  The Next Generation Science Standards (NGSS) require teachers to understand
aspects of energy degradation and the second law of thermodynamics, including
energy's availability and usefulness, changes in energy concentration, and the
tendency of energy to spread uniformly. In an effort to develop learning goals
that support teachers in building robust understandings of energy from their
existing knowledge, we studied teachers' impromptu conversations about these
topics during professional development courses about energy. Many of these
teachers' ideas appear to align with statements from the NGSS, including the
intuition that energy can be present but inaccessible, that energy can change
in its usefulness as it transforms within a system, and that energy can lose
its usefulness as it disperses, often ending up as thermal energy. Some
teachers' ideas about energy degradation go beyond what is articulated in the
NGSS, including the idea that thermal energy can be useful in some situations
and the idea that energy's usefulness depends on the objects included in a
scenario. Based on these observations, we introduce learning goals for energy
degradation and the second law of thermodynamics that (1) represent a
sophisticated physics understanding of these concepts, (2) originate in ideas
that teachers already use, and (3) align with the NGSS.
