds, or 1,440,000,000 seconds.

Very big number there.

So pause the video here and then play again when you are ready.

Okay, so hopefully you have got that and you've got that the correct way as well.

So remember, it's energy divided by power.

So it should have been 240,000 divided by 6,000.

Okay, well done if you've got those.

Next question, so a 40 watt bulb uses 4,800 joules of energy.

How long was it on for? So that's looking for time again.

Is it 192,000 seconds, 4,760 seconds, or 120 seconds? So pause the video here and then resume when you are ready.

Okay, so we should have got an answer of a 120 seconds for that one.

So that would've been the 4,800 joules of energy divided by 40, which would've been your power.

Fantastic.

Well done if you've got that.

So now you're gonna have a go at task four, rearranging the power equation.

Pause the video here and then play again when you're ready.

Well done if you had a good go at that and you've managed to answer all those questions.

We're just gonna go through the answers just now.

I'll pop all the answers on the screen just now for you and then I'll just go through some of them.

So for energy for the washing machine, you should have done power multiplied by time.

So that would've been 400 multiplied by 3,600.

Your mobile phone, you are working out time.

So you would have to do energy divided by power.

So then that would've been 36,000 divided by five.

For your TV, you're calculating energy again.

So you just had to remember to do power multiplied by time.

So you would've done 10,800 multiplied by 50.

And then for your laptop, you are working out time again.

So you'd do energy divided by power.

So you'd do 156,000 divided by 65.

So that's where we got those answers from.

Part B, you'd have to convert your times that you had from the table above into minutes.

So just remember to do this, you would need to divide your answer from part A in the time row, from seconds into minutes, you divide by 60, okay? So each of those answers were divided by 60.

So I hope you did well in those.

Well done for having a good go at those questions.

And keep going, keep up with the hard work.

For going through that lesson today.

Just go through a quick summary of the key points from this lesson.

So power is the rate at which energy is transferred.

Power is equal to energy divided by time.

We've also had a bit of practise at rearranging that equation.

Power is measured in watts, also written as a capital W.

One kilowatt is equal to a thousand.

And for the conversions for that, kilowatts to watts would be multiplied by a thousand.

If you're going from watts to kilowatts, we divide by a thousand.

And remember that one minute also equals to 60 seconds.

And the conversions for that would be from minutes to seconds, you would multiply by 60, and from seconds to minutes, you would divide by 60.

Now is your chance to have a little go at the exit quiz.

Thank you for using Oak National Academy and I hope you enjoyed your lesson.

And well done completing all the tasks and activities.

Bye for now.