Energy is a crosscutting concept in science and features prominently in national science education documents. In the Next Generation Science Standards, the primary conceptual learning goal is for learners to conserve energy as they track the transfers and transformations of energy within, into, or out of the system of interest in complex physical processes. As part of tracking energy transfers among objects, learners should (i) distinguish energy from matter, including recognizing that energy flow does not uniformly align with the movement of matter, and should (ii) identify specific mechanisms by which energy is transferred among objects, such as mechanical work and thermal conduction. As part of tracking energy transformations within objects, learners should (iii) associate specific forms with specific models and indicators (e.g., kinetic energy with speed and/or coordinated motion of molecules, thermal energy with random molecular motion and/or temperature) and (iv) identify specific mechanisms by which energy is converted from one form to another, such as incandescence and metabolism. Eventually, we may hope for learners to be able to optimize systems to maximize some energy transfers and transformations and minimize others, subject to constraints based in both imputed mechanism (e.g., objects must have motion energy in order for gravitational energy to change) and the second law of thermodynamics (e.g., heating is irreversible). We hypothesize that a subsequent goal of energy learning—innovating to meet socially relevant needs—depends crucially on the extent to which these goals have been met.
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We use the term “work” to refer to the mechanical transfer of energy from one object to another (i.e., a transfer by means of a force exerted through a displacement), including objects that may both be part of the same system.
Though a physics analysis tends to prioritize the mechanical conversion of kinetic energy into another form of mechanical energy—in this case, the elastic energy in the spring—the metabolic production of thermal energy is often the energetically dominant process in scenarios involving living organisms.
Figure 2 does not include, but could be modified to include, other processes that would be present in real phenomena such as transfer of thermal energy from the person to the environment via conduction.
In a scenario in which the magnets were pulled apart and then both released simultaneously, both magnets would gain kinetic energy and two distinct tracks would be required in the Energy Tracking Diagram corresponding to the release. Each track would start with an M in the shared zone and end with a K in one of the magnets.
An advantage of this approach is that it avoids the attribution of infinite negative energy to closely spaced magnets, due to representing only the magnetic energy associated with the given objects rather than all the magnetic objects in existence. Energy Tracking Diagrams are not suited to representing negative energies.
The string connecting the two masses is assumed massless and not shown in Fig. 5, but could be included as an intermediary.
Incandescence is distinctive in that energy transfer and transformation are simultaneous aspects of this single process. Light is distinctive in that it may be considered to be either a form of energy or a means of transporting energy. An alternative is to identify “light energy” as “electromagnetic energy,” and the blue arrow with radiation.
Teachers were told, “A meterstick is laid on edge on the floor and one end is fastened down. The other end is pulled back and used to propel a metal ring across a level floor. The metal ring slides to a stop.” Teachers also watched a video of the scenario.
Teachers were shown a diagram of a power plant and told, “In a ‘steam turbine power plant,’ coal is burned to produce steam that turns a turbine, generating electricity. The diagram shows various parts of the power plant.”