What are fuels?

Fuels are useful chemicals that we can burn to get things done. The fuel might be a can of petrol, a chocolate snack bar, or a cylinder of camping gas. Any of these fuels can be made to react with the oxygen in the air to get a job done. Here are some examples:

• The petrol tank empties as you make your way up the motorway in the car. Each explosive combustion in the engine burns up a carefully measured quantity of fuel with oxygen from the air.
• Boiling rice over a gas flame in the kitchen uses up the gas, a fuel transported to your home through a pipe.
• Frying bacon over an electric ring can also be driven by burning a fuel – but the burning is not in your house. The fossil fuel powered generating station may be burning coal several tens of kilometres away, so long as it is connected to your home by electrical wires. These wires form part of an electrical circuit, making the outcome of burning that fuel available to you in your home.
• Thinking hard draws on the food fuel eaten at breakfast time, as does the brisk walk to catch the bus to school in the morning. This kind of burning (in the human body) is less rapid, but still depends on combining a fuel with oxygen.

In each of these examples, the fuel gets used up as the different jobs are done, and this fits in well with the idea that you cannot get something for nothing. You can only burn a fuel once. Once the chemicals have combined with oxygen, that's it.

As any Yorkshire person will tell you: You can never get owt for nowt!.

Here's an example where fuel is burnt, and there is an energy change

You need to identify carefully what change you're discussing.

Not all jobs require a fuel to get them done

An important point to bear in mind here is that not all jobs require a fuel to get them done.

Lifting a book onto a shelf or a rocket into orbit do involve burning a fuel. The fuels are of different kinds, but both involve using up a supply of reacting chemicals. However, keeping the book on the shelf or the rocket in orbit do not require a supply of fuel. Changes are still happening, with the rocket zipping around the Earth and the book spinning around the Earth's axis along with everything else in the room. Both are moving even faster through our solar system, but none of these changes are powered; no fuel is used up.

In a similar way, dragging a sledge through snow or pushing a car along a road both burn up fuel, whereas the endless motion of gas molecules in the atmosphere or the planets' orbital motions around the sun involve no such continuous consumption. Burning a fuel gets some jobs done, but lots of changes happen without burning a fuel.

Here are some changes: some need a fuel and some do not.

Associating energy stores to fuels

You can describe the ability of a fuel, such as petrol, to get a job done by saying that it has an energy store associated with it. As the fuel does its job (maybe propelling a car along the road), energy is shifted from the store. Given the source of the energy (the petrol), we say that this is a chemical store.

We could describe this process purely in terms of the familiar everyday things involved: I put petrol in the car, and the car goes along the motorway.

Alternatively, we can look at the process from a different perspective (in precisely the same way that we encouraged you to look at the world through forces spectacles). When looked at from the energy perspective, we describe the process in different terms: As the car travels along the motorway, energy is shifted from the chemical store associated with the petrol.

It's important to remember that fuels such as petrol or chocolate need to react with other chemicals to shift energy, often by burning in oxygen. This shifting of energy is what empties the store. To have a store, both the fuel and the oxygen are needed. That is why rockets designed to work in space must carry both – they cannot rely on the oxygen being around once they have left the atmosphere.

Consider the burning of methane(methane + oxygen  →  carbon dioxide + water) as shown by the equation:

CH₄ + 2O₂  →  CO₂ + 2H₂O As the fuel is burned, energy is shifted from the chemical store associated with the methane and oxygen. If there is no reaction, there is no change in the store of energy.

In general terms you can say:

Teacher: Energy is shifted from a store when a fuel is burned

(Remembering that both fuel and oxygen are needed.)

Theoretical and physical ideas

It is important to recognise from the very outset that this description of the action of fuels, in terms of energy and energy stores, is theoretical or abstract in nature. It doesn't belong to discussion in the everyday, or lived-in world. You can't look down a very powerful microscope to find the chemical store inside the petrol! We think it is helpful to make a clear distinction between the everyday description and the energy description.

These are linked, but different:

Both the energy and the energy store are theoretical ideas, whilst the petrol belongs to the everyday world. It is the energy store idea that allows us to describe and make calculations about the burning of petrol in a car engine, but it is the petrol that you must carry along in a can if you run out of fuel!

You probably won't find yourself having conversations about stores of energy at the pump. But nevertheless the energy in these same stores have a very real and physical effect on what's possible (and what isn't).

Getting the job done

So far we have concentrated on combining chemicals to get jobs done. However burning fuels with oxygen is not the only way to get these jobs done. It is possible to use other devices to drive the car along the road, or to lift a book onto the shelf. These might work by allowing a spring to relax or something to fall.

We do not have many examples of rubber band powered cars, although the alternative technology centre in Machynlleth does have a transport system powered directly by falling water. Electric trains, for which the wires supplying the electric current are connected to a hydro-electric generating station are another example of a falling-object driven transport system. This time the connection is indirect; there is an electric circuit connecting one change to another.

It does not seem appropriate to refer to either a relaxing spring or a falling object as a fuel (even though they get jobs done).

In just the same way as fuels, we refer to these as having energy stores associated with them:

• A supply of fuel and oxygen has a chemical store of energy associated with it.
• Water held high in a reservoir (ready to run downhill) has a gravitational store of energy associated with it.
• A stretched rubber band, ready to be released, has an elastic store of energy associated with it.

This idea of stores of energy is fundamental to our treatment of energy. There are a number of changes associated with emptying energy stores. These are clues as to which kind of store will be relevant to the energy description.

A carefully chosen selection

The good news about describing the world in terms of energy stores is that there are a limited number of stores to think about. Here is a selection of stores, with those that might be used earlier listed first. In each case that there is more of less energy in a store is a semi-quantitative description, standing in for a useful calculation that can be done.

Energy stores can be more or less full

Imagine a car travelling down the road, the amount of energy in the energy store (in this case a chemical store) decreases. As changes occur, it is often the case that energy stores are filled or emptied.

It is best not to think of a store as empty or full – just as augmented or depleted during processes. Augmenting and depleting stores may be a precise way of expressing matters, but we'd be surprised to find this language being useful in classrooms. So it is better to talk in terms of increasing or decreasing the energy in a store, of filling up and emptying the store.

Filling and emptying

You'll need to become familiar with these, paying particular attention to identifying physical clues in a process suggesting changes in the quantity of energy in a particular store.

Filling stores-examples

• shifting energy to a chemical store: water and carbon dioxide rearranged to give sugars and oxygen
• shifting energy to a gravity store: water pumped uphill in a hydroelectric scheme
• shifting energy to a kinetic store: a cyclist speeding up as she sprints for the finish line
• shifting energy to a thermal store: a kettle warming water for a hot water bottle
• shifting energy to a thermal store: melting ice
• shifting energy to an elastic store: drawing back to the elastic on a bait catapult
• shifting energy to a vibration store: sea waves increasing their height as a storm passes, or a swing moving more and more as it is pushed
• shifting energy to a nuclear store: stellar reactions
• shifting energy to an electric or magnetic store: rubbing a comb against a woollen jumper, or pulling magnets apart

Emptying stores-examples

• shifting energy from a chemical store: petrol and oxygen combining to give water and carbon dioxide
• shifting energy from a gravity store: a meteorite falling towards the surface of a planet
• shifting energy from a kinetic store:A large tanker gradually coming to a halt as it drifts through the water with its engines off
• shifting energy from a kinetic store:
• shifting energy from a thermal store: a storage radiator cooling down
• shifting energy from a thermal store: steam turning to liquid water
• shifting energy from an elastic store: letting air out of your bike tire
• shifting energy from a vibration store: A wave driven generator providing power to the electrical grid
• shifting energy from a nuclear store: Fission reaction, where large nuclease split in power stations and bombs, or fusion reactions, where small nuclei join, such as within the Sun
• shifting energy from an electric or magnetic store store: electrons being fired towards the television screen, or allowing magnets to come together

Teacher Tip: Note that electric and magnetic stores are magneto-static and electro-static, so do not apply to electrical circuits.

A full set

Here are all of the stores on single sheets:

Support sheet

• Firstly, more empty, ready to be filled.
• Secondly, more full, ready to be emptied.

But how do you tell if the store is being filled or emptied during a process?

Calculating changes to stores

The different stores that are presented here cover all of the processes that you may care to describe from the energy perspective.

There is, however, a deeper significance to selecting these stores. For each of the chosen stores it is possible to calculate, and therefore quantify, the change in energy as processes happen and shifts in energy occur.

To see if the store is being filled or emptied, look for a physical clue:

Later these physical clues will lead to measurements that'll enable you to calculate the changes of the energy in the store.

What about light and sound?

The list of stores is likely to be familiar to you from previous experiences teaching about energy. One big difference is that here, we are referring to different stores of energy rather than different kinds of energy (we find find the description kinds rather too slippery to be helpful).

You may also have noticed that we do not list a light energy store. The reason for this is that whilst it is quite natural to refer to a fuel such as domestic gas as having a chemical store of energy, it is not helpful to talk about light as an energy store. The fuel can do the job of heating your home, but what can a beam of light do? Ultimately, the beam of light interacts with the surroundings (being absorbed, reflected or scattered) and produces a very slight warming effect. In other words it leads to the filling up of a thermal store of energy. We shall return to the case of light in episode 03 when we introduce the idea of power in pathways. It turns out that light is better thought of as a mechanism for shifting energy around rather than as a store … but more of that later.

Teacher Tip: Don't think of light: think of lighting, or maybe radiating.

In the SPT: Sound topic, we argued that sound was a good label for the whole process of vibrations travelling from source to detector. We hope you can see the link between this and light. When you go to the store, you buy your light bulbs and sound systems for their power output, usually quoted in watts. That measure reports the accumulations in a store in each second, not the quantity in the store. You might find it hard to think of sounding, rather than sound, but once the song is over (the process has finished), all that's happened, on the energy front, is that some thermal stores have filled. Much more on this in the SPT: Radiations and radiating topic.

Teacher Tip: Don't think of sound: think of sounding, or maybe radiating.

From one store to another

The next step in this energy story involves recognising that when one energy store empties, another energy store elsewhere must be filling. The energy is just shifted from store to store. Energy is not used up, but is conserved.

For example, combining chemicals to lift an ornament up onto a shelf gets the job done, but raising this ornament also sets up another kind of store. This store involves the ornament positioned higher in the Earth's gravitational field. From the energy perspective, this process is seen in the following way:

Energy is shifted from the chemical store of your muscles to the gravitational store of the ornament in the Earth's gravitational field.

You can then work with this gravitational store by allowing the ornament to fall to do another job. At this point, we must admit that there are few, if any, useful jobs we can think of that are based on falling ornaments, but pile drivers work in just this way as they hammer steel foundations into the ground. The general point is that the second store can be emptied to fill yet another store of energy as a job gets done.

Some questions

These ideas can lead to a whole host of questions:

1. Can emptying the second store (associated with the ornament in a higher position) do an equivalent job to that done by emptying the first (the chemical store associated with the muscles of your arm)?
2. If not, what happens to the energy not shifted from the first store to the second?
3. Is it possible to reduce this shortfall?

These are very practical questions concerned with getting the best results from designed devices and with not squandering the resources available to use. It was concerns such as these that led to the law of Conservation of Energy and to the subsequent growth in importance of the idea of energy.

Describing processes in terms of energy

You can develop an energy description using the following approach.

Select a part of the world to study: chose the process or change to analyse.

Define the before and after snapshots for the change that you want to study. If you choose different moments to take your snapshots, you'll end up describing different parts of the process.

Identify the energy stores between which energy is shifted during the process.

A simple example to consider

For example, the action of lifting an ornament onto a shelf and allowing it to fall back to Earth.

For our example, various start and end points are possible, defining two quite separate processes that jointly make up the action:

The lifting process

Start: Ornament on the ground

End: Ornament on the shelf

The falling process

Start: Ornament on the shelf

End: Ornament on the ground

For the lifting process, two energy stores are of interest:

• The chemical store associated with the lifting arm
• The gravitational store of the ornament in the Earth's field

Energy is shifted from the chemical store of your muscles to the gravitational store of the ornament in the Earth's field.

The general approach:

1. Choose a change to study.
2. Take a snapshot (before).
3. Take a snapshot (after).
4. Identify the stores.
5. Produce an energy description based on this analysis.

Building simple models

With any process, as well as identifying start and end points to the energy analysis, it is also worth thinking about the extent of what you choose to analyse.

For example, in relation to the lifting process, it is possible to focus purely on the lifting agent, and the ornament in the Earth's gravitational field. If you're lucky, you'll find that all of the energy shifted from the chemical store ends up in the gravitational store.

However it is more likely that this will not be the case and that you'll have to extend the boundaries of what you consider to include the surroundings. Here things become a little more complicated! Rather than assuming that all of the energy is shifted from chemical to gravitational stores, now we must investigate whether any energy goes to warming up the surroundings, thereby ending up in the thermal store of the surroundings.

If the amount of energy shifted to the thermal store of the surroundings is relatively small compared with that shifted to the gravitational store, a physicist might chose to ignore it and therefore model the process much more simply, thinking about only two stores, without too much error.

Shifting energy

The energy story that we are presenting here is rather different from the kinds of approaches that are commonly used in school. As such it is quite instructive to take a specific process and describe it in both the commonly used and energy store ways. Let's take lifting a book onto a shelf as the process.

Commonly used approach

Everyman: when I lift the book onto the shelf, chemical energy is transferred from my arm to potential gravitational energy in the book.

We suggest this approach:

Teacher: When I lift the book onto the shelf, energy is shifted from the chemical store of my arm to the gravitational store of the book.

The first approach refers to two kinds of energy, and the picture that is painted is of something tangible (chemical energy) leaving the arm and turning up in the book as something different (gravitational potential energy). There is a confusion and running-together of the physical description with the energy description, with chemical energy being portrayed as something physical that resides in the arm and gravitational potential energy as being something in the book.

The second approach refers simply to energy moving from one store to another. We like this because we see energy as identical, wherever it turns up it's still the same thing. Energy is a unified abstract idea and it does not make sense to talk about different kinds or forms of energy. In addition, energy is described as being shifted between energy stores. This approach keeps all of the theoretical ideas together in the energy description and does not confuse them with tangible things in the physical world.

Choosing terminology

We have chosen to use the word shifting to describe what happens when one energy store empties and another fills. We think that the idea of shifting helpfully supports the underlying understanding of energy leaving one store and turning up in another.

Of course, in many school physics books, the term transfer is used. If you are not happy with shift, then no inaccuracies ensue from replacing it with transfer. However, we thing it worth trying out the shifting approach. Shift is a simple and evocative word.

Predicting what is possible

The most important feature of the energy concept is that it can tell us if a particular change is possible. So:

1. Define carefully the part of the world to be studied.
2. Decide on a possible change.
3. Compare the energy before and after that change.

This comparison will not tell you if or how the change will happen, but if these energy considerations tell you that the change cannot happen, then it absolutely will not.

An example to think about is climbing Ben Nevis, Scotland's highest peak. We can set up a very simple model of this process. The tourist route up Ben Nevis rises from about 40 metre to 1344 metre, a gain of 1304 metre. A simple calculation shows that to raise a female walker (55 kilogram) by this amount takes about 704,000 joule.

Plain chocolate, according to the figures on the wrapper, provides 21,420 joule for each gram consumed. So if supplied with only 2 g the walker would not make it to the top, as the energy available from the chemical store is less than the energy that must be shifted to the gravitational store to allow the climb to happen. In practice, of course, the chemical store does not start completely empty, and many of us can climb Ben Nevis with no immediate food intake at all. The quantity of energy shifted from the chemical store shows how unwise this attempt is, however. It is not a recommended activity!

This example emphasises an important fact. The essence of working with energy ideas is that they ultimately involve quantitative manipulation; you have to work with numbers. The first question for energy is how much? Energy is concerned with the how much, and not with the how.

To describe processes from an energy perspective is to paint a relatively simple picture. In the Ben Nevis example, you can give a precise limit to the possible height gain for a given amount of chocolate (by working out how much energy the climber needs to shift) without needing to worry about how the climber's physiology and anatomy work together to enable her to climb the hill.

Some jobs simply cannot be done because there is not enough energy available. So how do we find out the amount of energy available?

For many fuels, from petrol to breakfast cereals, we can simply look up the energy shifted from the chemical store when these fuels are burnt. If you burn them in oxygen, then they shift a known number of joules from a chemical store for each kilogram of fuel burnt.

Explaining the causes of changes, such as the collision of a car with a wall, often involves modelling the situation with forces. Descriptions in terms of energy do not involve mechanisms.

Teacher: Tracking the quantities of energy through changes provides an account of: What is and what is not possible.

How much energy is available?

The quantity of methylated spirits burnt limits how much water can be warmed for the hot drink.

Conservation of energy

Richard Feynman, one of the most celebrated 20th century physicists, describes the idea of energy conservation eloquently (Feynman, Lectures in Physics vol. 1, 1963, p. 4–1):

Teacher Tip: There is a fact, or if you wish a law, governing all natural phenomena that are known to date. There is no exceptions to this law – it is exact so far as is known. The law is called the conservation of energy. It says that there is a certain quantity, which we call energy, that does not change in the manifold changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says that there is a numerical quantity, which does not change when something happens.

Feynman has a story about the conservation of energy, pointing to this abstract calculated nature. The story is also useful as a reminder that through many changes the energy tends to become more and more spread out, or dissipated, appearing in different stores, and in ways that make it harder and harder to use as a fuel.

He introduces the difficulties in tracking the energy, with increasing ingenuity needed to track down the blocks. But please remember that this is just a story – there are no tangible energy blocks!

It was also a story told to highly numerate American undergraduates, and therefore does not necessarily form the best basis for teaching UK teenagers.

Feynman's story

Imagine a child, perhaps Dennis the Menace, who has blocks which are absolutely indestructible, and cannot be divided into pieces. Each is the same as the other.

Let us suppose that he has 28 blocks. His mother puts him with his 28 blocks into a room at the beginning of the day. At the end of the day, being curious, she counts the blocks very carefully, and discovers a phenomenal law – no matter what he does with the blocks, there are always 28 remaining! This continues for a number of days, until one day there are only 27 blocks, but a little investigating shows that there is one under the rug – she must look everywhere to be sure that the number of blocks has not changed. One day, however, the number appears to change – there are only 26 blocks. Careful investigation indicates that the window was open, and upon looking outside, the other two blocks are found. Another day, careful count indicates that there are 30 blocks! This causes considerable consternation, until it is realized that Bruce came to visit, bringing his blocks with him, and he left a few at Dennis' house. After she has disposed of the extra blocks, she closes the window, does not let Bruce in, and then everything is going along all right, until one time she counts and finds only 25 blocks.

However, there is a box in the room, a toy box, and the mother goes to open the toy box, but the boy says No, do not open my toy box, and screams. Mother is not allowed to open the toy box. Being extremely curious, and somewhat ingenious, she invents a scheme! She knows that a block weighs three ounces, so she weighs the box at a time when she sees 28 blocks, and it weighs 16 ounces. The next time she wishes to check, she weighs the box again, subtracts sixteen ounces and divides by three. She discovers the following:

Number of blocks seen + weight of box

There then appear to be some new deviations, but careful study indicates that the dirty water in the bathtub is changing its level. The child is throwing blocks into the water, and she cannot see them because it is so dirty, but she can find out how many blocks are in the water by adding another term to her formula. Since the original height of the water was 6 inches and each block raises the water a quarter of an inch, this new formula would be:

Number of blocks seen + weight of box

In the gradual increase in the complexity of her world, she finds a whole series of terms representing ways of calculating how many blocks are in places where she is not allowed to look. As a result, she finds a complex formula, a quantity which has to be computed, which always stays the same in her situation.

What is the analogy of this to the conservation of energy? The most remarkable aspect that must be abstracted from this picture is that there are no blocks. Take away the first terms in [the equations ] and we find ourselves calculating more or less abstract things. The analogy has the following points. First, when we are calculating the energy, sometimes some of it leaves the system and goes away, or sometimes some comes in. In order to verify the conservation of energy, we must be careful that we have not put any in or taken any out. Second, the energy has a large number of different forms, and there is a formula for each one. These are: gravitational energy, kinetic energy, heat energy, elastic energy, electrical energy, chemical energy, radiant energy, nuclear energy, mass energy. If we total up the formulas for each of these contributions, it will not change except for energy going in and out.

It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and when we add it all together it gives 28 – always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.

Saving energy

We are all familiar with the SAVE IT campaigns which urge people to SAVE ENERGY. You might be forgiven for thinking:

Hold on! Energy is conserved! Why do we need to save it? It saves itself!

In fact, you'd be right in thinking along these lines. There is an instructive paradox here which follows from the different usage of words in science and everyday contexts. In scientific terms it is not energy but fuels that get used up and it is these that we should make every effort to conserve. Energy is conserved (and there is nothing that we can do to avoid this conservation) but we need to make every effort to conserve fuels.

If we burn a piece of coal, the energy that was in the chemical store becomes spread out amongst other stores, most of which will be thermal stores of energy, as the surroundings undergo heating. It is very difficult to do anything useful with these dispersed stores of energy. There is a difference between having plenty of something useful around and being able to do something useful with it. A few sticks can be used to make a cheerful fire, but with sawdust scattered over a square kilometre of forest floor, it is rather harder to perform the same trick (not impossible, but needing significant input before getting something back). You may have the same quantity of wood, yet probably only the sticks are thought of as viable fuel.

Energy as an orange fluid

Purists suggest that you should think of energy only in abstract mathematical terms, but for some this is not helpful. You can think of energy much as we have depicted it – an orange fluid which is contained in stores. We thing that not much harm will come from this in elementary studies, and it may help to make energy more intelligible. A feeling that you are talking about something real does help!

Teacher Tip: There are parallels that can be fruitful: orange fluid model  →  energy model can be moved around  →  shifted; can't just disappear  →  conserved; can be spread out to many locations  →  dissipated. You can model all of these in the laboratory with the orange fluid.

Different stores label calculations

Calculations of certain energy changes may not be possible at the time that you'd first want to analyse what's possible and what's not possible in terms of energy. The quantity of energy shifted to or from a store stands for a calculation that is possible, not too far in the pupil's future. Indicating the change of energy in a store between one time and another is the essence of developing this pre-cursor description, that will later become fully qualitative (here it is necessarily only semi-quantitative).

Requiring that a physical description is well-established before attempting an energy description is essential to establishing a well-founded representation of the process in energetic terms.