From: utzoo!dciem!mmt
Newsgroups: net.physics
Title: Re: adding cold milk to hot tea
Article-I.D.: dciem.136
Posted: Thu Dec 30 14:45:35 1982
Received: Thu Dec 30 14:56:09 1982
References: brunix.1163

This tea-milk problem is often given to first-year physics students
after they hear about Newton's law of cooling. Newton's law only applies
if the interface between hot and cold systems stays the same. If you
put milk into hot tea, there are two kinds of difference:
  (i) the total contact areas between (a) milk and room and (b) tea
      and room changes. If your milk was all in a small jug and was added
      to a larger cup of tea (no milk remaining in the jug), the total
      heat-transfer surface is probably reduced.
  (ii) Hot tea cools at the surface (and edges), inducing convective effects
      which increase the rate of heat transfer as more hot tea is brought
      to the surface. Cooling the tea with milk can reduce these effects.
The answer to the question of whether the tea will be warmer when drunk
after early or late milk addition depends on several things, including
the shape of the teacup, the shape of the milk-jug, and maybe other
things like how carefully the milk was added.