Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• link the idea of burning fuel to the dissipating of energy
• distinguish between the physical descriptions of the lived-in world and the descriptions using the idea of energy

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• build in the conservation of energy
• establish a list of ways to calculate energy, represented by words or diagrams
• select an appropriate representation of a quantity of energy for your teaching

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Burning fuel

Wrong Track: To make anything happen you have to burn a fuel like food or petrol.

Right Lines: Some changes happen without a fuel burning.

Using fuels as an introduction

Thinking about the teaching

We suggest starting with fuels because of their obvious usefulness, and because something is very clearly used up. However, thinking about fuels is only an introductory stage on the way to a full appreciation of energy stores, so it is not worth pupils spending too long on this section. Thinking about fuels also provides a useful link to a discussion of energy resources, which follows in episode 03.

For now, and particularly if pupils show signs of claiming that every change needs a fuel, you need a supply of examples where fuels are not needed. Here are some pairs that may be useful:

• Holding a book up using a shelf as opposed to a human supporting the same book on their hand (your muscle fibres need to keep on contracting and relaxing, so depleting their supply of ATP).
• An ice skater gliding along the ice as opposed to a car rushing along the motorway.
• A planet moving around the Sun as opposed to a fairground ride moving around and around.

We hope you spotted that burning fuel enables the second one of each pair, but not the first.

Accounting for friction

Wrong Track: Energy can't be conserved, and this proves it! When a pendulum swings back it doesn't go as high.

Right Lines: If you take into account the energy of the pendulum and that of the surrounding air, then energy is conserved. It is true that the pendulum does not swing back to its original height, but some of the energy from the gravitational store of the pendulum (in the Earth's field) is shifted to the thermal energy store of the surrounding air. In other words, as the pendulum travels through the air there is some heating of the air due to friction. Overall, there is as much energy around at the end of the process as there was at the start.

Energy is always conserved

Thinking about the learning

Pupils need to get hold of the idea that energy is always conserved.

Thinking about the teaching

Energy is conserved. But showing that this is so requires rather careful measurement. A good approach to take is to emphasise at all points that energy must be conserved. If some process happens in which there seems to be an energy loss, the cry should go up: Let's play hunt the energy! In virtually all of these cases, where there appears to be a loss of energy, something will be warmed (with the energy previously unaccounted for turning up in a thermal energy store).

You can help them in this by always talking of the changes of energy in a store, and avoiding qualifying the energy in any way, such as: dark energy; positive energy; negative energy; light energy; sound energy; electrical energy.

Teacher Tip: Just talk about the energy.

Not just fuels

Wrong Track: Things like petrol and coal are the main energy stores because you can burn them.

Wrong Track: How can a bag of sand on the roof be an energy store? You can't set fire to it or anything. It just sits there!

Right Lines: There are a wide range of energy stores if you look carefully enough.

Finding other energy stores

Thinking about the teaching

Many of the things we think of as fuels are things that get burnt, or at least oxidised. This is a good starting point but needs to be extended. This involves getting pupils to see that other things can act as an energy store.

• A camping gas cylinder, together with a supply of oxygen, is a store that gets run down when we use it to cook our evening meal.
• A stretched rubber band can be seen as a store, able to be emptied by allowing it to relax.
• A supply of water held high up behind a dam is a store, able to drive a turbine when it runs downhill.

Dealing with an abstract quantity

Strictly speaking energy is an abstract quantity with a calculated number and a unit of measurement. However, even with the brightest pupils of this age range, you will do few calculations, and even then only for a limited range of situations. Therefore, we have chosen the orange fluid to stand in place of the quantity. We hope this will engage the imagination. Although it does not have much predictive power, the model does have considerable value as a tool to think with. We think you'll need to use one like this, if not this one.

We have suggested that you concentrate on changes where comparatively few stores account for all of the energy changes. So no orange fluid can cross the boundaries of what you describe. The fluid may move around, and even become more spread out with time, but there is still the same quantity of fluid. Furthermore, this fluid behaviour is familiar, following the everyday behaviour of water.

Describing stores

Wrong Track: The ball on the roof has energy.

Right Lines: The ball on the roof is a gravitational energy store because of its position in the Earth's field. Moving the ball towards or away from the Earth alters the contents of the gravitational store.

Energy stores can depend on more than one physical …

Thinking about the learning

Pupils need to develop the idea that energy stores often depend on more than one physical object.

Thinking about the teaching

You need to treat this lightly, but firmly, taking care with your own descriptions so that you emphasise the whole, rather than just a physical object. In the case of fuels, the energy store is the fuel-plus-oxygen, not just the fuel or just the oxygen.

Describing an elastic band

It may seem a bit of a mouthful in class to talk about:

Teacher: The elastic store of energy associated with the rubber band.

However, it would be perfectly acceptable to refer to:

Teacher: The elastic store of the rubber band.

… or to hold up the rubber band and say:

Teacher: Here we have a rubber band, an elastic store.

Describing energy shifts

Wrong Track: When I lift the book onto the shelf, chemical energy is transferred from my arm to gravitational potential energy in the book.

Right Lines: When I lift the book onto the shelf, energy is shifted from the chemical store of my arm to the gravitational store of the book.

Energy is not a substance

Thinking about the learning

Very often, pupils describe energy changes as if energy were a real substance that was taken out of one object, transformed into a different kind of energy and then put into another object. Such descriptions mix physical and energy descriptions. The orange fluid we suggest as a teaching model is a physical analogue of the energy description: it's not an assertion about real flows of tangible stuff.

Thinking about the teaching

Differentiating between lived-in-world, physical and tangible objects and energy stores helps to keep the two levels of description separate.

• One level of description concerns the changes you can see and measure directly (lifting a book onto a table; throwing a ball at a wall). This involves the real, physical world.
• The other level focuses on tracking changes in the amount of energy in the stores associated with real objects. This involves the abstract world of theory.

You'll need to be explicit about what you're up to in order to keep the two kinds of description separate.

There's a good chance you could improve your teaching if you were to:

Try these

• working as if energy is, in essence, quantitative
• using energy as a constraint on possibilities, not a cause of a process
• using a clear teaching model
• always taking simply about energy and changes in the energy
• developing the idea of a store as a placeholder for a calculated energy change
• working from well-described snapshot to well-described snapshot, then discussing the energy changes

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• allowing an endless list of kinds of energy
• using energy as cause
• using qualifying adjectives in front of energy
• treating energy as a physical substance

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.