Abstract
Research has demonstrated that many students and some teachers do not consistently apply the conservation of energy principle when analyzing mechanical scenarios. In observing elementary and secondary teachers engaged in learning activities that require tracking and conserving energy, we find that challenges to energy conservation often arise in dissipative scenarios in which kinetic energy transforms into thermal energy (e.g., a ball rolls to a stop). We find that teachers expect that when they can see the motion associated with kinetic energy, they should be able to perceive the warmth associated with thermal energy. Their expectations are violated when the warmth produced is imperceptible. In these cases, teachers reject the idea that the kinetic energy transforms to thermal energy. Our observations suggest that apparent difficulties with energy conservation may have their roots in a strong and productive association between forms of energy and their perceptible indicators. We see teachers resolve these challenges by relating the original scenario to an exaggerated version in which the dissipated thermal energy is associated with perceptible warmth. Using these exaggerations, teachers infer that thermal energy is present to a lesser degree in the original scenario. They use this exaggeration strategy to productively track and conserve energy in dissipative scenarios.
Highlights
The Generation Science Standards (NGSS) [1] emphasize the importance of tracking and conserving energy through physical scenarios [2]
It follows that a person who accepts that energy is conserved would expect the perceptibility of that energy’s indicators to be “conserved.” For example, in the scenario of a ball rolling to a stop, the disappearance of a perceptible indicator without replacement by another perceptible indicator can seem to suggest the disappearance of energy and a violation of the principle of energy conservation
Changes in mechanical energy of about 1 J may be associated with perceptible indicators, but if all of that energy were transformed to thermal energy, it would only increase the temperature of a typical room (50 m3) by an imperceptible 10−5 K (10−5°F)
Summary
The Generation Science Standards (NGSS) [1] emphasize the importance of tracking and conserving energy through physical scenarios [2]. For example, changes in height and speed of the rollercoaster are the perceptible indicators used to track energy as it transforms from gravitational energy to kinetic energy This method of tracking energy by its perceptible indicators is useful in idealized scenarios that neglect dissipative processes (e.g., a rollercoaster moving on a frictionless track). Learners expect that kinetic energy associated with visible motion will transform into thermal energy associated with palpable warmth This expectation challenges their commitment to energy conservation when all energy indicators disappear from perception. Learners accept the presence of thermal energy in a rollercoaster scenario when they recognize that warmth is perceptible in a space-shuttle re-entry scenario We support these claims by first describing the physics of energy dissipation and the perceptibility of indicators of energy forms (Sec. II).
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