MadSci Network: Physics |
You performed a very interesting experiment! To think about this problem we need to consider what is "heat". We use the term as if it were really something, like a fluid (e.g., heat capacity) but this is a vestige of the caloric theory. It is better to think of heat as a metaphor for energy transferred through random molecular motion. When we casually speak of "heat rising" what we really mean is "hot air rises." This is so because the density of air, and most everything, decreases with increasing temperature. Following Archimedes' Principle, the lower density object experiences a buoyant force when surrounded by a more dense fluid, i.e, it floats. There are two relevant mechanisms for thermal energy transfer - conduction and convection. Conduction occurs through the local, random molecular motion. Convection occurs through fluid flow, which can be forced by a fan, for example, or occur from buoyancy (natural convection). If you heat a piece of metal from below, hot air will flow up around the object and transfer the heat more rapidly. Likewise, if you cool from above, the cool air sinks and cools the object faster than from conduction alone. How could you test this? Perform the experiment in weightlessness where there is no buoyant force. (In our research, we study chemical reactions in weightlessness, and the reduction in heat transfer is enormous) Or, do the experiment in a vacuum. Without air, no convection. This is the principle of the Thermos, in which an evacuated sleeve surrounds the liquid. Energy can only be lost through conduction through the ca
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