Energy and Thermal Physics

What stores do we need?

Perspectives IOP RESOURCES

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.

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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