bbanerje@sjuvax.UUCP (B. Banerjee) (05/20/85)
Pulled over from net.cooks... >> For God's sake, you *can't* freeze hot water faster than cold water! >> (for the same reason you *can't* boil cold water faster!) >> No argument from me on that. >> AGAIN: >> >> Newtons (thrid, I think) Law of Thermodynamics! >> >> which, stated roughly, says that an object (substance or whatever) that has >> mass cools (heats) in direct proportion to it's mass and the difference >> between the desired temps. >> Newton's law of cooling holds only for small temperature differences. For larger temperature differences between the object and the ambiant temperature, you need Kirchoff's law of cooling. This states that the rate of cooling is proportional to the difference in the *fourth* power of the body, and the surroundings. >> >> If you have any doubts, mail me - if I get enough replys, I'll look up >> the formulas in my old chemistry book and post it. >> I seem to recall that Kirchoff's results were for Black Body radiation. It's been about 5 years since my B.Sc, and I'm sort of fuzzy on the details. Perhaps someone more current could clarify. Regards, -- Binayak Banerjee {allegra | astrovax | bpa | burdvax}!sjuvax!bbanerje P.S. Send Flames, I love mail.
brian@digi-g.UUCP (Merlyn Leroy) (05/28/85)
>>> For God's sake, you *can't* freeze hot water faster than cold water! >>> (for the same reason you *can't* boil cold water faster!) > >No argument from me on that. > You will get one from me. Freezing water involves more than just subtracting energy from the water. It has to form crystals. Rapidly cooling warmer water is thought to make crystal formation easier, thus warmer water can freeze faster than cold water (in some circumstances). It is possible to "supercool" water, down to about -40*. Very pure, still, water slowly cooled can be cooled below freezing without forming crystals. Drop something in, or give it a nudge-nudge, and it freezes almost instantly. Water isn't just this stuff, I mean, it's got molecules & junk in it. This makes the real answer more complex. Try it in your fridge! Merlyn Leroy *F. or C., take your pick.
dgary@ecsvax.UUCP (D Gary Grady) (06/04/85)
Jearl Walker, the same Flying Circus of Physics guy who did the
firewalking, also argued with his mother over the question of hot water
freezing faster than cold. He told her about Newton's Law of Cooling
and so on, and to prove to her that hot water would not freeze faster
than cold, he did an experiment, which sure enough verified Newton's
Zeroth Law: Mom Was Right.
No explanation here. Walker has reported some tests involving covering
the tops of the containers that suggest evaporation is somehow involved.
Others who have speculated on this suspect convection currents...
So much for old spouse's tales always being wrong. By the way, a
popular old physicist's tale that turns out to be baloney is that water
in a sink or bathtub spirals down the drain in a direction determined by
the Coriolis force (e.g., counterclockwise in the Northern Hemisphere).
The Coriolis force is far too weak, even when tests have been conducted
in circular vats in which the water is allowed to settle for many days
before emptying. (Curiously, the direction of spiral tends to reverse
when the water surface gets down to about 5cm from the bottom...)
--
D Gary Grady
Duke U Comp Center, Durham, NC 27706
(919) 684-3695
USENET: {seismo,decvax,ihnp4,akgua,etc.}!mcnc!ecsvax!dgary