Energy Language Thoughts Part 2

15Feb17

The previous post was a summary or introduction – thanks to all those who have commented already – and tomorrow I’ll be moving on to stores in more detail. But for now…

Descriptions vs Labels: Processes

To make life easier, humans like to use shorthand for complex processes. These are categories or labels, not detailed descriptions. Many pathways or processes can be put into one of these categories, but the aim should always be end up able to describe what is actually happening.

  • Heating by particles
  • Radiating (aka heating by radiation)
  • Electrical working
  • Mechanical working

Longer explanation at SPT

How we choose these categories will alter our interpretation. For example, are sound waves a form of mechanical working? Or do we include all waves in the ‘radiating’ category? The physics description of what is happening is what we and our students should be concentrating on, because it doesn’t change. The ideas about Johnstone’s Triangle that I’ve read about via Michael Seery’s blog, from chemistry education, has obvious parallels.

johnstones-triangle-1024x573

Reproduced from Michael’s post, credited to University of Iowa.

If we can link the macro (observations in lab) and sub-micro (particles and interactions) levels, the symbolic can wait. A similar discussion is had on SPT about alternating between the lived in world and a theoretical model.

Avoiding using these categories – which by their very nature are imprecise – might be worth considering. It would be very easy for students to think they have to assign any physics process to one of the four listed above, without really thinking through what’s happening. (If you’d like to consider symbolic approaches, I’d suggest checking out the physical versions of energy bar charts as described here by Greg Jacobs.)

As pointed out by several – most recently Richard Needham on Twitter – changing how energy is described in physics lessons means nothing if we can’t apply this to biology and chemistry. And it needs to make more sense there too! In school chemistry, heating and radiating (in the form of light-emitting) will be the significant processes. The equations used later on for enthalpy change – endo and exo-thermic reactions and so on – work nicely with this framework. In school biology, the transfer of energy is usually about photosynthesis (a radiating pathway fills a chemical store by the production of glucose/oxygen) or nutrition/metabolism. (More about stores in the next ‘chapter’.) One of my jobs is to have a closer look at the KS3 specification for any mention of energy in chemistry and biology and see what I’ve missed – please let me know in the comments if this has been done already!

I’ve heard – and contributed to – discussions about other possible pathways, perhaps useful for younger students. The regular suggestion is reacting, which would include chemical reactions in cells (aka metabolic-ing) as well as the lab. The shift happens between two chemical stores. The physics process, if we look closely enough, is about electron exchange between atoms. But I wouldn’t want to have that level of explanation in a year 7 lesson! As ever, the question is about us choosing a realistic level of detail for our students at any particular time.

The Power of Processes

I wrote earlier that we weren’t interested in how quickly a process worked. That’s obviously not always true; rates are very important in physics! So the process can happen quickly or slowly, which changes the magnitude of the final change in the relevant stores. This tells us that processes are about power, not just energy. (Thanks to Brendan Ickringill who pointed out the word rate is important.) The analogy I use is that of the carbon cycle. Asking how much carbon is ‘in’ plant biomass at any point is a meaningful question, if not an easy one. But it makes no sense to ask how much carbon is ‘in’ combustion, or any other process. They are rates, not amounts.

My colleague Trevor Plant reminded me of the need to change how we use Sankey diagrams for this new approach. The width of the arrow can now describe the power of the process, transferring or shifting energy between stores. A lot of the same questions could be asked, and efficiency is still a helpful consideration. We’d now think about useful processes (with values in watts) and wasteful or dissipative ones. As ever, we’d need to distinguish between similar processes; for example, energy shifted to the thermal store of water in a kettle is useful, whereas heating of the air around it is not.

Effectively what we’re doing here is describing what the ‘magical arrow of energy transfer’ is symbolising. A useful resource is a set of laminated arrows which students can write on for descriptions of the physical processes. You could provide some with descriptions on them, but the danger is that the class – or the loudest member of it – will then choose a best-fit rather than something more accurate. If you also supply laminated cards – as boxes, not arrows – with the eight stores on them, they are ‘encouraged’ towards the new model. These might be particularly useful to analyse a chosen selection from the famous energy circus.

On this theme, I produced some cards to go on the electrical sockets in the lab. The idea is to remind students that the current comes from somewhere else, and that the electrical supply is a pathway/process, not a store. Download below.

power-stations as pdf

As before, I hope the discussion here is useful – and please respond in comments if there’s something I’m missing out! Next post will be looking at stores in more detail, then hopefully a last one at the weekend on practical approaches and ways to adapt what you used to use!

 

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5 Responses to “Energy Language Thoughts Part 2”

  1. “The width of the arrow can now describe the power of the process, transferring or shifting energy between stores.”

    The use of the word “power” in this sentence is a bit confusing. In a Sankey diagram the width of the arrow tells you about the efficiency or the proportions of the energy going to the different stores. It’s not the rate of movement of energy. Or have I misunderstood?

  2. I think the width of the arrow represents (is proportional to) the rate at which energy is transferred by that pathway (i.e. the power). The efficiency is useful power / total power so it’s the comparison of arrow widths that tells you about the efficiency, not the width of one arrow.

    However, I just tried to write some examples and got in a horrible mess. Can you help Ian?

    On a Sankey diagram for a filament lamp is it reasonable within your approach to talk about a 90W arrow for IR radiating and a 10W arrow for visible light radiating. If I use heating by radiation for the 90W, I have a problem with the 10W because of course that is heating by radiation too, just at a different wavelength. The old ‘heat and light’ is simple – if perhaps wrong. I’m yet to find the new approach as simple but maybe it will come eventually with familiarity.

    • Jenny, Matt, thanks for your comments. I’m afraid I’ve often been quite relaxed about my use of Sankey diagrams, but of course you’re right. The problem is that they get used in all kinds of ways, and this would be a good chance to clarify their use. Proportions is, of course, the way they should work.

      This is a good example of the four ‘official’ pathways being a bit of a blunt instrument. Yes, the filament bulb would involve two arrows which are both ‘heating by radiation’. So we’d need to give a better description, and treat that as a more general category (which is useful to recognise similarities, for example a reminder that wave properties will be relevant). I’d go with ‘heating by IR radiation’ and ‘heating by visible light radiation’. Adding wavelength values is probably over the top!

  3. I have regretfully concluded that the filament bulb example has to be ditched. Shame — I’d built up a nice set of resources. Incidentally, I couldn’t find a mention of Sankey Diagrams in the new AQA GCSE syllabus…

    • Hello – thanks for commenting! Yes, the filament bulb is a good example of an old favourite which needs extra care with the new approach. But it still works:

      chemical store of cell (via electrical pathway) to thermal store of filament

      thermal store of filament (via 2 x radiating pathway, IR and visible, plus heating by conduction pathway) to thermal store of room

      I know that suggesting two distinct and discrete EM radiations, IR and visible, is a bit of an oversimplification. But it works, yes? And it makes the difference to an LED much clearer…

      As far as Sankey diagrams are concerned, I suspect most colleagues will want to use them so kids are familiar in case they do get used in the exams. And they’re useful! Apart from anything else, they get used a lot in food chains, metabolism and similar.


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