gjphw (01/17/83)
In the discussion about the equivalence of adding milk to hot tea and let-
ting that cool to allowing the tea to cool before adding the milk, I would like
to relate an experience that some of my fellow physics graduate students had in
a graduate laboratory course (at SUNY at Stony Brook, NY). This laboratory
exercise involved adding cream to hot coffee.
As occurs for hot tea and milk, the coffee-cream mixture remained warmer
if the cream was added first to the hot coffee than allowing the coffee to cool
a time before adding the cream. The experiment began with equal amounts of
coffee and cream, and the temperatures were recorded at equal time intervals.
None of the graduate students received any credit for this lab since they all
failed to describe the processes involved to the satisfaction of the instruc-
tor.
According to the professor, adding cream to hot coffee will allow the mix-
ture to remain warmer longer for two reasons:
1. With the temperature difference between the room and the coffee
decreased, the rate of heat loss is reduced.
2. Of perhaps greater importance is that with the addition of the cream, a
layer of milkfat forms on the surface of the coffee-cream mixture which
inhibits evaporation. Evaporation is an effective means of heat loss for
hot water (and human beings).
No student in the class mentioned the milkfat and evaporation factors, so
no one got any credit for that particular lab (the lab instructor was not a
favorite faculty member). Sometimes, we are led astray by the success of the
idealizations used in science and overlook other factors.
Pat Wyant
*!iheds!gjphw