As you would expect, this follows on from Part 1 (Introduction and Summary) and Part 2 (Pathways/Processes). Even if you’ve read them, you might want to look back at the comments readers have made – many thanks to everyone who has been able to take the time.
Descriptions vs Labels: Stores
The stores are not simply renamed ‘energy types’. A lot of them use similar words, but that’s because they’re trying to describe the same physics. They represent the changing properties of a part of the system, caused by it gaining or losing energy. When a steel block undergoes physical processes, it changes in a measurable way. It might change shape. Its temperature might change. It might be moved away from the Earth’s surface. It is a shame that exam boards are taking different approaches, but the eight suggested by the IOP are:
- chemical store
- thermal store
- kinetic store
- gravitational store
- elastic store
- nuclear store
- vibration store
- electric-magnetic store
More details at SPT
Like the processes, there are sometimes more than one way to consider what is happening. If a gas is heated, the change could be considered in terms of the measured temperature change (thermal store), or in the increased movement of the particles (kinetic store). Realistically, there are not many situations where two stores will seem equally appropriate. And when they are, this is actually a strength. The two approaches will give values for the energy change which are the same. Energy is energy, whether it is considered in the context of a thermal or kinetic store. The whole point of using energy as a ‘common currency’ is that is translates between contexts.
The stores, as discussed in my introductory post, are each a way of considering a physical measurement and an associated equation. The idea is that you consider the ‘before’ and after’ situations for relevant stores, as snapshots. (The exception, for school-age students at least, is a chemical store where the values are found empirically.) I produced, based on some ideas from IOP colleagues, some energy store ‘bookmarks’ which bring together the different aspects. They wouldn’t take long to put together, but you’re welcome to my version:
stores-bookmarks as pdf
The vibration store can be considered as a kinetic store which oscillates. The easier measurement is not speed but amplitude and time period. Imagine trying to find a meaningful value of the speed of a swinging pendulum, for example. But some boards are omitting it, which is fairly easy to justify.
I’m less happy that at least one exam board (AQA) miss out the nuclear store entirely. This seems like a huge mistake to me as it uses the one equation pretty much everyone knows from physics, E = mc2! It would also make it impossible to start with the sun, which makes most biology a bit tricky. (From nuclear store via particle heating processes to sun’s thermal store then via radiating processes to Earth’s thermal store and biomass chemical store)
The electric-magnetic store – not electromagnetic – is about the position of objects within two kinds of field. Now, I know they’re related – Maxwell’s equations and all that – but I think for most students it’s a lot easier to consider two separate stores, the magnetic and the electrostatic. The upside is that this means you can clearly link them to gravitational stores and so cover fields as a ‘meta-model’. The downside is that it makes the stores list look even more similar to the old approach. If you take this tack, make sure you emphasize that it’s an electrostatic store to clearly distinguish from the electric current pathway.
Which brings me to…
What about light/sound/electricity?
The SPT resources have some very good explanations on this. My reasoning is that they are processes which only have meaning if we think about duration. To describe them in numbers, we use power in Watts rather than energy in Joules. So they are, obviously, real physics effects. But they fit best into this model as processes shifting energy between stores, rather than stores themselves.
Disclosure: my issue with this is that a very strict interpreation of thi would seem to rule out kinetic stores as well. The snapshot approach – comparing the change to stores in between two static frames – makes it hard to reconcile a moving object with a single instant. Hmm. Although we have no problem with considering momentum at a moment in time, yes? Contrariwise, students may have an image of light as being made up of photons as moving objects, or when older the equation E=hf. Hmm again. And what about latent heat? This is best considered as a special term of the thermal store, but it’s not obvious. (Thanks to my colleague Lawrence Cattermole for reminding me of this today.) Of course, no model is perfect. The test is whether this approach is better than the ‘types’ of energy approach that has been so pervasive.
‘Better’, of course, is not a very scientific term! It is more accurate when describing the physical processes. The words are a closer narrative match to the equations students will need to use as they develop their physics. The model is different to what we and our students are used to, but objecting to it on that basis seems short-sighted. As I originally said, you could argue that the timing is unfortunate, with new specs and grading systems, but I don’t think we’d ever be at the point where all science teachers welcomed a change with open arms.
As always – please comment, respond, shout angrily at me using the field below.