Re: Critical Thinking

[Richard Langer <rlanger@slps.k12.mo.us>]

At 08:22 PM 1/21/97 EDT, you wrote:
> 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
> supported by the block of ice for more than 24 hours. 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 traveling 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. 


I applaud your efforts to "debunk errors in the textbook" - I am more
interested in teaching critical thinking than in teaching PHYSICS.

BUT - doesn't the classic loaded wire at room temperature involve the ice
refreezing above the wire?  If the wire merely conducted heat into the
block of ice, where did that heat go to so that the ice could refreeze?
Did we merely increase the temperature of the ice?  

Is it possible that the effect of pressure is to LOWER the freezing point,
and that when you  did the demo outside at -5 C, the wire only lowered the
freezing point to -3 C??

If we go ice skating, do you believe that there is a layer of liquid water
between our skate blades and the ice?  But we are outside - where did the
heat that the metal blades conducted to the ice come from?

Is there significant heat going into the ice?  How does it get there?  I
suspect conduction - can the loaded wire experiment be repeated with a
NON-CONDUCTING wire?  Perhaps nylon monofilament fishing line?  Worth a
shot in my opinion.

I look forward to your reply
Richard M. Langer                  Gateway High School,
Physics Teacher                    5101 McRee Ave.
rlanger@dtd1.slps.k12.mo.us        St. Louis, MO 63110