J. Josˇ Bonner
Energy is a very difficult concept. In conversational English, energy is what I have when I feel like moving around a lot, and what I don't have when I'm tired. We also hear references to energy in a vague overall kind of way, taking in electricity, fossil fuels, and "renewable" energy. In science, energy is recognized at a molecular or even subatomic level, which is so different from the average conception of energy that it is almost incomprehensible. With energy, as with a number of other subjects, developing a more accurate understanding requires building on and eventually replacing students' original preconception. If we simply tell students the scientific view of energy, they will hold it in mind separately from their initial preconception, and eventually forget it.
Let's first ask our students "what is energy?"
Typically, even for college students, this elicits "what I don't have when I'm tired."
From this response, we can infer that energy somehow has to do with movement. When I don't have enough energy, I can't move. [Note: even at the molecular level, this is a valid inference. The more energy a molecule has, the more movement it has--whether we're speaking of kinetic energy with the molecule jiggling and twirling, or of electrons in a higher energy state. Thus, this first question and the inferences we draw from it build students' new understanding from their current understanding.]
Now, we can ask "how do you feel after you've moved around a lot?"
We should expect two answers: "tired" and "hot." "Sweaty" may also come into this, which will lead to a bit of a digression; sweating is the body's way of using energy to pump water to the skin surface, so the water can cool us as it evaporates.
"Tired" of course is essentially what we've started with; "hot" is new. From this response, we can link energy and heat. Somehow, energy of movement turns into energy of heat. [Note: at the molecular level, they are the same. Heat is a measure of the movement of molecules; the hotter they are, the more they jiggle and bump. And, now that we have brought in heat, we can investigate heat per se.]
How else can we get heat?
* Sunlight. Things in the sun get hot.
Light must be a form of energy. [Note: with the discussion of movement being converted to heat, and now light being converted to heat, we begin to build the concept of energy conversion or "transformation" from one form to another. This will eventually be important, because one of the laws of physics is that energy cannot be created or destroyed. But we don't want to talk about "energy interconversion" yet, or the laws of physics. We want to give our students a more intuitive sense of how energy works. Besides, the big words often get in the way of learning the science.]
* Burn wood (or oil or gasoline or coal, but we don't usually see these burning; we can watch wood burn). Does burning wood create energy, or does it release energy that's somehow "stored" in the wood?
[We can't really distinguish these right now. But as teachers, we know that energy is not created when we burn fuel, but is released through the chemical reactions of burning. We can't go into the chemistry at this point, but we can talk about how wood is built, and from this develop our reasoning.]
To answer this, we should look into how wood is made.
Photosynthesis (which we will examine later) is the process by which plants obtain energy and grow. What do we already know about how plants grow? It's likely that one or more students will know that plants need light. To make this more fun than a simple question/answer session, ask the students to talk in small groups about what they know about how plants grow, and then have a larger discussion that identifies what the students have said. Once we have light on the list of things that plants need to grow, we can ask the next question:
What do plants do with light? Again, ask the students to kick around ideas. They may not know, but in science, we usually don't know the answers to questions we ask. We propose various explanations, and then think about which ones are more likely. In the end, the teacher may have to think out loud, something like this: "We just said that light is energy...but plants don't get very hot from sunlight, so perhaps they use the energy to do something else. What could that be...?" Can the students carry on from here? If not, think out loud again: "They might use the energy to build wood, and other plant materials. Hmmm....I wonder if we can test this idea." [Until we study photosynthesis, students will probably have to take this on faith, but if we start a simple photosynthesis experiment, we not only model the thinking, we also model the process of science. We develop an explanation, but aren't certain about it, so we devise a test. Here, we can grow seedlings with and without light, until there is enough difference to conclude that light is essential for plants to grow larger.]
This tells us there's some sort of mechanism for "storing" energy.
[This is a conclusion that students will not be likely to reach on their own. The following may help illustrate it.}
Here's an analogy: using light energy to build wood is somewhat like using our own energy to lift things up high. The things we lift up stay there, not doing anything, until they fall. We can build a tower of blocks to illustrate this (using wooden blocks helps relate this analogy to the mystery of trees somehow holding wood up in the air.) When they fall, they "release the energy" of lifting them, turning that energy into movement.
Similarly, plants use energy to build wood; when wood burns, it "releases the energy" of wood-building, turning that energy into heat.
[It will seem mysterious and magical to students that plain old boring wood has energy stored in it somehow. This means it will be important to examine photosynthesis to get more information and understanding. For now, it may be sufficient to say that plants build wood and other plant materials from small molecules--chemicals--that they take out of the air (CO2) and from the earth (H2O). In general, assembling large things from small parts requires energy. Think, for example, of the tower of blocks. Also in general, when large things break apart (as in burning wood), energy is released. Again, think of the tower of blocks. When it falls, it is releasing the energy we put into it when we built it.]
The Rationale Behind the Strategy
"Science" is not simply learning facts; it is building understanding from the evidence. The evidence need not be complicated; here, we ask students to think about a number of observations that they have undoubtedly made many times before. This is evidence from which they can build their understanding. To work with evidence, it is necessary to build explanations that can explain the observations. It is these explanations that we call Scientific Knowledge.
The strategy here is to solicit students' prior knowledge of things they have experienced, such as getting hot when sitting in the sun. Then, we build upon this knowledge by building explanations for these observations. These are small steps, but even so, they may not be easy for students to come up with on their own. It may require that the teacher think out loud to model the thinking that we would like our students to be able to do. The concept of energy is tricky enough that we may need to do a fair amount of thinking out loud, but there are a few places where we can bring in the students more actively. We should take advantage of these places.
Note that it is tremendously important to ask students to construct their own understanding from the evidence (even if it requires some verbal thinking by the teacher). This is essential to students' long-term learning, and to their development of skills in problem-solving and scientific reasoning. (If we simply tell them the answers, they may be able to answer test questions in a few days, but they are unlikely to retain the learning for a significant length of time. Nor will they have the opportunity to practice the thinking skills that they will eventually need in the world they will enter as adults.) However, we do not leave their knowledge-construction to chance. We designed the learning experience to reach a goal; we need to ensure that it gets there.
A useful assessment strategy may be to ask students to think of a number of different kinds of things that use energy. Then ask them to explain how the energy is used. Does it cause movement? Does it make heat? Does it allow smaller things to be assembled into larger things? Does it cause all of these (think of helping Dad build a brick wall...you move, you get hot, and you assemble something)?
An interesting reinforcement might be to consider how thermometers work, or how bread dough expands when we cook it. In each case, the molecules (or "tiny pieces") that make up the liquid in the thermometer or the air pockets in the bread dough warm up. They absorb energy (heat) from the surrounding environment. As they absorb heat, they wiggle and twirl and jiggle and bump faster. The faster they move, the harder they bump into each other, which causes the material to expand. As the liquid expands, the column in the thermometer gets taller. As the air expands, the bread dough puffs up. (Once the bread is cooked, of course, the protein and starch become stuck together, and make a semi-solid network that retains its shape even after we cool it.)
The picture above is a piece of coal. It is also a fossil of a fern. Because coal deposits are often full of fossils of ferns and trees, we infer that coal is produced by geological processes acting on thick deposits of dense forests.
Coal burns. Like the plant material from which it is made, it is a fuel.
Because it is a fuel composed of fossils, we call it a fossil fuel.
Because the geological processes are very slow, we cannot replace fossil fuel after we burn it. It is a non-renewable resource.
Oil, such as that used to produce gasoline, is another form of fossil fuel. It is produced from long-dead plants, animals, and microbes, but through a geological process that is more extreme than the process that produces coal. The material is smashed and heated while deep underground. Eventually the material is converted into a thick, black gooey liquid--oil.
Oil, like coal, is a non-renewable fossil fuel. Every time we drive our cars, we burn and destroy a little bit more of this resource. What will we do when it's gone? [This is a serious issue for our students. We have reached the peak of world-wide oil production. All of the major oil fields are producing less oil each year (see the BP annual report for hard data). So far, we have managed to offset the decline by discovering new, smaller oil fields. We aren't discovering very many these days, and the ones we find are small. In a few years (2? 10?) it will become clear that the supply is decreasing, while demand is increasing. This will make the post-Katrina prices look like peanuts. Note that ethanol and biodiesel are not solutions. With current technology, we use about a gallon of fuel for each gallon of fuel we produce. We also use up food that we could otherwise eat.]