4 ms·
> Second, many coffee urns contain heating elements and thermostats, so as you take more coffee out of them, they actually perform a bit less work. There’s just
by foobarbecue 4y ago
> Second, many coffee urns contain heating elements and thermostats, so as you take more coffee out of them, they actually perform a bit less work. There’s just less coffee left to keep warm.
I know this is just an analogy... but I don't think that's how heat transfer works. Heat loss here is primarily a function of temperature difference across the urn walls, and and thermal conductivity of the urn walls. I don't see how replacing coffee with air would slow down the heat loss.
- knubie 4y agoI think he means it takes less energy to heat a smaller volume of coffee.
- foobarbecue 4y agoFrom cold, sure. But he's talking about an urn that started full of hot coffee, and then saying energy input required to maintain that temperature will decrease as the coffee volume decreases. What's the mechanism for that supposed to be? The more I think about it, the more I think he's just wrong about the coffee thing. Contents of the urn have no effect on energy required to maintain its temperature.
- Zanovis 4y agoI think OP is correct. Think about the extremes of the urn, full vs 1mL in it. It will require much less energy to heat the 1mL state than the full state. Regarding the contents, the thermal conductivity of liquid (Water mostly for coffee) is higher than that of air, so it transfers heat out of the urn faster than an urn full of air. It also has higher heat capacity which requires more heat energy per degree change of temperature. Heat capacity is given by (C) = heat absorbed (Q) / temperature change (ΔT); heat capacity increases with volume