Instructors_Guide_Ch10 - 10 Energy Recommended class days 2...

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10 Energy Recommended class days: 2 minimum, 3 preferred Background Information Energy is a very abstract concept. The full importance of energy was not recognized until Joule’s experiments of the mid-nineteenth century, nearly two hundred years after Newton’s flash of genius. And unlike the well-defined idea of momentum, , p mv = r r we keep “inventing” new forms of energy—kinetic energy, potential energy, thermal energy, chemical energy, nuclear energy, and so on. Energy, at least to beginning students, is an amorphous, ill-defined concept. It’s not at all obvious how 1 2 2 mv has any connection to chemical energy or nuclear energy, yet we lump them all under the same heading. And students are certainly not helped by the everyday use of the terms work and energy . After all, why should we be worried about “conserving energy” if energy is supposed to always be conserved? Yet out of this, we want students to understand that conservation of energy is one the most hallowed and best established truths of physics! Energy and its conservation are difficult ideas. There’s no thing you can put your finger on and say, “Here, this is energy.” It’s just some number, calculated by adding a little of this and a little of that, that for some hard-to-fathom reason never changes. Perhaps, if we can put ourselves back in the mind of a freshman, we shouldn’t be surprised at the difficulties most students have learning to use energy conservation. There is little research on students’ concepts and understanding of energy. The paper of Lawson and McDermott (1987) cited in the last chapter asked students to compare the kinetic energies of two dry-ice pucks that had been accelerated by compressed air. Their performance on the energy comparison was slightly better than on the momentum comparison, but, even so, only 50% of honors students could answer correctly without being given hints. The investigators concluded that the majority of students—including those who can work standard energy problems—cannot use the concepts of work and energy to reason about a real situation. This research finding is in accord with the observation of many instructors that their students rarely make spontaneous use of energy conservation to solve a problem. Given an exam problem that can be solved either with Newton’s laws or (usually much more easily) with energy conservation, only a minority will use energy. Difficulties with energy resurface later in the course. The electric potential, defined as the potential energy per unit charge, is surely one of the most difficult concepts for students in the entire introductory physics course. Elementary quantum physics makes extensive use of potential-energy diagrams, yet there’s good reason to think that few students can “read” or interpret these diagrams.
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