Re: pinhole Ice forming on running water

Date view Thread view Subject view Author view Attachment view

From: pauld@exploratorium.edu
Date: Fri Jan 30 2004 - 13:03:59 PST 

Message-ID: <36731.63.81.67.222.1075496639.squirrel@www.exo.net>
Date: Fri, 30 Jan 2004 13:03:59 -0800 (PST)
Subject: Re: pinhole Ice forming on running water
From: pauld@exploratorium.edu

Hi Christopher

I've been out on vacation, skiing on ice cold water crystals.

This is a bit tricky.

It's not the motion of the water that matters it's the relative motion or
shear flows.

Consider a calm lake that is freezing.
Run past the lake, from your rame of reference the water is moving yet it
still freezes. This is like the water at the surface of a river, it's all
moving aong together and should freeze.

However, if there is turbulence to the flow then one parcel of water is
moving relative to another parcel of water. This is shear flow. The shear
flow leads to eddies, the eddies break up into smaller and smaller eddies
with time and then eventually add to the random thermal motion of the
water, increasing its temperature a little, detering ice formation. New
large eddies are always forming and decaying.

The famous poem is:

Big wirls have little whirls that feed on their velocity
and little whirls have lesser swirls and so on to viscoscity.

Paul D

> In the Hewitt video for heat, temperature and expansion he asked why ice
> does not form on flowing water that is "ice cold. " Hewitt does not
> know the answer in the vidoe although he does sya it can and does.
>
> My students have asked this question in there follow-up video
> questions.
>
> I think I have an answer but want to make sure.
>
> I think it is for 2 reasons. One, the flowing water (i.e. rapidly
> moving molecules) are moving too fast for ice crystals to form. So the
> water must be much colder for a significant number of molecules to form
> crystals. Two, related to the first, the motion of the stream
> provides addition KE that is added to the internal energy of the water,
> meaning the water has to be colder.
>
> Help.
>
> Thanks
>
> Christopher Reese
> Science Teacher
> Woodside High School
> 199 Churchill Ave
> Woodside, CA 94062
> www.seq.org/~creese
> 650-367-9750 x8660
>
> ---------------------------------------------------------------------------
>
> To unsubscribe from pinhole, send an email to requests@exploratorium.edu
> with the words 'unsubscribe pinhole' (without the quotes) in the SUBJECT
> of the email.
>
> To subscribe to the digest and only get 1 combined message a day, send an
> email to requests@exploratorium.edu with the words 'subscribe digest
> pinhole' (without the quotes) in the SUBJECT of the email.
>
> Check out what your colleagues have written on Pinhole in the Pinhole
> archives at: http://saturn.exploratorium.edu/ti/alumni/pinhole.html
> ---------------------------------------------------------------------------
>
>

Date view Thread view Subject view Author view Attachment view

This archive was generated by hypermail 2.1.3 : Mon Aug 02 2004 - 12:05:35 PDT