Energy and Thermal Physics

Energy in the New Curriculum

Perspectives for 5-11 11-14 14-16 16-19

Guidance on the way that we represent energy to students from Charles Tracy, Head of Education at the Institute of Physics.


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An opportunity for change

The first in a series of posts written by Charles Tracy about energy in the new curriculum.

Energy and Thermal Physics

An opportunity for change

Perspectives for

Where this series has come from

I have had many discussions about energy over the years and know that these discussions are usually, to say the least, uncomfortable. So, in starting this series, I am bursting with caveats and caution.

The first caveat is that the overriding aim of this series is to be helpful; I will be discussing ways in which I think energy discussions can be improved and I will highlight a number of concerns with the prevalent paradigm. But that is not to be critical of individuals.

Secondly, this is not about banning specific words. I hope it is more about taking the opportunity to consider the pictures that those words paint and how we can make those pictures as helpful as possible to children coming to energy ideas for the first time.

Thirdly – and at the very least – it is an opportunity to have a look at the new energy statements in the Revised National Curriculum and consider the implications for what is now required (and also what is no longer required) in the energy topic.

Where it is going

The changes in the revised National Curriculum can be summarised as there being a move towards:

  1. energy as a quantitative tool rather than energy being a substance
  2. explanations that explore processes rather than rely on energy
  3. start and end points in energy analyses
  4. quantifiable terms
  5. heating (as a process) rather than ‘heat’ as a substance
  6. differences as the cause of change
  7. dissipation and ideas from the second law

Each of these is to be welcomed and each one provides an opportunity for talking about energy in a more helpful way. I will work through these opportunities in this series of postings.

Some personal comments

When I started teaching in the mid 1980s, there were articles and discussions about the teaching of energy. There was concern about the use of the words ‘transform’ and ‘convert’ when talking about ‘types’ of energy. But these concerns was part of a broader unease: that the existing paradigm gave energy substance and suggested that energy made things happen by being interconverted between different types. However, discussions about reframing the way we talk about energy had little impact in classrooms – mainly because official documents continued to insist on types, transformations and energy’s causal powers; to the extent that, at one point, the QCA documentation specified that there are 9 types of energy. I’ll call this the ‘9 types’ paradigm.

It was with some relief, then, that, in 2006, I was introduced to the approach advocated in the Institute’s SPT resource. We will return to that later but it is worth saying that it is an approach I strongly support but did not develop. It is also an approach that works with the statements in the revised National Curriculum (at both Key Stage 3 and Key Stage 4) and also the GCSE criteria – which determine how Awarding Organisations have to present energy ideas in their specifications. So, after decades of having official documents that have hobbled innovation and improvements, we now have official documents that support a more helpful (in my view) approach to talking about energy.

IMO and mainly IMHO

I have thought about all these issues quite a lot over the years. I don’t have all the answers but, as you will see, I do have some views. I won’t preface every single one with ‘IMHO’, but please take that as read.

A gentle introduction to concerns

So, there are positive moves. But I do need to dwell momentarily on some negatives and discuss why some of us are troubled by the prevalent ‘9 types’ paradigm. In the following examples, please do think about what picture is being painted of energy, the role that energy is playing in the explanation and whether either of those is helpful.

Here are some examples of how the 9 types paradigm manifests itself in the media:

  • The steam [from a volcano vent] is converted into energy and transported to Europe via a 1,200-mile sea-floor cable. (The London Paper)
  • Carbonaceous matter is converted to heat or other forms of energy (a physics magazine)

And then in slightly more official documents:

  • Energy makes things happen (ASE Big Ideas)
  • The moving pencil uses kinetic energy (QCA)
  • The bulb lights because energy flows from the battery to the bulb (Sophie, Year 9, KS3 SATs sample responses)

In all of the above, I would suggest that energy is given some substance – so much so that steam and coal can be converted into energy (1). The substance can flow and it is used up to make things happen. Additionally, the use of energy-based terminology is employed to make the explanation sound a bit more scientific. But, in fact, explains away the actual processes and mechanisms that are what really ‘make things happen’ (2). There are ambiguities (3) and even spurious ‘types’ of energy that have been invented to fit into the paradigm (4). The most deceptive of these is ‘heat’ or ‘heat energy’(6).

A harsher development of some concerns

The examples above were, perhaps, from easy targets. However, they a mirrored – or even developed – by the language that is used in the study of the sciences up to 16. All of these examples are from GCSE text books or exams:

  • The useful energy input of a food processor is electrical energy and its useful energy output is kinetic
  • Some of the energy released by the Sun is converted into electrical energy
  • A microphone converts sound energy to electrical energy
  • Machines are noisy. They transfer energy and some of the energy is transformed into sound
  • Energy is the capacity to do work
  • Green plants capture light energy and use it to grow
  • The cells in animals and plants all use energy for the jobs they do.
  • Chemical energy from hydrocarbon fuel is converted into motion
  • It is a striking example of a chemical reaction producing heat and work.

Again: energy is presented as a substance (that exists and can flow); it is ‘released’ and ‘produced’; it is converted and does useful jobs.

Above all, the examples state (or imply) that it is used up. Yet, we also tell children that energy is conserved.

And finally... the second law

So, on top of all the concerns about the 9 types paradigm, we are presenting students with an incomplete (and paradoxical) picture of an energy that is both conserved and runs out. It’s not altogether surprising because people’s everyday language often implies that energy is used up. However, we should try to get it right in science lessons. And that means addressing students’ experience – generally and through the language they hear: that something useful is used up to make things happen (fuels, food etc) (6).

However, whatever it is that runs out, it is not energy; and this distinction can be made through discussing ideas relating to the second law of thermodynamics  – ideas that are accessible and, perhaps, whose time has come (7). Certainly, the revised National Curriculum provides some opportunities to talk about the second law – so maybe we should; and I will return to this towards the end of the series.

Next steps

In this introductory posting, I have highlighted some concerns, hinted at how they might be addressed through the way we talk about energy and suggested that the revised National Curriculum will not hinder that rethink and may even help it.

Attached is a summary document that lists the concerns (labelled 1 to 7), summarises how they might be addressed and links them to opportunities in the National Curriculum statements.

I will explore the opportunities in more detail over the course of this series of posts.

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Starting out

Here we introduce two useful rules of thumb and the 'three-step approach' for talking about events or phenomena that might be susceptible to an energy analysis.

Energy and Thermal Physics

Starting out

Perspectives for

Picking up some threads

The two main threads I will pick up from the previous post are that:

  1. Energy analysis is about calculations;
  2. Energy discussions are prone to explaining away the interesting, beautiful and revealing process and mechanisms that make things happen.

Why bother with energy?

In the previous post in this series, I highlighted some of the concerns about the way we talk about energy. So, if discussions about energy are so fraught, why do we bother with it? The answer is that it enables us to do calculations. These calculations often allow us to determine whether something is possible:

  • Can the LHC create a Higgs boson?
  • Are Catalytic Crackers hot enough to break C-C bonds?

And also provide quantitative answers to questions:

  • How many Mars bars do I need to climb up Mount Everest?
  • How much fuel is needed to lift a satellite into orbit?

These are questions that an energy analysis can usefully address. Whereas, an energy discussion is little help in answering a question like “How does a microphone work?”

First rule of thumb

I suggest that this is a useful rule of thumb:

the purpose of an energy analysis is to prepare for or to do a calculation.

It is not a hard and fast law; nor a proscription. Rather, it is simply something that is worth keeping in mind when talking about energy with students; and I will return to it in a later posting. It is also addressed in the first episode of the SPT resource on energy which uses the gentle approach of right lines and wrong tracks to address concerns.

At Key Stage 4, students will start to carry out calculations. At Key Stage 3, it is useful to:

  • Introduce the numerical nature of energy analyses through looking at fuel values;
  • Introduce a way of talking about energy that can lead to calculations.

In the latter, it is important to choose examples that could lead to calculations (even if they do not perform those calculations at KS3). And to develop a way of talking about energy that prepares for those calculations (I will return to this in week 4). In particular, we need to be careful about bringing in the language of energy where it can be a distraction: in explanations.

The microphone is a case in point. It is unlikely that a school student would perform an energy calculation involving a microphone. The only possible result of invoking energy (for a microphone) is to explain away its operation.

For example

Here is a typical response to the question about how a microphone works:

a microphone converts sound energy to electrical energy. 

It appears to provide an answer but it does not answer the question. All discussion is closed down because there are no physical referents for the terms ‘sound energy’ or ‘electrical energy’. They cannot be explored any further, they do not help understanding and they do not relate to any more fundamental physical phenomena. They are simply labels.

The following response relates more directly to what is happening:

a microphone converts a sound wave into an electrical signal.

This response is richer: each term in the sentence relates to something physical that can be explored, described and investigated more deeply: sound, sound waves and electrical signals. Similarly, it is open to questions about the mechanisms within the microphone: how does it convert a sound wave to an electrical signal? This question can be addressed through discussion about the diaphragm being attached to a coil in a magnetic field; about pressure differences making the diaphragm move; and about an EMF being induced in the moving coil (because it is in a magnetic field); and so on.

Authentic physics

There is a rich story hiding in the microphone. And it is a story that develops and employs some of the unifying ideas and methods of physics. For example:

  • reductionism and explaining phenomena at a more fundamental level;
  • economy – aiming to use as small a number of fundamental laws as possible;
  • a desire to understand phenomena deeply – not being satisfied with the superficial;
  • the use of models;
  • the need to synthesize ideas from across many topics.

Discussing the microphone with students in this way will not only give them more insight into how the microphone works, it will give them an insight into how physics works. That is, they will get an authentic experience of what it is like to think like a physicist.

By contrast, the labelling exercise that is “sound energy to electrical energy” gives them no insight into either the microphone’s operation nor what physics is really like.

A hint of a biological example

The classic example from biology is photosynthesis. It is terrifyingly easy to explain away this amazing process with a description like:

photosynthesis converts light energy to chemical energy.

Once again, there is little in this sentence that is open to investigation. It provides a magic box description with input and output labels and little chance of investigation. I will return to photosynthesis in a later post.

Second rule of thumb – Separating out processes

The examples above show how it is preferable to avoid invoking energy in explanations of phenomena. Satisfying explanations are more likely to come from discussions of physical processes and mechanisms. These are open to deeper investigation – often through a reductionist approach – and will lead eventually to a discussion based on a fundamental piece of physics (or science). So my second rule of thumb is:

use physical processes and mechanisms (not energy) to investigate and explain phenomena.

A three step approach

I’m going to propose an approach that can be used at Key Stage 3 to discuss events and phenomena in a way that separates explanations from energy analyses. The example has been chosen because it could lead to a calculation. So an energy analysis will be genuinely useful preparation for Key Stage 4. The discussion is split into three distinct steps:

  1. An observation. In everyday language.
  2. An explanation. Relying on mechanisms and physical, chemical and biological processes. 
  3. An energy analysis. Using a start and end point (I will develop this further next week).

Example of three step approach

Imagine we want to discuss when a falling apple is travelling fastest. A three-step discussion would be (in brief):

Observation: The apple falls towards the ground (because of gravity).

Explanation: When the apple is let go, there is an unbalanced force acting on it. The force is downwards, so the apple accelerates towards the ground. It keeps accelerating and will be at its fastest just before it hits the ground. On its way down, the apple bashes into air particles, making them move faster, increasing its own temperature and the temperature of the air around it (a little).

Energy analysis: I will develop this section more fully in the next post. For now, let’s say, the system loses energy stored gravitationally and the apple gains kinetic energy; also, the energy stored thermally by the air and the apple have risen a bit.

Note that there is no mention of energy in the explanations. And there is no attempt to explain anything in the energy analysis. Explanation and energy analysis are kept deliberately distinct. The sole purpose of the energy analysis is to set up a calculation.

We will return to the third aspect of this three step approach (energy analysis) in the next post.

National Curriculum statements

The approaches I have suggested here support and are supported by statements in the revised National Curriculum at Key Stage 3:

  • comparing energy values of different foods (from labels) (kJ)
  • domestic fuel bills, fuel use and costs
  • using physical processes and mechanisms, rather than energy, to explain the intermediate steps that bring about such changes.

And, very pleasingly, on page 14 of the draft for Key Stage 4,  there is a preamble that refers to some of the unifying ideas and methods of physics.

Up next

Starting and ending

This post picks up on the third of the three step approach given in the 'Starting Out' post: start and end points in an energy analysis.

Energy and Thermal Physics

Starting and ending

Perspectives for

In this post, I will pick up on the third point in the list from An Opportunity for Change, the use of start and end points in an energy analysis.

In many ways, this is the strongest recommendation that I will make. Of all the changes in the way that we talk about energy, this is possibly the easiest to implement and the one that can have the biggest effect on cleaning up the language. Many other improvements flow from it. Even if you do not adopt all of the ideas of SPT, you will find it helpful to refer to and define start and end points in your energy discussions.

Eliminating ambiguity (of chains)

A typical exam-style question might be to describe the energy changes as a man (Sisyphus) pushes a boulder up a hill.

And a typical answer would be something like:

chemical energy ⇒ kinetic energy ⇒ gravitational potential energy + heat energy

An alternative might be:

chemical energy ⇒ kinetic energy ⇒ gravitational potential energy + heat energy + sound energy

There are various other alternatives. 

However, what is clear is that there is no single, unambiguous answer on which everyone will converge. I’d suggest that this lack of a convergent answer is un-science. It arises because an energy discussion cannot provide any insight into or explanation of the journey. And we should not be drawn into mixing energy terms into the discussion of the journey. This relates to the suggestion in the previous post to separate the explanation from the energy analysis. But it goes further. The energy analysis is only useful for comparing two (or maybe more) fixed points on the journey – not for discussing the continuous process of climbing the hill.

The most obvious fixed points to choose are the start and end of the journey.

Eliminating ambiguity with start and end points

As we saw in the last post, the purpose of an energy analysis is to do calculations. So energy is a useful tool to invoke if we wanted to know how many chocolate bars he has to eat to ascend the hill; for which there are two important points in time: before he begins his climb and after he has finished it. So we have a:

start point: at the bottom of the hill

and an

end point: at the top of the hill.

We choose these two points because they will be useful in an energy analysis. And, once we have chosen them, the analysis is unambiguous:

  • the energy associated with the chemicals in the system (food and oxygen) has decreased;
  • the energy associated with Sisyphus being in the Earth’s gravitational field has risen;
  • the thermal stores associated with the temperatures rises of Sisyphus and with the surroundings have also risen.

We might show these changes on a bar chart.

The language above is a bit cumbersome for Key Stage 3. Phrases like “the energy associated with the gravitational field...” do not lend themselves to basic discussions with 12-year-olds. One way around this is to use the idea of energy stores. This is an idea that is developed in the Supporting Physics Teaching (SPT)resources.

The terminology of stores

SPT uses the terminology of energy stores. In the example above, we might say that there is a chemical store that empties during the journey. And there are thermal stores and a gravitational store that fill up. Importantly, the amount the stores fill up is equal to the amount that the chemical store is depleted.

This is explicitly and knowingly representational. There is no suggestion that there really is an energy store with orange stuff in it. But that that this is a useful way of illustrating that there is energy associated with the chemical at the bottom of the hill. We have found that the language of stores is a helpful way of introducing energy at Key Stage 3. It is up to you whether you adopt this language completely. You may prefer to take on the idea of start and end points without adopting the language of stores – and that is perfectly possible. Although the idea of stores can work and it provides a break with the past, you have to feel comfortable with whatever paradigm you adopt.

Other advantages of start and end point analysis

We have seen that defining a start and end point in an analysis gets rid of ambiguity. However, you will notice that it has also eliminated the reference to kinetic energy that appeared in the unscientific approach at the top. Let’s think why kinetic energy has dropped out of the analysis and also why that is a good thing. Although Sisyphus is moving, his kinetic energy is (a) constant and (b) transient. Its value would not affect any of the final values; so it would not form a part of any calculation. At the start, he is stationary and at the end he is stationary. At each of our snapshots (start and end) he has no kinetic energy. So we can ignore it in the analysis. This is helpful because it was, after all, a distraction in the energy chains at the top of the page.

An even more transient kinetic store

What is the energy analysis for a car journey – let’s say from London to Birmingham?  I am going to use the three step approach from the previous post. Our start point is London. And our end point is Birmingham.

During the journey, the engine gets hot and heats the surroundings. Also, the car does work to push air out, thereby raising the temperature of the air (and the outside of the car) – by a very small amount. Every so often, we have to brake – to slow down. The friction in the brakes raises the temperature of the brake blocks and discs and, in turn, these get hot and heat up the surroundings.

When I get to Birmingham, I have used up some petrol (and oxygen form the atmosphere) and have raised the temperature of the car, its engine and the surrounding air – albeit by a small amount. I have also increased the amount of carbon dioxide in the atmosphere.

  1. Observation: We get in a car, fill it with petrol and drive from London to Birmingham. The car is a very convenient way to get from one place to another at a time that suits us and reasonably quickly.
  2. Explanations: The internal combustion engine burns the petrol with oxygen (from the air) in its cylinders. The reaction produces water and carbon dioxide at a high temperature – so high, that they are both gases. Therefore, there is an immense pressure in the cylinder which is used to turn the crankshaft and the back wheels of the car. The turning wheel push the car forwards. The cost of making the car move is that we use up petrol and produce carbon dioxide.
  3. Energy analysis: 
    • start point: departure from London
    • end point: arrival in Birmingham
    • a chemical store has emptied
    • thermal stores have been filled (one for the car and one for the surroundings)

As a histogram:

In terms of energy stores:

Notice again that there is no reference to kinetic energy because it was constant and transient.

Choosing another end point

Remember that we choose the end point that is most convenient for a useful energy analysis. The analysis above might be useful if we needed to know how hot the engine might get. However, we could choose an end point sometime after the car has stopped. By which time the engine will have cooled and heated the surroundings some more.

As with Sisyphus, the energy analysis tells us nothing (and cannot tell us anything) about the journey itself. In terms of energy, a car journey is distressingly simple: it empties a chemical store and fills up a thermal store of the surroundings. All of the explanations of processes and mechanisms (how the car works, how it raises the temperature of the surroundings etc) appear in the middle part of the three part analysis.

Further links

More about energy stores: shifting-and-conserving-energy

Up next

What stores do we need?

This resource considers what stores are helpful in energy discussions and for before-and-after analysis.

Energy and Thermal Physics

What stores do we need?

Perspectives for

What stores do we need

This is a question that often comes up in workshops that we run. So, this post will consider what stores are going to be helpful in our energy discussions and for the before-and-after analysis. But, before doing so, it’s worth remembering a few points about what we are trying to do here:

  • We are aiming to develop a helpful way of talking about energy with students. We are not developing a model; and we are certainly not defining a set of laws. We are trying to find a language that is helpful to their understanding and is a good preparation for what comes next.
  • We are mainly thinking about Key Stage 3 (up to age 14). Once students can carry out calculations (in GCSEs, A-levels or their equivalents), the language can become more sophisticated.
  • We are not performing a linguistic trick and simply using the old ‘types’ of energy and renaming them stores.

Taking those points into consideration, any store should:

  1. be quantifiable: we need to know that we could calculate changes in its value (even if we don’t do so at Key Stage 3).
  2. have a value at a snapshot in time (corresponding to a start or end point); i.e. we should be able to calculate its value from properties of the object or the system at an instant.
  3. have a physical referent: i.e. relate meaningfully to some physical features of the system.

Doing work

I will return in more detail to the idea of working in a subsequent post. However, I will use it here because it is a useful way to get into discussions about energy (because it is easily quantified) and it will help determine which stores will fulfil the criteria above. We can do work on (or within) a system to change its energy profile. For example, by lifting something up.

Gravitational store

If I lift a ball onto a shelf, I do work (in a technical sense). I can leave the ball on the shelf for a while and, at a later time, allow it to fall and do the work back for me. While the ball is on the shelf, the system (ball plus the Earth) is storing energy thanks to the gravitational force between its two constituents.

So, it looks like it is going to be useful to refer to a gravitational store that is filled a bit when we raise an object and empties a bit when it falls.

Let’s see if it matches the criteria above.

  1. We can calculate the amount that a gravitational store fills and empties (mgΔh); so we can calculate the amount of energy that is stored (albeit in a relative sense);
  2. The value depends only on the position of the ball and its mass; so we can calculate the value at any instant that we choose;
  3. The store clearly refers to the physics of the situation – the position of the ball in a gravitational field.

Elastic store

We can follow a similar reasoning for an elastic store. Let’s say the ball is made of rubber. We can do work to stretch (or compress a rubber ball). It is elastic and will return to its original shape. So, we will get back at least some of the work that we do on it.

Therefore, it looks like it will be helpful to be able to refer to an elastic store that fills and empties as the ball is deformed and returns to its original shape. Referring to our criteria, we can calculate the energy associated with the deformation at any instant and the store refers to the work involved in the deformation. So let’s add it to our list.

Kinetic store

The idea of a kinetic store is less obvious. Some people are not comfortable with thinking of a moving object storing energy. However, we need to remember that the ‘store’ terminology is simply a way of talking about the energy associated with a system (and its behaviour) to younger students.

Also, it is the case that we can do work to make something move (for example by propelling a ball upwards). And the moving object will do work back for us (in this case, to raise the ball in the Earth’s gravitational field). Therefore, there is certainly energy associated with moving objects.

Does it fulfil our criteria? Yes: it will be helpful (because so many systems involve constituents that start and stop moving); we can quantify it and we can do so at any instant (as long as we know the speed and mass of an object).

When is it useful (and not) to refer to the kinetic store

It is often the case that the motion of an object is transient so that we do not need to consider the kinetic store in our analysis. There are two examples in the previous blog: pushing a boulder up a hill and the car journey. In each case, there is no need to consider the kinetic store. However, this can cause some concern so let’s look at it in a couple of ways.

The main reason that we can ignore the movement in the energy discussion is that we have used the start and end analysis. And there is no movement at the start and no movement at the end. So any kinetic term is zero at each of the analysis points – thereby obviating the need to consider a kinetic store (or kinetic energy).

The speed is constant during the journey so it is not playing any part in the changes that occur (after all, we do not consider all the other stores that are constant).

Having said all that, there may be situations in which we would use the kinetic store in our analysis. For example, to determine the braking distance of the car – right at the end of its journey. This example shows why it is important to a) define our start and end points and b) choose them depending on the calculation that we might want to perform. The important feature is that we (and our students) do not get tangled up in chains of energy stores.

Thermal store

We can do work to raise the temperature of a system (or to change its state). In each case, the energy associated with the system has increased due to the internal arrangement or movement of its particles. We do not really have a common school-level term for how the system is storing energy. I have previously suggested “internal” store (because it relates to the term ‘internal energy’ that is used at a higher level; and it does not imply ‘heat’). However, "thermal store" is a better term and can be used to cover both temperature rises and changes of state. It is used in SPT.

The most important point is that, whatever is stored, it is not ‘heat’. I will return to this in a future post. For now, suffice to say that heat is to do with a process rather than the way that energy is stored or associated with a system.

Four more stores

To round off fairly quickly, the other four stores that are useful, calculable and identifiable (at an instant) are chemicalnuclearvibrational, and electric/magnetic. So here is a complete set of stores that should be sufficient for most of our discussions (certainly at Key Stage 3).

Note that there are three (four if you include ‘heat energy’) of the old ‘types’ of energy that are not featured above. They are: ‘electrical energy’, ‘sound energy’, and ‘light energy’. I will say more about these spurious labels next week.

Closing remarks and observation

It is worth noting that:

  • as we might expect, we have stores associated with the two fundamental forces that act on a macroscopic scale: gravitational, electric/magnetic. In each case, it is possible to set up a system in which the force acts between objects, to do work against that force and for the system to store energy in a way that is associated with that force.
  • we have stores associated with the two main ways of binding at a particle level: nuclear and chemical. This makes sense because both chemical and nuclear reactions bring about differences in the ways that energy is associated with a system.
  • some of the stores can be though of in terms of others. For example, a chemical store could be thought of as an electric/magnetic store (chemical bonds are electrostatic). A vibrational store could be through of as an interchange between an elastic and a kinetic store. And a thermal store could be thought of as a combination of kinetic and other stores. However, in each case, they can be quantified in their own right; and therefore they are useful in calculations and in our analysis.

These ideas are developed further in Episode 2 of SPT.

Up next


Here we discuss the idea of pathways, which link physical processes and energy analyses.

Energy and Thermal Physics


Perspectives for


In the last two blogs, I have introduced the idea of ‘stores’ and, in the previous post, suggested that referring to eight stores will be helpful in discussions at Key Stage 3.

This post introduces the idea of pathways – the link between the physical processes and an energy analysis.

(Not too hard) work

Let’s try the three step approach on a clockwork motor lifting some masses.

We will choose a start point after the spring has been wound and an end point when the masses come to rest at the top.

Start point: there is an elastic store associated with the wound spring;

End point: there is a gravitational store associated with the masses. This store has filled as the elastic store has emptied. Some thermal stores have also filled a little.

  1. Observation: the spring unwinds and lifts the masses. The spring may have got a little warmer – though it would be hard to notice that in a small system.
  2. Discussion/Explanation (in brief – as ever, you can drill into the physical processes and mechanisms in as much detail as you choose): the coiled spring exerts an upwards force on the masses that is bigger than their weight. Therefore there is a net upwards force and they accelerate. They quickly reach a steady speed and the spring does work to raise them. It also does work against friction and air resistance and raises the temperature of the mechanism and surrounding air (ever so slightly).
  3. Energy analysis:

    We will choose a start point after the spring has been wound and an end point when the masses come to rest at the top.

    Start point: there is an elastic store associated with the wound spring;

    End point: there is a gravitational store associated with the masses. This store has filled as the elastic store has emptied. Some thermal stores have also filled a little.

The changes have come about through the spring doing work or working. To be more precise, we might call it ‘mechanical working’.

Points to note

  • It is worth trying to use the term ‘working’ rather than ‘doing work’ (at least some of the time) because it is clearly active and it will align with ‘heating’ later on.
  • There is (as ever) no role for a kinetic store in this analysis. The masses were moving (and that is discussed in step 2). But the movement is transient.
  • The initial temperature rises were brought about by working (as opposed to heating).
  • It is the springs that are working (not ‘elastic energy’). The job of working has to be done by a physical entity that exerts a force. It is not done by an energy store (or a type of energy). I.e. it is unhelpful to say something like: “the ‘elastic energy’ does work to raise the masses”.
  • Working is not a store. Or a ‘type’ of energy. We are going to call it a pathway.
  • The pathway has meaning in a physical discussion (step 2 above) or in an energy analysis (step 3).

Working a bit harder (electrically)

Now let’s get an electric circuit to do the same job.

  1. Observation: Attaching a cell to a motor drives the motor round and it lifts the weights. The motor gets hot (especially if it is run for a long time). And, eventually, the battery will run down.
  2. Discussion/Explanation: I will keep this discussion simple for the time being – it could get as involved as you like (there is nothing here that closes it down).

    The cell contains chemicals which produce an EMF across its terminals. The EMF can push charges around the circuit. These charges are already in the wires and form a continuous loop. As soon as the circuit is switched closed, the charges start to move and they do work in the motor to turn its shaft. Therefore the motor lifts the masses. The flow of charge is also part of a chemical reaction between the substances in the cell. This chemical reaction changes the useful chemicals into less useful products. Eventually, the reacting chemicals run out and the cell stops producing an EMF.

  3. Energy analysis:

    We will choose a start point before switching the circuit on and an end point after the masses have been raised.

    Start point: there is a chemical store associated with the cell.

    End point: the chemical store has emptied a bit and a gravitational store (associated with the masses) has been filled a bit; also, thermal stores associated with the wires, motor and air have been filled a little.

What is working now?

In this case, the working was done electrically by the cell and the circuit. So, it seems reasonable to say that the circuit was working electrically, i.e. this is a special case of working.

Points to note 

  • The circuit is working electrically rather than ‘carrying electrical energy’.
  • Electrical working is a pathway in the same way that mechanical working is a pathway.
  • The electric current is transient (in the same way that the movement of the masses is transient).
  • We do not need an electrical store in the analysis.

Let’s explore some of those points in more detail because I know some people feel uncomfortable that there is no electrical store – or equivalent of ‘electrical energy’ – in this analysis.

What happened to ‘electrical energy’?

I suggest that the usual analysis using the 9 types paradigm would have been something like:

chemical energy ⇒ electrical energy ⇒ gravitational energy

We are so used to this that many people are tempted to invent an ‘electrical store’ to use in the new analysis. However, although it is a difficult idea to drop, there are very good reasons why we don’t need such a term – and why the term ‘electrical energy’ is misleading in this context. They are:

  • There is no physical referent for ‘electrical energy’ in this circuit – i.e. there is no sense in which the moving charges store or ‘carry’ energy in the circuit.
  • There is certainly no way of calculating, at an instant, the ‘electrical energy’ associated with an electric current. The amount of work done depends on time – how long the circuit is switched on.
  • It is a spurious use of the term ‘electrical energy’. That term – like our electric/magnetic store – relates to the separation of one or more charges (or their position in an electric field). In circuits, we only need to invoke our electric store when we are considering capacitance (which is unlikely to happen at Key Stage 3).
  • The circuit is switched off at the start and end points. Therefore we don’t need to take the charges or their distribution into account. 

So, to summarise and emphasise, the charges are not storing or carrying electrical energy. Instead, they are enabling the circuit to do work (electrically) thanks to the fact that they form a continuous loop.

Being consistent

This analysis is consistent with our model of a continuous loop of moving charges in an operating electric circuit. It is also consistent with SPT’s rope-loop illustration of this model.

Heating as a pathway

To complete our toolkit, we need to introduce the idea of heating as a pathway. I’ll do this very briefly here and return to the idea in a subsequent post.

Heating is a well-defined idea in thermodynamics: it is the transfer of energy from an object at a higher temperature to one at a lower temperature. It is clearly helpful if we can use the term in a similar way at school level.

There are two mechanisms for enabling a hot object to heat a cooler one:

  • Heating by particles (when the bodies are in contact; AKA conduction)
  • Heating by radiation (when they are not in contact).

A cup of tea will heat its cooler surroundings by both conduction and radiation.

So, jumping straight to the energy analysis (for brevity)...

Start: there is thermal store associated with the tea

End: the tea’s thermal store has emptied and the thermal store of the surrounding has filled a bit. The pathways are heating by particles and heating by radiation.

Points to note

  • The word ‘heating’ makes it clear that this is a process (it is not a store). Nor does it sound like it might be a store.
  • The ‘ing’ formation makes it tally with working and electrical working.

Summary of pathways

We have discussed four pathways:

  • Working
  • Electrical working
  • Heating by particles
  • Heating by radiation


These will cover our needs and are discussed further in the SPT resources. Here are some points to note:

  • Pathways lead to energy stores being emptied and filled – i.e. they are the processes that bring about a change in the energy profile of a system.
  • They are not stores themselves because we cannot calculate the energy associated with them at an instant.
  • They bring about change by occurring over a period of time.
  • They fall into two pairs: working and heating.
  • They are consistent with the first law of thermodynamics – that there are two ways of changing the internal energy of a system: working and heating:  dU = dQ + dW

In summary – a complete toolkit (almost)

We now have a complete toolkit for discussing energy at Key Stage 3 – although I have skirted over some aspects here. It comprises:

  • The three step approach to discussion and analysis. Or, if you prefer, another way of distinguishing between the physical processes and the energy analysis.
  •  A set of stores associated with the ways that systems can store energy. It looks like we need just 8 of them.
  • A set of 4 pathways that describe how the physical processes lead to some stores being emptied and others being filled.

Note that this toolkit is not a model or a set of laws of physics – although it is consistent with some important physical laws and models – including some that we use at school.

Up next

Resisting the old language

This resource discusses the difference between the old language of 'types' of energy and the newly introduced concept of 'stores'. 

Energy and Thermal Physics

Resisting the old language

Perspectives for

Resisting the old language

In a previous post, I proposed a set of stores that are going to be useful for energy discussions at Key Stage 3.

When we arrive at this point in workshops with teachers, we find that there are two common reactions:

  1. “Isn’t this the old ‘types’ but there are eight instead of nine?”
  2. “What happened to sound energy, light energy, electrical energy and heat energy? We would like to have stores that correspond to those types.”

In this post I will try to address those comments and hint at why the four old ‘types’ above are not helpful and are, in most cases, spurious.

Is this simply new for old (stores for types)?

This point came up in a discussion in What Stores do we Need? Have we simply changed the language: replacing ‘types’ with ‘stores’ and having 8 instead of 9? Naturally, I don't think so.

Firstly the idea of a store is very different from the way in which use types.

Stores vs. Types: a rationale for stores

The stores are knowingly and deliberately illustrative. They are not intended to represent a physical entity – i.e. some kind of actual store. Instead, they represent a value. A store is rather like a bar on a histogram. In Starting and Ending, I used a histogram and a set of stores to analyse an energy change. The height of each bar illustrates the value of a quantity that we can measure or calculate. Similarly, the level within a store illustrates the value (and change in value) of a calculable quantity.

Although that quantity refers to a physical attribute (speed, temperature, distortion etc.), there is no suggestion that the store itself is (or is representing) a physical entity.

In summary, a store is a way of illustrating a value of a physically meaningful attribute of a system. And that value can be calculated.

So each store represents a way in which energy can be associated with a system (movement, temperature, distortion, position in a field and so on).

Therefore, I would suggest that there is a rationale for choosing each of the stores. We chose each store because

  • it refers to an attribute (behavior or state) of the system that contributes to the system’s total energy. For example, position in a field, motion, distortion and so on;
  • we are able to calculate the change to the total energy of the system as a result of that attribute changing. For example, lifting a mass in a gravitational field, changing the speed of a tennis ball, stretching an elastic band etc.

Taking one example

Let’s look at the example of the mass from Pathways.

When we lift the mass, the energy of the Earth-mass system increases; this is because there is an attractive force between the Earth and the mass and they have been pulled apart.

The gravitational store is clearly not a physical entity. Neither is it anything fundamental in physics (although it is linked to one of the fundamental forces). It is simply a way of illustrating the change in the way that the system is storing energy.

Note that, in this example, the general bullets above are exemplified as follows:

  • there is clearly a physical referent – the mass in the Earth’s gravitational field;
  • we can calculate the energy associated with the change at an instant (mgΔh); and we can do so by simply making some measurements at that instant.

As we might expect, the bullets above are true for all of our ‘stores.’

For each of the eight stores, there is a formula for calculating the change in its value. We can do so at an instant (i.e. at any end point) by inspecting or measuring values in the system; i.e. the (change in) value depends on the state of the system not on how long something has been running.

Although students will not perform any calculations at Key Stage 3, the ability to quantify changes in each store is reassuring – and is, of course, deliberate. It means that any discussions that use, with reasoning, this group of stores must be rooted in thinking about energy as a quantitative tool. And therefore must be preparation for doing calculations which is, as we saw in Starting Out, is the main purpose of studying energy.

The same cannot be said for the terminology and labels used in the 9 types paradigm.

The ‘missing’ types

People are (understandably) keen to hang onto some of the old ‘types’ of energy and give them an equivalent store. Most notably: sound, light, electrical and heat. This desire is kind of natural – it is hard to let go of familiar (even habitual) language! However, these terms never earned their place. They came about because the ‘9-types’ labelling system was being used to describe and explain (or, more likely, explain away) phenomena. Therefore, the labelling system required some energy-based terms to refer to perfectly good physics ideas: sound, light, electric current and internal energy. New terms were invented and shoe-horned into the labelling system – usually by tacking the word ‘energy’ onto those perfectly good physical ideas. So we got ‘sound energy’, light energy’, electrical energy’ and ‘heat energy’.

So, instead of a rich, meaningful and reducible phrase like:

a tuning fork makes a sound

we have meaningless phrases like

a tuning fork releases sound energy

Similarly, instead of “plants absorbing light”, we hear people talk about “plants absorbing light energy”.

The energy-based descriptions above do not prepare people for calculations. They sound scientific but are not: they do not provide any insight into the phenomena, they are not reducible and they do not prepare students for meaningful calculations.

Other problems with these invented terms are, variously:

  • There is no physical referent
  • It is usually a corruption of a useful idea (by adding ‘energy’ on the end)
  • There is no formula for calculating the value at an instant (...but anyway why would you want to?)
  • They are often transient so have no use in a start-and-end analysis
  • They generally require us to include a consideration of time (i.e. they represent power values rather than energy values)

Over the final weeks, I will develop those concerns by looking at the four ‘missing types’ and considering why they were always spurious. To summarise, we should abandon them from all discussions about energy and we certainly do not need them in our revised paradigm. Therefore, there are no equivalent stores in the group above.

What if I don’t like the language of stores?

There are a number of big changes in all of the proposals in SPT and in these blogs. They might be summarized as:

  1. Separate explanations (using physical processes and mechanisms) from the energy analysis.
  2. Use start and end point in the energy analysis; identify the relevant energy values that change between those two points.
  3. Employ the idea of pathways to analyse and quantify the ways in which the energy profile of a system changes between the start and end point.
  4. Use only those labels that have a physical referent and lead to calculations (i.e. get rid of the four spurious types)
  5. Use the idea of stores to represent the energy profile of a system rather than energy types. The latter tend to reinforce the idea that energy is a substance that takes on different manifestations

Although it is easy to focus on the changes brought about by the language of stores, the first four points are the more fundamental set of proposals and they are not dependent on that language. They constitute a paradigm that will, many people think, improve the way that we talk about and understand the idea of energy.

The adoption of the language of stores is different. It is a teaching tool. It is (I think) a useful teaching tool, but it is not integral to the new paradigm. One of the reasons it is helpful is because it is highly visual and illustrative. But another, important, reason is that it is deliberately new language and therefore represents a break with what went before. It is, possibly, easier to change the way we think about energy if we start to use a different language for discussing it.

But, having said that, if you don’t like the language of stores (and that is allowed – of course), and would prefer to stick to referring to ‘forms of energy’ or ‘types’ (if you must), then that is fine.

The important changes are the first four points above (process v analysis, start and end points, pathways and no spurious types). And you can take them on (which I strongly urge) without necessarily adopting the language of stores (which I urge but less strongly). 

As ever, the discussion here is based on ideas developed in the SPT materials.

Up next

Intermediate stages in a process (energy chains and ladders)

 In this post, we discuss the temptation to include 'intermediate' steps in an energy process, why it is better to avoid them, and how to do so.

Energy and Thermal Physics

Intermediate stages in a process (energy chains and ladders)

Perspectives for

This is the first of three posts about situations in which people are tempted to include intermediate stages as part of a chain in an energy analysis. These are sometimes called ‘ladders’ or ‘chains’. In these posts, I will explain why it is better to avoid them and how to do so.

This first post is an overview and I have expanded some of the examples in the following posts on HEP and circuits (like appendices).

Reasons why chains and ladders are unhelpful

When discussing how we represent energy at school, a question often comes up about the role of intermediate processes and whether they need to be included in an energy analysis. It is certainly the case that, historically, we have included them. And therefore, we are used to seeing them. But that does not mean that they are either required (for an energy discussion) or helpful.

In fact, I suggest the reverse is the case. In an energy analysis (for the whole process), intermediate, transient stores serve no useful purpose. In fact, they only complicate and confuse any discussions or activities – especially with younger students. And therefore, it is better to omit them.

Typical situations in which this question comes up are:

  1. Lifting a mass with a clockwork motor
  2. A hydro-electric power station
  3. Lighting a light bulb
  4. Car making a journey
  5. Heating a room
  6. Pedalling a bike
  7. A loudspeaker playing music

In teaching schemes and assessments of the recent past (using types and transformations in the way that it had come to be used), an analysis of each of the above would probably have included an intermediate ‘type’. For example:

  1. Kinetic energy of the rising mass
  2. Kinetic energy of the water, the turbines etc.
  3. ‘Electrical energy’ in the circuit and ‘heat’ stored in the lamp’s filament
  4. Kinetic energy of the car, its wheels etc
  5. ‘Heat’ stored in a radiator
  6. Kinetic energy of the chain, the wheels etc
  7. ‘Sound energy’

(I have used inverted commas to signify terms that are best avoided).

Including these intermediates as part of a chain is compelling: partly because we are used to them; and partly because they appear to be important (or essential) to the operation and/or fundamental purpose of the device. However, these are not good reasons for doing so.

The role of energy

In all of the examples above, it is certainly the case that, in order to operate, a device has to get up to speed, reach a certain temperature or produce a sound. There are rich and interesting physics explanations about mechanisms of these on-going processes. They will include: moving masses, balanced forces, electric currents, temperature rise in resistors, hot wires radiating across the EM spectrum, vibrating diaphragms producing sound waves and so on.

However, trying to replicate the processes with a labelling exercise relating to energy does not shed any extra light on the phenomena or their processes. Instead, it introduces arbitrariness, confusion and ambiguity. Whilst also masking the underlying physics. Furthermore, it does not achieve any useful learning outcomes in terms of understanding or using energy; i.e. it does not prepare students for calculations or for having meaningful discussions about world energy resources.

In other words, including the intermediate stages is a distraction from the underlying physics without laying any helpful foundations for energy analyses. Therefore, such chains should be avoided.

Let’s see how this applies to the example of the rising mass.

A. The rising mass.

I’ll quickly define the situation. We use a clockwork motor (pre-wound) to lift a 100 g mass through 80 cm from the floor to the bench. The mass is attached to the motor by a light steel wire. Let’s say it takes 16 seconds.

Now, it clearly has to move in order for it to be lifted from the floor to the bench (as it happens, in this example, it moves at an average 5 cm/s).

In an old assessment item, given that it is moving, we might have expected a student to say that the energy story was something like:

elastic energy ⇒ kinetic energy (plus ‘sound energy’) ⇒ gravitational potential energy (plus ‘heat’)

Why is this chain of energy unhelpful?

This emergent chain of energy stages is a tell-tale sign that we are forcing energy into a role for which it is not fit. It is an attempt to replicate or mirror those processes and mechanisms with chains of energy stores (or types). And that is unhelpful. Here are some reasons why.

Including intermediate stores in a chain is:

  • Obscuring: it operationalises the energy analysis and hides all the interesting underlying processes and mechanisms – i.e. the physics.
  • Misleading: The chain implies that, somehow, there is a kinetic store that is emptied in order to fill a gravitational store. And that this is happening throughout the upward journey – i.e. that the gravitational store can only fill if energy has passed through a kinetic store. i.e. that energy has somehow been carried by the moving masses. This picture does not reflect the physical reality. A more helpful representation is to refer to the clockwork motor exerting a force on the mass and, because that force is moving, the system is working (mechanically) to lift the mass against gravity.
  • Arbitrary: if we were to include kinetic energy, it would be an arbitrary choice. There are other ways in which energy is temporarily stored but we have chosen to ignore them. For example, the supporting wire will be in tension. An elastic store fills (a little) at the start, stays constant and empties at the end. Omitting the elastic store and including the kinetic store would be entirely arbitrary. If we (quite rightly) ignore the minimal amount of energy stored elastically, then the consistent approach is also to omit the energy stored kinetically as it ascends.
  • Unenlightening: it does not reveal anything interesting about the way in which the device operates. Nor does it achieve anything positive relating to learning about energy. It does not provide any tools to help students think about, explain or analyse the situation. Specifically, it is...
  • ...not relevant for any calculation: If we wanted to calculate the speed of the masses or the time it takes to rise, we would not use an energy analysis to do so (at no point is the amount kinetic energy gained equal to the amount that the energy stored elastically has decreased). Similarly, there is no (useful) calculation in which we would think of the kinetic store emptying and a gravitational store filling. One way of thinking about this is that, the amount of energy stored due to its movement will not affect the final outcome (please note, I am aware that its speed will affect the final outcome – because of increased friction – but that is different).
  • Minimal: not a reason on its own to exclude kinetic energy; but worth noting. It accounts for about 0.01% of the increase of energy stored gravitationally.
  • Ambiguous and confusing: once it is included, there are many other stores of similar magnitude that could be included. And many ways of including them. There is no unambiguous solution to the energy analysis. This is more obvious in an example of, say, a hydro-electric power station. For a student who is given a basic set of tools to think with (whether it is stores or types), it wouldn’t be possible to reason their way to an unambiguous solution for the chain of stores that arises at a HEP station (or any new situation). And, given that such is the case, it’s not much like physics – it is simply a labelling exercise with ad-hoc, imposed rules.
  • It has led to spurious forms of energy: the terms ‘sound energy’, ‘light energy’, ‘heat energy’ and ‘electrical energy’ have been introduced in order to try to mirror physical processes in the labelling exercise. None of these is helpful, some of them are misleading and some have no physical referent.

A more helpful approach

These concerns have arisen by trying to describe an on-going process with an energy story that mirrors the description based on physical processes and mechanisms. And in doing so masks that physics. A more helpful approach is to analyse the journey in terms of forces and motion. And reserve the energy analysis for determining the change in the system between the beginning and end of the on-going process. i.e. to use start and end points – chosen in a way that keeps the discussion simple.

First let’s look at the mechanisms and processes for the rising mass:

Mechanisms and processes

In brief, the coiled spring in the motor exerts a force on the mass (bigger than the gravitational force). The unbalanced force gets the mass moving but, within a very short time, friction in the system balances the net upwards force and the mass reaches terminal velocity. It continues to be dragged up at a constant speed. All the while, the frictional forces cause the temperature of the gears and axle to rise a little and they, in turn, raise the temperature of the surroundings.

This discussion can be expanded, reduced, embellished, challenged and recast until there is an agreed version. Because it is based on physics rather than on different interpretations of labelling rules.

Trying for a more helpful and elegant energy story?

The first step is to choose start and end points. Let’s start at the point where the motor is fully wound and the mass is on the floor; and choose an end point when it has reached the top. This makes the energy story simpler, less confusing and (I hope) easier to reach agreement.

Between the start and end points:

There is a reduction in the energy stored elastically and an increase in both the energy stored gravitationally and the energy stored thermally. During the process (of rising), the clockwork motor is working (mechanically) to haul the masses up against gravity. The temperature of the gears rises (through working) and the hotter gears rise the temperature of the surroundings (through heating).

If you are using the language of stores, then the phrasing becomes simpler again:

An elastic store has emptied (a bit) and, through mechanical working, the levels have increased in a gravitational store, a thermal store (associated with the mechanism) and a thermal store associated with the surroundings.

It might look like this in a diagram.

Summary of suggestions

  • Avoid chains of intermediate stores. To do so will lead to making arbitrary choices or devising arbitrary (unphysical) rules.
  • Introduce the idea of start and end points and choosing (or allowing students to choose) them carefully.
  • Beware of trying to replicate the physics story with a chain of energy stores and how they are linked up.
  • As ever, I suggest separating the physical description from the energy analysis: 
    • talk about processes and mechanisms to tell the physics story – there will be a lot in it and it will always be worthy of discussion. This should always be the first step – so as not to lose the physics by over-operationalising the task of energy analysis.
    • introduce energy as an analysis tool. Defining a start and end point is helpful. It allows us to have an unambiguous and agreeable description of how the energy is stored before and after an event.

This approach does not require a change in the way that you represent energy (i.e. it is not imperative to talk about stores and pathways). However, it works well if you do so. And then choose start and end points that make the story simple and allow you to make the point(s) that you would like to make without getting bogged down in filling in gaps.

I have gone into some of the examples (HEP and circuits) in more detail in the following posts.

Also it is always worth checking out the SPT resources on energy.

Up next

Energy chain example: hydro-electric power station

Putting everything together, this resource outlines an example.

Energy and Thermal Physics

Energy chain example: hydro-electric power station

Perspectives for

This is the second post about situations in which we are drawn into including intermediate stores as part of an unhelpful chain or ladder. This post is a bit like an appendix, in which I will look at the example of a hydro-electric power station. So it is worth reading the overview first.

Here’s a quick summary anyway.

Quick summary

There are a number of situations for which there is a temptation to introduce chains of intermediate stages when a process is still occurring. They include:

  1. Lifting a mass with a clockwork motor

  2. A hydro-electric power station
  3. Lighting a light bulb
  4. Car making a journey
  5. Heating a room
  6. Pedalling a bike
  7. A loudspeaker playing music

In the previous post, I looked at the example of the rising mass. In teaching schemes and assessments of the recent past (using types and transformations in the way that it had come to be used), student would be expected to reproduce an analysis something like:

elastic energy ⇒ kinetic energy (plus ‘sound energy’) ⇒ gravitational potential energy (plus ‘heat’)

I made the case for this chain being confusing and unphysical. And that spending time in teaching students to reproduce such chains is unhelpful because they are:

  • Obscuring: drilling in the operational details of the labelling task hides (from learners) the interesting physical descriptions;
  • Misleading: they paint an unhelpful picture of what is happening physically;
  • Arbitrary: there is no physics-based reason for choosing one intermediate store over any others;
  • Unenlightening: they shed no light on the physical processes nor do they provide any helpful tools for discussing or using energy ideas.
  • Not relevant for any informative calculations.
  • Ambiguous and confusing. there is no physics-based way for a student to reason their way to an expected answer.

I also suggested ways in which the analysis can be made more physical and educationally helpful. And they are to:

  • Keep the energy analysis separate from the interesting physics stories;
  • Identify start and end point that keep the energy analysis straightforward and relevant to the question.

I will endeavour to illustrate this approach with some of the other examples and hope to show that the resulting analyses are more helpful for learning good physics. First, let’s look at HEP. In the next post, I will look at electric circuits.

B. The hydro-electric power station

A good example for illustrating the ambiguous nature of chains is a hydro-electric power station.

Mechanisms and processes

The physics story (in brief) is: during a day, water drops from a high reservoir to a low one. In doing so, it turns turbines that drive generators, which generate an EMF. The output of the generator is connected to the National Grid. When someone switches on a kettle, the element draws a current from the national grid and its temperature rises as electrons are forced through its high resistance. The hot element then raises the temperature of the water by heating. Furthermore, during the time it takes to boil the kettle, a number of parts of the system (including the wires, the transformers and the water itself) will have got hot and raised the temperature of the surroundings by heating. Some bits will have been noisy; however, the sound waves will have been absorbed by the surroundings and raised their temperature a little.

All of the paragraph above is about mechanisms and processes. They may well need refining and each mechanism could be explored in more (or less) detail (of our choice when we tell the story). But we could probably agree on the physics.

Now let’s move to the energy analysis.

Energy story

First we need to choose start and end points that make the story simple and illustrative. I suggest taking a start point just before the kettle is switched on and an end point as the water boils.

The basic energy story is that

  • the energy stored gravitationally at the HEP station is reduced (its water has ended up in a lower place)

  • the energy stored thermally by the water in the kettle has increased (we have raised its temperature by electrical working).

You could show this change graphically:

We can embellish the story a little by including the idea that the hot water will raise the temperature of the surroundings (a little). Putting this into the language of stores, we can say:

  • a gravitational store has emptied (a little)

  • the energy in a thermal store (associated with the water in the kettle) has increased

  • the energy in a thermal store (associated with the surroundings) has also increased.

Again, this can be represented nicely in a diagram:

By choosing start and end points that pass over all the intermediate considerations, we have, I suggest, avoided obscuring the physics, misleading students, confusing them with arbitrary choices and ambiguous decisions. Instead, the approach is enlightening and, amongst other things, prepares them for performing informative calculations using energy ideas.

By contrast, the approach from the recent past, using a chain of types of energy and conversions or transformations:

  • obscures the story about processes and mechanisms. By attempting to mirror the physical processes, the chain of types is actually a distraction that covers them up and explains them away.

  • misleads. There is a suggestions that, in order to boil the kettle, energy has had to flow through the system or change forms. Or somehow that the kinetic energy of the moving water carries energy from the reservoir to the turbines (it does not – instead it transmits a force that allows the system to transfer energy by mechanical working). Furthermore, this type of analysis has led to the need to invent and to present students with spurious ‘types’ of energy such as ‘sound energy’ and ‘electrical energy’ (and expect them to know about them).

  • is arbitrary. The decisions about which ‘forms’ of energy to include has no basis in physics. Students might have included: kinetic energy (water), kinetic energy (turbines), sound energy, electrical energy and so on. The list would be (almost) endless. But probably have left out the energy stored elastically by the turbine blades, the energy stored by the water under pressure, the kinetic energy of the axles, coils etc. Importantly, the decision on which ones to include and exclude would be made purely by what an examiner or question setter was (arbitrarily) expecting – and not by any physical reasoning. Students would be coached in which ones to include through precedent and habit rather than physical reasoning.

  • is ambiguous. Neither we, nor students, will arrive at a single agreeable result. Physics cannot tell us the exact sequence in which we string together the different ways in which energy is stored. In other words, producing the chain is a purely a rule-based labelling exercise rather than physics.

  • sheds no light on either the physical processes or the energy story. Whilst it is true that the water is moving (as are the turbine blades) and that there is a sound and there is an electric current, all of these aspects are a part of the physics story and do not enlighten or inform the energy discussion. Nor does including them in an energy discussion help with their understanding of the physical processes.

  • distracts from calculation. The energy analysis using start and end points is a preparation for a calculation. It is illuminating and interesting to calculate the changes to each of: the energy stored gravitationally, the energy stored thermally (by the kettle’s water), and how much energy has been transferred by electrical working. By contrast, the kinetic energy of the flowing water is constant and transient; it does not inform or illuminate any calculation. Nor does its value (at any time) relate to any of the interesting changes in the system.

  • confuses through all of the above; especially through the picture it paints that energy is carried from one place to another by the constituents of the system – specifically the water flowing down the pipe. However, here is an oddity: when the water passes to the downside of the turbine, it will speed up. So, in fact, its kinetic energy will increase. So the picture of the water as an energy carrier is deeply flawed and confusing.

As I mentioned above, it is better to think of the HEP station as a transmitter of a force and to discuss energy being transferred through mechanical working rather than energy being carried by an intermediary.

A similar problem arises in electric circuits – where there is a temptation to paint a picture of charges carrying energy around the circuit (rather than energy being transferred by electrical working). I will follow this up in the next post.

Up next

Questions about energy

These questions can be used to explore students' ideas about energy at age 16 and address the new criteria for GCSEs in England.

Energy and Thermal Physics

Questions: Energy

Diagnostic Questions for 14-16

What the Activity is for

These questions can be used to explore students' ideas about energy at age 16 and address the new criteria for GCSEs in England. However, they are not intended to replace sample material from the awarding organisations. And, whilst they are set up to work as summative assessment tools, they can be used equally well for formative assessment and discussions with students.

The questions can be used for three main purposes:

  • To assess a student's ability to use ideas about energy in calculations.
  • To analyse physical changes.
  • To construct arguments in situations where energy analysis sheds some light on decisions or activities.

What to Prepare

  • Printed copies of the question sheets (see below).

What Happens During this Activity

The learning intentions of the questions have been linked to assessable learning outcomes (ALOs) and GCSE criteria where possible.

The questions review what we might expect students to be able to do with energy ideas at the end of KS4, including:

  • Gravitational energy stores
  • Kinetic energy stores
  • Chemical energy stores
  • Thermal energy stores
  • efficiency calculations
  • Cost calculations
  • Power calculations
  • Energy resources
  • Energy transfer calculations
  • Specific heat capacity calculations
  • Work done calculations
  • Insulation

The question sheets contain many multiple choice and exam-style questions. Therefore, they can be used for summative assessments, formative assessments and classroom discussions.

Some sample questions from the document are shown below.

Question: Calorie Intake/food
A cyclist has a combined mass (with her bike) of 62 kg. The mountain stage of a race involves a total climb of 1,900 m to the top where the race finishes.

(a) Which of A, B, C, or D describes the changes in the way energy is stored between the start of the climb and the end of it?

Energy stored chemicallyEnergy stored gravitationallyEnergy stored thermally
BIncreasesDecreasesNo change
CDecreasesIncreasesNo change

(b) Calculate the change in the energy stored gravitationally by the cyclist and her bike between the start and end points of the climb. Give your answer in MJ.

(c) In training, the cyclist has measured her total efficiency when cycling up hill to be 20%.

Calculate the change in energy stored chemically between the start and end point.

(d) It is recommended that the cyclist consumes food during the race to top up the energy stored chemically in her body. Her favourite is called ‘Energy Zap Gel’. The energy stored by one sachet is given as 365 kJ.

Calculate how many sachets she should consume during the climb.

Question answers: Calorie Intake/food
(a) D

(b) Change in gravitational potential energy
 =  mass  ×  gravitational field strength (g)  ×  height
 =  m  ×  g  ×  h  =  62 kg  ×  10 N/kg  ×  1,900 m
 =  1,178,000 J  =  1.2 MJ

(c) efficiency  =  energy stored gravitationallyenergy stored chemically  =  1.2 MJenergy stored chemically  =  20100
Energy stored chemically  =  10020  ×  1.2 MJ  =  6.0 MJ

(d) Depletion of chemical store  =  6.0 MJ
Each sachet will refill it by 365 kJ
Number of sachets  =  6.0 MJ365 kJ  =  16.4  =  17 sachets

Question: Car/train comparison
An electric train is powered by a set of electric motors. Over the duration of a journey lasting 45 minutes and covering a distance of 50 km these motors operate with a total combined average power of 920 kW.

(a) Calculate the energy transferred during the journey. Give your answer in MJ.

(b) The electricity is generated by a gas-fired power station. The network for distributing the electricity is 80% efficient. At the power station the gas plus oxygen behaves as a chemical store of energy.

Calculate by how much this chemical store has been depleted by the journey.

(c) When it is full, the train carries 480 passengers. Calculate how much the chemical store was depleted for each passenger’s journey.

(d) A small petrol powered car is used to carry a single passenger over a similar 50 km journey. The chemical store for this mode of transport is fuel plus oxygen. The chemicals store 34 MJ per litre of fuel.

During the journey the car burns 3.6 litres of fuel. Calculate how much the chemical store has been depleted by this journey.

(e) Using your answers to parts (c) and (d), discuss which type of transport, train or car, is more energy efficient in terms of transporting passengers.

Question answers: Car/train comparison
(a) Energy transferred by electrical working  =  power  ×  time
 =  920  ×  103 W  ×  (45  ×  60) s
 =  2,480 MJ

(b) efficiency  =  useful energy outputtotal energy  =  energy transferred by electrical workingenergy stored chemically  =  2,380 MJtotal energy  =  80100

energy in chemical stores  =  10080  ×  2,480 MJ  =  3,100 MJ

(c) energy per passenger  =  3,100 MJ480 passengers  =  6.46 MJ/passenger

(d) 34 MJ/litre  ×  3.6 litre  =  122 MJ. This is for one passenger.

(e) Figures indicate that a train is more efficient in terms of energy costs than a car for this journey (about 20 times);
The train would not always be full so its energy costs would be greater than the figure calculated.
The car can carry more than one person so when full the energy costs per passenger would be smaller than the figure calculated.
A full car has an energy cost approximately five times more in than a full train.

Question: When hands are rubbed together, their temperature goes up. Which of these describes the process that lead to the temperature rise?

  1. Working against the frictional forces raises the surface temperature.
  2. Heat is generated by friction.
  3. The frictional forces convert work to heat in his hands.
  4. Chemical energy is converted to heat energy.

Question answer: A

Question: An electric kettle is switched on for 1 minute. The house is supplied by a gas fired power station.
(i) Which of the following describes the changes in the way that energy is stored after the kettle has boiled compared with beforehand?

  1. Electricity has been used up to increase the energy in the system.
  2. Electrical energy has decreased to produce sound and heat.
  3. Solar energy is used to produce electricity, which is turned into heat.
  4. A chemical store has been depleted and the energy stored thermally has increased.

(ii) Which of the following describes the energy processes in the circuit?

  1. Electricity produces heat in the kettle’s element.
  2. The circuit transfers energy to raise the temperature of the element and water.
  3. There is a chemical reaction in the element that heats the water.
  4. The element creates energy, which passes into the water.

Question answers:
(i) D
(ii) B

A table that aligns the learning intentions of these questions with assessable learning outcomes, including GCSE criteria, is included in the downloadable questions below.

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