Thanks for the advice on calculating K, but my students are doing the
experiment suggested! I would like to compare class data to a "known" value.
In fact, we are discovering that, for water temperatures in the 70 C range,
preliminary student data suggests that evaporation is the primary culprit
responsible for the cooling of water in a Styrofoam cup. We are looking into
the relative contributions of conduction, convection, radiation, evaporation.
The kids are in charge of the process, the proper procedure is not immediately
obvious, and the kids are having a ball!
(Radiation is a slippery topic here, depending on emissivity(?) and external
temperature(?) of the cup and surroundings).