Lesson video

- I'm Ms. Hall Smith and I'll be your physics teacher for this lesson on conservation of energy.

You will need a calculator for this lesson, because later on we're gonna be doing some calculations of efficiency.

Let's get started.

Today's lesson is about conservation of energy and efficiency.

We're gonna learn about the conservation of energy, identify useful and wasted energy transfers, and be able to calculate efficiency.

Here's some keywords for this lesson.

Energy can be described as being in different stores.

Energy can be transferred between the different energy stores.

Conserved means that the total amount stays the same.

Efficient appliances are better at transferring energy to useful energy stores.

This lesson has got four learning cycles: conservation of energy, useful and wasted energy, calculating efficiency, and Sankey diagrams. Let's start off with looking at the conservation of energy.

The law of conservation of energy.

This law states that energy can be transferred usefully, stored, or dissipated, but that it cannot be created or destroyed.

And this is true for every single energy store and transfer that exists.

Energy can never be created or destroyed, only transferred among stores.

Energy transfers can take place within a system, and this is where there's a group of objects that interact.

An example of an energy system would be a ball held in the air.

When the ball falls to the ground, energy has been stored and transferred within this system, and the amount of energy in the system is conserved throughout this scenario.

The energy in this scenario was transferred from the gravitational potential energy store of the ball to the kinetic energy store of the ball as it fell to the ground.

Let's take a look at the conservation of energy or using numbers to represent the amount of energy in each store.

If the ball had 100 joules of gravitational potential energy before it was dropped, then it would have 0 joules of kinetic energy because it was stationary.

Then the ball is dropped, and just before it hits the ground, all of its gravitational potential energy has been converted into kinetic energy, so it's now gonna have 100 joules of kinetic energy.

Energy was transferred from the gravitational potential energy store, so the kinetic energy store of the ball.

And we can see that this energy was conserved.

Before the ball was dropped, the total amount of energy was 100 joules, as there was 100 joules in the gravitational potential energy store and none in the kinetic store.

After the ball was dropped, there's still 100 joules of energy overall, it was just transferred from the gravitational potential energy store to the kinetic energy store.

This means that the total energy before the scenario happened and the total energy after the scenario happened were totally equal, which shows us the conservation of energy law.

Let's check that we understand the law of conservation of energy.

The law of conservation of energy states that energy can be transferred usefully, stored, or dissipated, but energy cannot be created or destroyed.

Well done for remembering the law of conservation of energy.

True or false: a chemical energy store of 30 joules can transfer 35 joules of energy to a thermal store.

Excellent.

This is false, and that's because energy cannot be created, only transferred.

So if there was a chemical energy store of 30 joules, it could only transfer a maximum of 30 joules of energy to any other store.

Excellent on remembering how to apply the law of conservation of energy.

Now it's your turn to practise your knowledge of the conservation of energy with these questions.

Pause the video now, have a go at the questions, and then press play when you're ready to hear the answers.

Good luck.

Okay, let's go through the answers.

State the law of the conservation of energy.

Energy can be transferred usefully, stored, or dissipated, but it cannot be created or destroyed.

Well done if you remembered the law of conservation of energy.

It is quite a long one, but it's just really important that you know that energy can definitely be transferred, but not created or destroyed.

Well done.

Complete the sentence.

Within an energy system, the total energy before an interaction is equal to the total energy after the interaction.

You could have also put is the same as, that would've been fine as well.

Well done if you put that.

This is representing the law of conservation of energy.

Finally, an object with 560 joules of energy in its gravitational potential energy store can transfer a total of 560 joules of energy as it falls.

Well done.

You've applied the law of conservation of energy.

Now it's time to move on to the second learning cycle.

We're now gonna look at useful and wasted energy.

When energy is transferred in a system, some transfers can be wasted to unwanted energy stores.

Let's take a look at a real life example of a hair dryer to explain this in more detail.

When the hair dryer is switched on, it transfers energy usefully to thermal energy and kinetic energy.

We want the hair dryer to dry our hair when it's on.

However, lots of energy is wasted in the form of sound radiation.

We don't necessarily need the hair dryer to make a noise, but it does when it's switched on.

So this is an example of wasted energy.

Wasted energy is often dissipated and this can be in the form of infrared radiation, by heating the thermal energy store of the surroundings, or by sound radiation.

Let's check that we understand wasted energy and useful energy.

Which two of the following are useful energy from a TV? Excellent.

It's light and sound.

We want our TV to light up and we want there to be plenty of sound so we can hear what's going on.

Infrared radiation would be an example of how the TV wastes energy.

We don't want it to heat up the environment, but it does.

What is the wasted energy from a light bulb? Well done, it's infrared radiation.

We don't want the light bulb to heat up the surroundings, but it does get a bit warm.

We want the light bulb to only give us light radiation in a perfect scenario.

How does a light bulb dissipate energy to its surroundings? That dissipation is the energy transfer by heating.

Well done if you've got all those correct.

You've done a really good job at identifying useful and wasted energy stores.

Which energy store increases when a tumble dryer dissipates energy to its surroundings? Excellent.

It's the thermal energy store of the surroundings.

Now, the thermal energy store of the clothes inside the dryer is an example of a useful energy store.

But where the energy was dissipated or wasted is to the surroundings, because we don't need the tumble dryer to heat up the surroundings, but it does.

Now is your turn to practise your knowledge of useful and wasted energy.

Have a go at the questions on the screen.

Pause the video and press play when you're ready to hear the answers.

Let's go through the answers.

Describe the term dissipation.

The transfer and spreading out of energy stores into less useful forms. The key bit there in the answer was the less useful forms. Dissipation is not useful.

So why is dissipated energy often referred to as wasted energy? That's because it has not been transferred to a useful form.

Now have a go at completing this table.

You need to fill out the useful energy and wasted energy stores in the three examples of an electric kettle, a tablet computer, and hair straighteners.

Pause the video now to complete the table and press play when you're ready to see the answers.

Let's see how you did.

The useful energy store in an electric kettle is the thermal energy store of the water.

We want the kettle heat up the water.

However, it wastes energy by emitting infrared radiation and heating up the surroundings and also sound radiation.

We don't want the kettle to do these things, but it does.

The useful energy in a tablet computer is light and sound radiation.

We want the tablet computer to do both these things so we can watch a video or do our homework, but the tablet does waste energy again in the form of infrared radiation, because it's going to heat up the surroundings even though we don't want it to.

The useful energy in the hair straighteners is the thermal energy store of the metal plates.

We want them to get hot so they can straighten our hair, but the hair straighteners do waste infrared radiation dissipated into the surroundings again.

Amazing job if you completed that table correctly.

There were lots of different energy stores to identify there.

So you've done a fantastic job if you've managed to get them all right.

Now have a go at the final task.

Describe the energy stores and transfers in this scenario and explain how energy has been dissipated.

A battery powered toy train going around a track.

Pause the video now and press play when you're ready to see the answers.

Make sure you write your answer carefully and think about the different energy stores and transfers.

Let's have a look at the answer.

Now, you might not have written word for word what's been written here, but as long as you had similar ideas, then that's the main thing.

So the chemical energy store in the battery is transferred mechanically into the kinetic store in the wheels because the train is gonna move around the track.

Some energy is also transferred to the thermal energy store in the wheels as well, and that's an example of dissipation because the energy's being transferred to a less useful store and we can say that it has been wasted.

Amazing job if you've managed to identify the different energy stores and transfers in that scenario.

Time to move on to the third learning cycle, looking at calculating efficiency.

All new appliances have efficiency ratings and that shows people how good the appliance is at transferring energy to its useful store.

You might have seen stickers like that one on the screen on your washing machine or maybe on your fridge when it was bought.

An efficient appliance will dissipate very little energy.

That means that it can transfer as much energy as possible into useful energy store.

So can any appliance be more than 100% efficient? No, this isn't possible because of the law of energy conservation.

An appliance can't transfer more energy to a useful store than what was put into it.

So every appliance is less than 100% efficient, but the closer to 100% it is, the better it is at transferring energy usefully.

Efficiency can be calculated using an equation.

Efficiency is equal to the useful energy output divided by the energy input.

And both those energies are measured in joules.

Efficiency is often converted to a percentage, and we can do that by multiplying the number we get by 100.

Let's have a look at an example of calculating efficiency in a scenario.

Calculate the efficiency of a kettle that has 5,000 joules of energy input and transfers 4,000 joules to the thermal energy store of the water inside.

So first, we need to write down the equation.

Efficiency is useful energy output divided by energy input.

Now, in this situation the useful energy output was the 4,000 joules transferred to the thermal energy store of water and the input energy was 5,000 joules.

If I divide those two numbers together, I get an efficiency of 0.

8.

And multiplying that by 100 tells me that this kettle was 80% efficient.

Now I want you to have a go.

Calculate the efficiency of a toaster that has 3,400 joules of energy input and transferred 1,700 joules of energy to the thermal energy store in the bread.

Pause the video now and press play when you're ready to see me go through the answer.

Okay, so the efficiency is equal to the useful energy output divided by the energy input.

Now, the useful energy output in this situation was the 1,700 joules of energy that was transferred to the bread.

So we can divide that by the 3,400 joules of energy that was input into the toaster.

That gives us an efficiency of 0.

5, and multiplying that by 100 tells us that the toaster was 50% efficient.

Well done if you managed to do that by yourself.

Now it's your turn to have a go on your own at three more efficiency calculations.

Remember to read the questions carefully and identify the useful energy output and the energy input before you put them into the equation.

Pause the video now and press play when you're ready to go through the answers.

Let's take a look at the answers.

Calculate the efficiency of a phone charger that has 50 joules of energy input and transfers 40 joules of energy electrically to the chemical store in the phone's battery.

We'll start by writing down the equation.

Efficiency is useful energy output divided by energy input.

Now the useful energy output was 40 joules, so we can divide that by the 50 joules that was input into the phone charger.

This gives me an answer of 0.

8.

And converting that into a percentage tells me the phone charger was 80% efficient.

Excellent job if you've got that calculation right.

Calculate the efficiency of a smartwatch that transfers 6 joules of energy by light radiation when 10 joules is input.

We'll start again by writing down the equation.

Efficiency is useful energy output divided by energy input.

And six joules of energy was output by the smartwatch to light radiation when 10 joules were input.

This gives me an efficiency of 0.

6.

And converting that to a percentage tells me the smartwatch was 60% efficient.

Excellent job.

Finally, calculate the efficiency of a lawnmower that dissipates 3,000 joules of thermal energy to the surroundings when it has an energy input of 12,000 joules.

Now this question was a little bit tricky, so if you didn't get it right, don't worry, but if you did, then well done for identifying that the 3,000 joules of thermal energy here was dissipated.

That means to calculate the useful energy output, I need to take away this dissipated energy from the total input energy.

So the lawnmower transferred 9,000 joules usefully into kinetic energy to cut the lawn.

When we now write down the efficiency equation, useful energy output divided by energy input, the calculation you had to do was 9,000 divided by 12,000.

Remember, the 9,000 joules was the useful energy that the lawnmower transferred, and 12,000 joules was the energy input.

This gives me an efficiency of 0.

75.

And converting that to percentage tells me the lawnmower was 75% efficient.

If you got that correct, really, really good job.

That was a tricky one there that I put at the end just to test your knowledge of dissipation.

So if you've got that correct, give yourself a big pat on the back.

Time to move on to the final learning cycle in this lesson, Sankey diagrams. Sankey diagrams show how all the energy in a system is transferred into different stores.

And this is an example of a Sankey diagram for a stair lift.

Now, if the stair lift had 5,000 joules of energy input, this would go at the beginning of the arrow on the left hand side.

And then the arrows that come off, the larger arrow represent how the energy's transferred.

So if the stair lift transferred 2,000 joules of kinetic energy lifting the chair, but wasted 3,000 joules of energy, then I'll draw two arrows like this on my diagram.

You'll notice that the size of the arrow represents how much energy is being transferred to that store.

So the kinetic energy lifting the chair was 2,000 out of 5,000.

So it's two fifths of the size of the original arrow.

And the wasted energy, which is 3,000 joules, is three fifths of the original arrow.

The larger the amount of energy that gets transferred to each store should mean that the arrow should be thicker for that energy store.

And it's important again because of the conservation of energy that our arrows coming out of the Sankey diagram equal to the arrow going in.

So we can see here that if I add 2,000 joules to 3,000 joules, that does make 5,000 joules, which is input.

So I know the Sankey diagram's been drawn correctly.

Here is a worked example of a question that asked us to draw a Sankey diagram showing the energy transfers in a rollercoaster.

If the rollercoaster started off with 6,000 joules of gravitational potential energy, then 3,500 joules were converted to kinetic energy.

So I've put that arrow on the right hand side and it's gonna be thicker than the energy dissipated because it was a larger number.

The energy dissipated to the environment was 2,500 joules.

Now, we always draw the energy wasted or dissipated coming off the bottom of the Sankey diagram.

So that Sankey diagram represents the 6,000 joules input and the 3,500 joules converted to kinetic energy with the 2,500 joules dissipated or wasted to the environment.

Let's check your knowledge so far in this learning cycle.

Which diagrams are used to show how all the energy in a system is transferred into different stores? Well done.

It's a Sankey diagram that shows that.

True or false: a Sankey diagram needs to have the amount of energy input labelled.

This is true.

Well done.

And that's because the arrows have to represent the energy transfer amounts.

So I need to know the input energy or I'm not gonna know how much has been transferred to other stores or if anything's been wasted.

Well done for getting that question right.

Let's test your knowledge of Sankey diagrams. Sketch a Sankey diagram for a washing machine with an input of 20,000 joules of chemical energy if it transfers 15,000 joules to thermal energy and 5,000 joules to kinetic energy.

Pause the video now to have a go at this and press play when you're ready to have a look at the Sankey diagram.

Okay, let's go for the answer.

So the washing machine had 20,000 joules of chemical energy input.

So that starts off the arrow of my Sankey diagram.

15,000 joules were transferred to thermal energy and 5,000 joules were transferred to kinetic energy.

Now here we haven't got any dissipated energy represented, so we can just draw two arrows that represent the amount of energy.

The 15,000 joule arrow should have been a lot thicker than the 5,000 joule arrow.

Excellent job if you drew that diagram.

Let's go through a summary of this lesson on conservation of energy and efficiency.

The law of energy conservation states that energy can be transferred usefully, stored, or dissipated, but cannot be created or destroyed no matter what.

Energy is dissipated when it is transferred to a less useful energy store.

Efficiency can be calculated using the equation efficiency equals useful energy output divided by energy input, and we often convert this to a percentage.

And Sankey diagrams show how all the energy in a system is transferred into different energy stores.

Well done for completing this lesson.

See you next time.