Lesson video

Hello and welcome to this lesson on transferring energy.

My name's Mr Norris.

This lesson is from the forces topic.

Energy is such a powerful concept and understanding that energy can be transferred between different stores is so useful, so powerful, it can help you understand living things, it can help you understand chemical reactions, it can help you understand stars.

So energy is a really, really useful concept to understand from a scientific point of view.

So let's get going.

The outcome of this lesson is that by the end of the lesson you'll be able to identify when stores of energy change and describe what causes those energy transfers between different stores of energy.

The keywords that will come up in this lesson are store of energy, surroundings, and energy transfer.

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

There are two sections to this lesson.

The first section looks at certain example situations where energy stores change and the second section we'll then look at the reasons why energy stores change in different situations.

So let's get going with the first section.

So the starting point for this lesson is the idea that there are different ways or different reasons that different objects store energy, or we say that there are different stores of energy that objects could have such as gravitational stores of energy.

When objects are high up we say they have a gravitational store of energy.

When objects have a temperature, we say they have a thermal store of energy and the hotter they are, the higher the temperature, the more energy they have because of their temperature.

When things are stretched, we see they have an elastic store of energy, they have energy because they're stretched.

When things can undergo a chemical reaction, we say that they're a chemical store of energy.

And when things are moving, we say that they are a kinetic store of energy.

Now hopefully those words and ideas should be familiar to you from your earlier studies, but if they're not, then it is probably a good idea to go and study energy stores in more detail first before continuing with this lesson which is looking at energy transfers between those different stores.

So if you need to do that, go and find a way of studying different energy stores first and then come back to this lesson which is gonna look at what happens when energy transfers from one of these stores to another.

So, let's look at a first situation.

The situation is when a ball falls.

When a ball falls, its height decreases and it, well it gets lower, and its speed increases, it gets faster.

So here's a before snapshot where the ball is being held high up and it's not moving yet, it's stationary, it's about to be dropped and at that snapshot there the ball is lower, but of course it's now got a speed downwards, it's now moving.

So it's got faster 'cause it wasn't moving before and now it is moving.

So in terms of which energy stores have changed, its really important that when you're trying to answer that question, which energy stores have changed, you're really clear on the two points that you're looking at, the start point and the end point.

So between which two points did energy stores change? So I'm giving you the two points really clearly here, I'm giving you a start point and an end point.

So between these two snapshots, between the start point, when the ball's high up and stationary, and the end point, when the ball was lower and moving now.

Which stores of energy changed? Well, hopefully you might've spotted that because the ball is lower, then it's gonna have less energy because of its height.

So, its gravitational store of energy has decreased.

But it's now got movement, when it wasn't moving at the start point, so the ball's kinetic store of energy has increased because it wasn't moving and now it's moving with a speed.

Let's do a quick check about those ideas about gravitational and kinetic stores of energy.

So there are four statements below the table, put each statement into the correct box in the table.

You should pause the video now if you need to.

Okay, I'll go through the answers.

When does a gravitation store of energy increase? That's when an object is raised higher.

When does it decrease? When an object is lowered.

Gravitational store of energy is all about how high something is in a gravitational field.

And kinetic stores of energy increases when an object speeds up and decreases when an object slows down.

So hopefully that should've been fairly straightforward.

But what you've done there is you've begun to identify situations where stores of energy change, where they increase and store more energy or where they decrease and store less energy because of different factors like height or speed.

Let's look at another situation now.

So, this is gonna be stretching a spring.

To stretch a spring, a person's body has to work, do a bit of work to stretch that spring is, and like the bigger and stiffer the spring, then the more work your body has to do to stretch it, it's harder to do.

So there's our before snapshot, the spring is un-stretched.

And there's our after snapshot where the spring is stretched and it's stationary at the start and it's stationary at the end.

It was moving in between but that doesn't matter, we've just got a start point and an end point.

And the question is, how does stores of energy change between that start point and that need point? So between the two snapshots, well the person stretched the spring and whenever a person does anything, their chemical store of energy goes down.

The chemical reactions in your body are what enable your body to do anything.

And what other energy store changed? Well the spring's elastic store of energy increased because the spring was less stretched and it's become more stretched, so it now stores more energy.

The elastic store of energy has increased because the spring is now more stretched.

Let's look in more detail at chemical stores of energy changing.

So we looked at the human body, but actually all of these involve a chemical store of energy changing.

So burning a fuel, that's a chemical reaction, probably something's getting hotter so a chemical store of energy is decreasing whenever a fuel is burnt 'cause you end up with less fuel so a smaller chemical store of energy, less energy.

If you take in food, your body is increasing its chemical store of energy.

If a plant grows, then a plant is increasing its chemical store of energy.

If a human body moves or pushes an object, then that is gonna decrease the human body's chemical store of energy.

Any time your body does anything, even within your body, that is enabled by chemical reactions, so a chemical store of energy decreases within your body every time your heart beats, or every time your chest moves up and down so you can breathe.

And, any time anything uses or recharges a battery.

Using a battery is gonna run down the battery's chemical store of energy, and recharging a battery is gonna replenish the battery's chemical store of energy.

So, it's important to recognise that all of these examples are examples of the same thing, a chemical store of energy changing, either increasing or decreasing.

So let's do a check on chemical stores of energy changing.

So for each of these, I think there's gonna be eight situations, do they cause the chemical store of energy to increase or decrease? So here they are, a candle burning, grass growing taller, a person doing a push-up, a person eating, a torch that's left on all night, a person walking up stairs, a person standing still in a lift, and a mobile phone charging.

All you need to do is put increase or decrease for each of those for whether it increases or decreases a chemical store of energy.

Pause the video if you need to.

Off you go.

Okay, let's see how you got on.

So a candle burning, that's a chemical reaction which is gonna transfer energy to the surroundings, so the candle's chemical store of energy, you end up with less fuel, it's gonna decrease the chemical store of energy.

Grass growing taller, you're effectively ending up with more grass, so the grass is increasing its chemical store of energy by growing.

A person doing a push-up, they're decreasing their chemical store of energy, that's what enables them to move.

A person eating, they're taking in food, so that's increasing their chemical store of energy.

A torch left on all night is gonna run down the battery, so that's decreasing the battery's chemical store of energy.

A person walking up stairs is gonna decrease the person's chemical store of energy.

A person standing still in a lift.

Now remember, just even when someone's standing still, their muscles of their heart are still moving and the muscles in their chest are still moving to enable the person to breathe and all of those reactions or all of those processes are enabled by chemical reactions so involve a chemical store of energy decreasing.

And a mobile phone charging is gonna replenish the battery's chemical store of energy, so the battery's gonna end up with more energy after it's been charged, so it's increases for the last one.

So hopefully they were fairly straightforward and well done if you got most of those right.

Let's do another check now.

Describe how the stores of energy of the person and the toy changed when the toy was lifted onto a shelf.

So we've got a before and an after snapshot here and your job is to describe how do which stores of energy change? So between the two snapshots the person's which store of energy decreased? And after the second snapshot, the toy's what store of energy increased? Pause the video now, make sure you can fill in those two gaps.

Okay, I'll see how you got on.

So for this you had to remember the names of the different stores of energy that objects can have, so gravitational, thermal, elastic, chemical, and kinetic.

Now this is a person lifting a toy up.

Whenever a person does anything, their chemical store of energy decreases.

And, the toy has been lifted higher up, so it's gonna be gaining energy into its gravitational store, so we can say the toy's gravitational store of energy increased.

Well done if you got both of those.

Okay, let's look at another example now of energy stores changing.

So this is a cup of tea cooling down.

When a cup of tea cools down, it transfers some energy to the surroundings, and for this cup of tea, the surroundings is just anything that's not the cup of tea that is around the cup of tea, so the air in the room and the tabletop are part of the surroundings around the cup of tea.

So there's our before picture and here's our after picture.

So 10 minutes after, the cup of tea has cooled down a bit, you can't see the steam anymore.

So, between those two snapshots, how do stores of energy change? Well, the tea has cooled down, so its thermal store of energy has decreased.

But, the air surrounding the cup of tea and the tabletop will be a little bit warmer than it was before the starting point because the hot tea will have warmed the surrounding air and the hot tea will have warmed the tabletop that it's sat on a little bit.

So if the air and the tabletop are a bit warmer than they were before, then they have increased their thermal store of energy.

Okay, your turn to do another one of these.

Describe how stores of energy of the candle and the surroundings changed between the two snapshots.

So look at the two snapshots.

The candle's which store of energy decreased? And the surrounding's which store of energy increased between those two snapshots which are 30 minutes apart? Pause the video now, see if you can fill in the blanks using your energy stores, gravitational, thermal, elastic, chemical, kinetic, which of those fill in the blanks? Have a go.

Let's see how you got on.

So, when a candle burns, that's a chemical reaction and you'll end up with much less fuel left than you had at the start.

The fuel for a candle is the wax.

So you're ending up with less and less wax as the candle burns.

So that's decreasing the candle's chemical store of energy 'cause it's a chemical reaction.

Burning is a chemical reaction.

And what's the candle doing? Well the overall effect of what the candle does is the effect is that it warms the surroundings, so the surroundings' thermal store of energy increased.

The room will be a bit warmer 30 minutes later than it was 30 minutes before.

So, the idea of the surroundings' thermal store of energy is a really important one because energy often ends up in the surroundings, and in fact, objects often transfer energy to the surroundings' thermal store of energy in addition to what we normally think of that object as doing in addition to the intended effect.

So I've got lots of examples of that, lots of examples of objects or processes that warm the surroundings, so lead to the surroundings' thermal store of energy increasing.

So our first example is friction.

Any time two surfaces rub together, each surfaces gets a little bit warmer than it was before.

So that causes energy transfer to the objects themselves and to their surroundings.

Any electrical device will warm up the surroundings a little bit and sometimes a lot.

I'm sure you'll have had the experience of using a laptop and realising that the laptop has warmed your lap after maybe half an hour or an hour or something like that.

That's because any time electricity flows through any kind of circuit, there's always a heating effect, even if it's just a very tiny one.

Some electrical devices actually cause a large, unintended heating effect, such as using a vacuum cleaner.

Again, you might well have had the experience of vacuuming a room and realising at the end of the process that the vacuum cleaner itself has raised the temperature of the room.

This is why lots of electrical devices have fans inside, like a laptop or a computer has a fan inside, or a vacuum cleaner has vents on the side to make sure it doesn't get too hot that it would become dangerous.

Hot parts of machines also can transfer energy to the surroundings' thermal store of energy, causing the surroundings' thermal store of energy to increase.

The movement of living things also increases the surroundings' thermal store of energy.

That's because every time you move, that does increase the temperature of your muscles a little bit and you do certainly feel hotter after you exercise, but your body has very clever ways of making sure your body temperature doesn't rise too much and your core body temperature is basically constant.

So your body has multiple clever ways of decreasing its thermal store back down to normal again, so the energy gets transferred to the surroundings, increasing the surroundings' thermal store of energy.

So the energy of any movement that you make eventually ends up in the surroundings' thermal store of energy.

And the final example is objects that emit light or sound.

The light waves or the sound waves that these objects emit, well where do they go? They go off into the surroundings where they get absorbed by the surroundings.

So the final effect of anything that emits light or sound is that energy is being transferred into the surroundings' thermal store.

Just take a moment now to look over these five ways that energy can be transferred into the surroundings' thermal store because any time you see any of these processes, friction, electrical devices, machines that get hot, movement in living things, anything that gives out light and sound, you can be thinking straight away they are gonna cause the surroundings to warm up a bit, they are gonna cause the surroundings' thermal store of energy to increase.

So, let's do some checks of this idea that energy often gets transferred to the surroundings.

In this question, describe how the stores of energy of the car and the surroundings changed between the two snapshots.

So in the before snapshot, the toy car's moving and afterwards, the toy car has come to a stop probably because of frictional forces, maybe some air resistance too.

So fill in the gaps using the correct stores of energy, gravitational, thermal, elastic, chemical, kinetic, which stores of energy changed between those two snapshots? Pause the video, have a go.

Let's see how you got on.

I think the first bit should be quite straightforward because the car was moving and then it stopped moving, so the car's kinetic store of energy has decreased, it's no longer moving.

And where does the energy go? The energy goes to the surroundings' thermal store of energy, so the surroundings' thermal store of energy increased.

Well done if you got both of those.

Let's try another.

This LED torch is accidentally left on for 24 hours.

An LED torch just uses more modern light bulbs where there isn't really any temperature difference when the bulb's on and when the bulb's off.

The battery then goes flat 'cause it's been left on.

So describe how the stores of energy of the torch and surroundings changed between the two snapshots.

Have a go at doing that please.

Very similar to before, off you go.

Let's see how you got on with this one.

So the torch stores its energy in the battery which is a chemical store of energy, so well done if you got that.

And where does the energy go? Well the torch works by electricity and electricity has a small heating effect, but the torch is also giving out light and when light goes into the surroundings, the surroundings absorb that light and get maybe a tiny bit warmer over time, so the energy has basically gone into the surroundings, and the we know that surroundings has a thermal store of energy.

So the surroundings' thermal store of energy increased.

Well done if you got that one.

I think let's try one more.

Mr Orange does 20 minutes of high-intensity exercise.

Describe how stores of energy of Mr Orange and the surroundings changed from before the exercise to 10 minutes after.

There's three pictures here, you can basically ignore the middle picture which is straight away after exercise.

We're only interested in comparing before exercise, when Mr Orange is stationary with a core body temperature of 37 degrees C to 10 minutes after he's finished when he's stationary again and his core body temperature is back to 37 degrees C again.

So how do stores of energy change between the before picture and the 10 minutes after picture? Pause the video if you need to, have a go.

Let's see how you got on with this one.

Well this is a person exercising, so it's gonna decrease their chemical store of energy and 10 minutes afterwards, where has that energy going to have gone? Well it's going to have gone into the surroundings' thermal store of energy like we described earlier.

Well done if you got both of those.

Okay, time to do a task on these ideas now.

Now, every single one of these questions is exactly like the checks for understanding that we've just done where there's a before picture and an after picture, and you've just got to spot what's changed.

So, which store of energy has changed? And there will always be a store of energy that's decreased, and there'll always be one or two, possibly, other stores of energy that increase.

So for every question where you've got to describe the stores of energy that changed, you can always write it as, this thing's store of energy decreased and this thing's store of energy increased.

But of course, you've got to say what the object is and you've got to say what kind of store of energy was it that increased or decreased, gravitational, thermal, elastic, chemical, or kinetic each time.

For example, in question one, a person spun a spinning toy.

We've got the before picture with the toy still and the after picture with the toy now spinning, so that's moving.

So, first thing to think about, what store of energy would've decreased when a person spun a spinning toy? Well the person's doing it, so think about what store of energy the person has which is gonna decrease and then you can write the person's, which type of store of energy is it, the person's mm store of energy decreased.

And then you ask yourself, well what store of energy increased? And you look at the pictures and well it started off not moving and now it's moving, so you should be able to work out what store of energy, which of the toy's store of energy increased in that example.

And you do the others in exactly the same way.

If you're still not 100% sure what I'm asking you to do, then go back to the previous couple of slides and look at the examples there that we did.

If you are ready, then pause the video now and have a go with your best effort on this task.

Off you go.

Welcome back and well done for your effort on that task.

I'm gonna go through these answers quite quickly.

So, when a person spins a spinning toy, the person's chemical store of energy decreases and the toy's kinetic store of energy increases.

Some energy goes to the surroundings, so the surroundings' thermal store of energy increased too.

What about when the spinning toy stopped? Well the toy's kinetic store of energy decreased, it's moving less and less fast then it stopped, and all of that energy ended up in the surroundings, the surroundings' thermal store of energy increased.

And that's because the toy slowed down because of friction between the moving part and the axle and we know that friction transfers energy to the surroundings' thermal store.

For number three, when the stretched catapult is released the catapult's elastic store of energy decreased and the ball's kinetic store of energy increased.

And of course some energy goes to the surroundings, perhaps due to the sound waves given out into the surroundings, perhaps because of some air resistance as well.

In number four, when the toy car rolls down a slope, this time it was going from higher up to lower down, so the gravitational store of energy decreases, the kinetic store of energy increases because it's got faster, and some energy of course has gone to the surroundings' thermal store of energy because of friction.

And for number five, when a petrol car gets faster, it's burning petrol in the engine, petrol is a chemical store of energy so the car's chemical store of energy decreased and it got faster so the car's kinetic store of energy increased, but of course some energy went to the surroundings' thermal store of energy, which increased.

And finally, when a camping stove heats water from eight degrees C to 100 degrees C, the stove's chemical store of energy decreased because you might've said the gas' chemical store of energy decreased 'cause you've burnt some gas, the pan and the water's thermal stores of energy increased 'cause they've both got hotter, and the surroundings' thermal store of energy has increased too.

So very well done if you got lots of that right.

Lots of that is quite tricky, so well done if you're on the right path.

Time for the second part of this lesson, the causes of energy transfer.

So, what we've not said yet is that when a store of energy decreases, other stores of energy must increase but by the same amount.

So for this candle burning, these bars represent initial amounts of energy in the candle's chemical store and the surroundings' thermal store, and they're gonna change, but the amount that one decreases is the same as the amount that the other is gonna increase.

So when the candle's chemical store decreases, say that amount, then that amount of energy is gonna be the amount that the surroundings' thermal store increases by.

So this is called an energy transfer because it looks like an amount of energy has moved out of one store and into another because one store decreased by the same amount that the other store increased by, it looks like energy's actually gone from one to the other.

This is linked to the idea that energy cannot be created.

A store of energy can only increase if another store of energy decreases by the same amount to give that first store the energy that it gained.

So, for a falling ball that's only falling under a gravitational force with no other forces acting, like air resistance, so let's ignore air for the moment.

The falling ball gets lower and it gets faster, so the gravitational store of energy is going to decrease and the kinetic store is going to increase, but this is not possible.

The gravitational store can't decrease by that amount and the kinetic store increase by much more because where would the ball's kinetic store get that extra energy from? There's no other store that would've decreased.

So, energy cannot be created, the energy gained by the ball's kinetic store must be the same as the energy lost by the ball's gravitational store because otherwise energy would've appeared from nowhere which isn't possible.

And similarly, energy cannot be destroyed or used up.

Energy can only be transferred into another store, so this means you can never actually get rid of energy, it always has to go somewhere, usually the surroundings.

In this case, again, we'll look at the falling ball where there's no air resistance.

So the ball gets lower and it gets faster and there's no other forces acting.

Again, the ball's gravitational store is gonna decrease and the ball's kinetic store is gonna increase, but it has to be by the same amount.

This is not possible for the ball to decrease in the gravitational store this much and a ball to only increase its kinetic store this much in this situation where there's only the gravitational force acting and no air resistance.

Where would the energy that was initially in the ball's gravitational store go if it's not all gone to the ball's kinetic store? And that can't happen, there is nowhere else it could go if there's no air resistance.

So energy cannot be created and energy can't be destroyed.

So let's look at which of these diagrams could show the correct changes in energy stores for a falling ball.

Is it A? Is it B? Or is it C? Remember, energy can't be created or destroyed, so when the gravitational store decreases the kinetic store should increase by the same amount which shows that, A, B, or C? Five seconds to decide.

Well done if you identified that A shows the correct energy transfer for a falling ball.

The kinetic store increases by the same amount that the gravitational store decreases by, so no energy is created or destroyed.

B shows energy being lost or destroyed completely and C shows energy being gained or created, neither of which can happen.

So the idea that energy cannot be created or destroyed, only transferred between stores, is called the law of conservation of energy.

The law of conservation of energy can tell us what's possible and what's impossible in a situation.

So for example, for a ball on a slope, if you release the ball here from rest, then it will never be able to roll above the line, because where would it get the energy to do that from? It would have to be given an extra push or something like that, but then it's not released from rest.

So, energy can tell us what's possible and what's impossible, and everything in the universe obeys this law of conservation and energy, which is why it's so useful.

A rolling football has a kinetic store of energy 'cause it's moving.

As the football rolls, it's gonna hit blades of grass, making them move, and some of its energy is transferred to the grass.

The football also hits air particles as it rolls, some of its energy gets transferred to those air particles, so to the air.

So as the ball slows down and stops, its energy is not used up or lost, its energy gets transferred to the surroundings, the blades of grass and the air particles that surround it, because of the forces of friction and air resistance.

So, which is the best explanation of what happens to this car's energy? The toy car's moving and then the toy car has come to a stop.

Is it A, the car's energy was used up keeping the car moving? Is it B, the car's energy disappears? Is it C, the car's energy disappears into the surroundings? Is it D, the car's energy is transferred into the surroundings? Which is the best option? Well done if you said D, the car's energy it had because it was moving gets transferred into the surroundings and that's why it comes to a stop.

So, we've seen that energy transfers can be caused by forces like gravitational forces that accelerate a falling ball, make it go faster as it falls.

We've seen that energy transfers can be caused by friction forces because that can transfer energy from a car's kinetic store into the surroundings.

And we've seen that energy transfers can be caused by pulling forces that might stretch a spring, causing energy to be transferred into a elastic store.

But energy transfers can also be transferred by all of these things, chemical reactions such as burning or chemical reactions in a battery or chemical reactions within your body.

Energy transfers can also be caused just by contact between two objects that are at different temperatures.

If you've got a colder thing and you put it in contact with a hotter thing, then the colder thing gets warmed up because it's touching the hotter thing.

That's an energy transfer.

Electricity causes transfer of energy from one store into another, we've seen that.

And also absorbing or producing sound or light causes an energy transfer from one store into another store, often into the surroundings' thermal store of energy.

So which causes energy to transfer from a chemical store to the thermal store of the surroundings in a firework? Is it electricity, a chemical reaction, or is it a force? Choose A, B, or C now.

Five seconds to decide.

Well done if you said B, a chemical reaction.

In a firework, there's a chemical reaction which causes the nice lights and sound, so that causes an energy transfer from the chemicals in the firework to the surroundings.

What causes energy to transfer from the gravitational store to the kinetic store of the skydiver as they fall? Is it electricity, chemical reaction, or a force? Five seconds to decide.

Well done if you said a force, in this case it's a gravitational force of course.

Time for the final task of this lesson then.

For each example, I'd like you to do what you did in task A in the lesson and describe how the named stores of energy have changed, but this time I want you to add on part B, what caused the energy to be transferred out of our list of forces, electricity, chemical reaction, contact between objects at different temperatures, and absorbing light or sound.

Have a go at this now, part A and part B for each situation one, two, and three.

Pause the video now, off you go.

Well done for your effort on that task, let's see how you got on.

So, for situation one, it's the candle burning.

The amount of energy in the chemical store has decreased and the amount of energy in the thermal store of the surroundings has increased between the two pictures.

And what caused that? Part B, this was caused by the burning of the wax which is a chemical reaction.

You might have mentioned this was also caused by the candle giving out some light which was absorbed by the surroundings, that's fine too.

Okay, situation two.

How did the stores of energy change? Well the amount of energy in the gravitational store decreased 'cause the car got lower, the amount of energy in the kinetic store increased and some energy of course went to the surroundings too 'cause of friction.

What caused the energy to be transferred? Well I've mentioned friction already, transferred some energy to the surroundings, but also the gravitational force is what moved the car from higher up to lower down and increased its speed.

So well done if you got those.

And situation three.

Which stores of energy changed? The amount of energy in the person's chemical store decreased and the amount of energy in the giraffe's gravitational store increased 'cause they moved higher up.

And what caused the energy to be transferred? Well it was caused by a force from the person lifting the toy giraffe using their muscles.

You might have mentioned chemical reactions in the person's body which enabled the person to provide that force which lifted the giraffe.

So well done if you included any of those ideas in your work.

So that's the end of this lesson on transferring energy.

Very well done, there were some tricky ideas in this and you'll revisit them again as you move through your school science studies.

So here's a summary of the lesson.

Stores of energy can increase or decrease when there are changes in speed, height, temperature, length, such as the length of a spring, and during chemical reactions.

When stores of energy decrease, other stores of energy have to increase by the same total amount.

That's because energy cannot be created or destroyed, only transferred.

And, what can cause this energy transfer? Well forces is the first thing, forces can cause energy to be transferred between different stores of energy, and energy can also be transferred when an electric current flows, when there's a difference in temperature between two objects, when there's a chemical reaction, and when light and sound waves interact with matter and get absorbed.