Date: Thu, 7 Nov 2002 17:18:04 -0500

Author: Wolfgang Rueckner

Subject: Re: Hot Chocolate Effect - On Video Encyclopedia of Physics


> This demo is 3D30.77. Maybe the references will provide a
>lead. A web search for "hot chocolate effect" yielded 8 hits,
>including the one listed below which explains the effect:
> Jerry

The link above gives the explanation that I have always offered in
the past to my students until one day I tried something new. Now
when I hear the explanation I say, "not so fast -- let me show you
something ..."

Try this for yourself. You need your coffee cup, a metal spoon, and
fresh hot water out of the tap. That's all -- no other ingredients.
Fill the cup with hot water (it comes out of the tap aerated) and tap
the lip of the cup with your spoon (continuously). You'll hear the
pitch go DOWN, stay down for a while, and then go UP (a couple of
octaves or so). The pitch depends on the speed of sound in the fluid
and that, in turn, is dependent on the overall density of the fluid.
But if the change in pitch is due only to the bubbles in the fluid
(which are rising to the surface), why does the pitch go down first
and then up? BTW, this doesn't happen with hot chocolate stirred in
-- it just goes up.