Energy and Thermal Physics

Resisting the old language

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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.

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