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ice and wire experiments



I composed this message from the files I saved last winter. You saw most
of them in the ice skating thread. Posting it because references to my
experiments were made today. Delete without reading, unless you want to
read old messages about melting of ice below blades (winter 1996/1997).
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Subj: A myth about ice skating
Date: Tue, 3 Dec 96 06:13:41 EDT
From: kowalskil@alpha.montclair.edu

Do you recall that the tripple point curve for water has a negative
slope separating the solid and liquid phase? And you know that many
textbooks tell us that the negative slope (lowering of the melting
point by pressuse below the blades of a skater) is responsible for
the formation of liquid water there. According to a new book by Clifford
Swartz (see below) this is a myth; the pressure is usually too low for
this. What do you think?
........................................................................
HERE IS WHAT CLIFF SWARTZ SENT ME ABOUT ICE SKATING AFTER THE LAST
REGIONAL AAPT MEETING

From: APOLLO::WINS%"CSWARTZ@ccmail.sunysb.edu" 30-OCT-1996
To: KOWALSKI@alpha.montclair.edu

Hello, Ludwik
Let me expand the information about ice skating by citing the
section on the topic from my new book "Teaching Introductory Physics",
published by the American Institute of Physics.(p 93)

"Ice skates experience low friction because a thin water film is
produced between blade and ice. A popular legend has it that this
effect is caused by the lowering of the melting point due to the pressure
exerted by the blade. It is true that one of the unusual features of
water is that it expands during freezing. Consequently, if we prevent
the expansion by increasing the pressure, we prevent the freezing and
thus lowwer the melting point. The usual pressure-temperature diagram
for water, Fig 4-5, shows the boundary line between solid and liquid
arching back to the left from the triple point. However, the actual
effect is very small.

The slope of the boundary line is -(1.2x10^7 N/m^2/C) It would take an
increase of 120 atmospheres to lower the melting temperature 1 Celsius
degree. For the typical skate blade, the area is 27 cm x 4 mm = 11 cm
If the full weight of a skater with mass 65 kg is exerted on one blade,
the increased pressure would be about 6 atm. Sharpening the blades
does not decrease the contact area appreciably, since the blade sinks
down into the relatively soft ice.

What does produce the water film between blade and ice? There are two
plausible explanations. When the leading edge of a blade strikes the
ice, the resulting friction energy can melt a trail for the rest of the
blade. A more important effect stems from a phenomenon first noticed
by Faraday and then largely ignored because it was not understood.
We now know that at the interface between ice and air there is a fhin
film of water. The thickness increases from monomolecular to several
hundred molecules as the temperature rises from -10 to 0 C. Since the
reduction of friction depends on the the water film, you might conclude
that the fastest skating could be done on ice close to the melting point.
However, warm ice is soft ice, allowing the blades to sink in more. On
the other hand, cold ice, which is hard, has only a thin film of surface
water. These two competing effects yield a minimum of friction for
speed skating at about -7 C. (For further details. see James White, The
Physics Teacher, 30, 495 ((1992))."

Also note that any explanation must account for the very low friction
experienced by the puck. I hope this explanation will either satisfy
people or stir up further controversy!

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Date: Thu, 2 Jan 1997 04:33:35 EDT
From: Ludwik Kowalski <kowalskil@ALPHA.MONTCLAIR.EDU>
Subject: a myth about ice skating

Do you recall our exchange of ideas about ice melting under pressure? The
discussion was triggered by C. Swartz who wrote about the subject in his
new book. I finally had a day cold enough to perform the promissed
experiment.

A copper wire (diameter 1 mm), loaded with 4.5 kg masses on each side, was
placed over a cylindrical block of ice. First I did this indoor (+27 C)
then outdoor (+7 C). The difference was dramatic.

1. Indoor data. After about 20 minutes a groove below the wire was already
as deep as its diameter. An hour later the top of the wire was already
about 4 mm below ice surface and a layer of ice was formed above it
(regelation). The water was dripping. You probably saw this demonstration
or read about it.

2. Outdoor data. After one hour the wire was still sitting on top of ice
and no groove was formed. The same status at midnight, two hours later,
when the temperature was still +7C. The groove, if any, was too shalow
to be noticed. I left the wire hanging and went to sleep. The morning
temperature (at 7 a.m.) was still the same and I expected the wire to
be on top of ice, as I left it. But that is not what happened. The wire
was deep inside ice (penetration and regelation of 80 mm). The entire
surface was wet but the diameter of my ice cylinder did not change by
more than 2 mm. The wire would probably go through ice but the weights
reached the ground before this happened. It is unlikely that the outdoor
temerature went up for a short time during the night.

[an experiment in below the freezing air is described on Jan 21, below]

Conclusion? Swartz was right; lowering of the melting point of water by
pressure does exist but an enormous pressure is needed to observe A wire
penetration into ice is due to the conduction of heat along it, from the
warmer section below to the colder section above. The driving difference
of temperature was 27 C indoor and only 7 C outdoor.

During the first two hours the surface temperature of my block was still
below the melting point (ice, taken from the freezer at t= -8 C, was dry).
But during the night the ice temperature did come to 0 C and the wire
started to penetrate into ice (thermal contact improves considerably after
the surface becomes wet.) Progressive penetration was not observed but I
believe that its rate of was much slower than indoor. I predict that the
loaded wire will not penetrate ice when the outdoor temerature is less
than zero degrees C.

Here is a typical quotation from a textbook (Physics by A. Beiser, 3rd
edition, page 334). Ice "is unusual in that its melting point decreases
with increased pressure. Hence, it is possible to melt ice by applying
pressure to it as well as by heating it. An ice skater makes use of this
fact in an interresting way. ..." You know the story.... I used to say,
incorrectly, that "crystals are crushed to the liquid state by pressure".

My blocks of ice were made in common plastic soda bottles (2 liters) filled
with water and placed into a freezer for two days. I cut away plastic and
had nice cyliners of ice. Two tables was used to support the structure indoor;
outdoor it was supported by a simple wooden stand. A

IT IS FUN TO PLAY A SCIENTIST. A student project can easily be formulated
on the basis of a common textbook description of melting under pressure.

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Date: Fri, 3 Jan 1997 08:35:18 EDT
From: Ludwik Kowalski <kowalskil@ALPHA.MONTCLAIR.EDU>

On 1/2/97 Tom Gordon wrote:

I am reminded that as a kid I had once gone up to VanCourlandt Park early
on a V-E-R-Y cold Saturday morning in January, with borrowed skates. Some
big guys were there before usand were skating, but when we attempted to join
them , it was like trying to skate on concrete. We were told to wait 'til
the sun came out, We did, and we skated after a while. Colder needs more
pressure?

I tend to recall similar experiences. This can be interpreted in two ways.
Ether the coefficient of friction increases as the temperature goes down
or heat generated from friction is not sufficient to melt some ice. Can
somebody suggest a student activity which may help to decide which of these
two possible interpretations is correct?

In view of what I learned at our regional AAPT meeting, and observed now,
I no longer think that melting of ice below the blades is due to pressure.
The effects of the loaded wire on ice, at different temperatures, are easier
to interpret than ice skating because they are not dominated by frictiction.
One variant of a student-oriented project on this topic is to place two
equally loaded wires on ice at the same time. A copper wire should be
cutting through ice fasted than a wire made from a material which is less
conductive. Will this expectation be confirmed? What material would be
the best?

Use the Kelvin scale. What is wrong with Celsius? Ludwik
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Date: Mon, 6 Jan 1997 12:07:56 -0800
From: Nick Cabot <ncabot@HALE.SSD.K12.WA.US>

I thought the ice skating issue had been resolved once and for all when it
was noted that there is always a thin layer of water on ice. The
thickness of the layer is proportional to the temperature and varies
between about 3 to 40 atoms in thickness and exists as long as the
temperature is between about 0 and -10C. Something to do with vapor
pressure equilibrium at a surface. I thought it was you, Ludwik, who told
us this, but I guess not. Now I'll have to remember where I saw this.
**********************************************************************

Date: Sat, 11 Jan 1997 21:37:25 EDT
From: Ludwik Kowalski <kowalskil@ALPHA.MONTCLAIR.EDU>

On Jan 3 Nick Cabot wrote:

I thought the ice skating issue had been resolved once and for all when
it was noted that there is always a thin layer of water on ice. The ...

Real scientists can say this. But teachers who want students to play
scientists are in a different situation. ... At the AAPT meeting I did ask
a plenary speaker, an expert on the phase transitions of water, about the
reason of this thin layer of liquid on ice. He did not know but promised
to look into this and to let me know by e-mail. I will distribute his reply.

Somebody else came with a good suggestion for a student project on this
subject. Instead of performing experiments with one copper wire at two
different temperatures (as I did) one can use copper and nylon "wires"
side by side in a warm room. Same pressure under "wires" but the ice is
probably going to melt much faster below the Cu. Can somebody verify this?

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Date: Tue, 21 Jan 1997 09:42:52 EDT
From: Ludwik Kowalski <kowalskil@ALPHA.MONTCLAIR.EDU>

The outdoor temperature was below the freezing point and I did verify that
my copper wire (supporting two 4.5 kg weights, as described before) does
not penetrate ice at low temperatures (between -2 and -5 C). It has been
suspended over the block of ice for 20 hours. Recall that it took only
20 minutes to cut over several millimeters indoor (at +27 C).

The meaning is clear, melting of ice under a loaded wire, often demonstrated
at room temperatures, is not caused by pressure; it is caused by heat moving
along the wire. PLEASE DO NOT SAY "SO WHAT". A common textbook misconception
is a great teaching resource. Help your students discover that "this book
statement is wrong" and they will feel great about studying physics. What
is better to establish the "ownership" of ideas?

Are there other examples of textbook misconceptions which our students
can "discover" in high school and college physics labs? My repertoir is
limited to two (distribution of charges on capacitors in series and melting
of ice under pressure). Critiacal thinking is worth cultivating.
***********************************************************************

Date: Fri, 24 Jan 1997 11:22:13 -0500 (EST)
From: Sthammack@aol.com
To: kowalskil@alpha.montclair.edu
Subject: Re: Ice stuff

Ludwik,

Nice job on the ice experiments. The scientific method works!! Publish this
stuff in the context of providing a learning opportunity for kids. Bravo!
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