Energy and Thermal Physics

Starting out

Perspectives IOP RESOURCES

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.

appears in the relation ΔEΔt>ℏ/2 ΔQ=mcΔθ E=hf E ∝ A^2
has the special case Photon Energy
is used in analyses relating to Emission/Absorption Spectra Phase Change
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