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Re: Pb



Replies to various replies:

Orig. Post

I drop a steel ball onto a one of
the temperature sensitive color changing post cards (the Exploratorium
sell these). The card must be on a hard floor. A color change occurs at
the point of impact. Pressing the ball against the card and floor with
great force and effort causes no change in color. Dropping the ball on a
piece of wood with the card in between also causes no change in color.
Demo leads to great student discussion regarding where energy is being
transferred.

"John S. Denker" <jsd@MONMOUTH.COM> wrote:

That's a tricky demo, and requires a very special discussion lest the
students draw diametrically-wrong inferences from it.

First of all, there's no reason to believe the card measures energy "being
transferred". The card measures temperature, which is something quite
different.


Whoops, I forgot about the sacred heat cow on this list.

We tell students that GPE changed to KE during the fall, but I wish to show that
after the fall there is still evidence of energy. The energy "goes" (verb of your
choice) to another form. Temperature is _evidence_ of the increase in thermal
energy not the energy itself. Footprints are not feet

Secondly, I suspect the measurement causes a pretty big perturbation in the
collision phenomenon being measured. I'll bet the steel ball hitting a
hard surface bounces markedly higher without the card than with the card.


Yes, but not much.

In this situation, we have neither the ideal inelastic collision, nor the
ideal elastic collision. That's fine; it's good to look at non-ideal
situations. My concern is that the card measures only one of the
nonidealities, while other small effects (and indeed big effects) go
unmeasured.


I wish to show that the energy did not just disappear. This demo provides
_simple_ evidence that a change did occur and something is still moving even if
it cannot be seen or felt directly.


If the objective is to demonstrate energy transfer, aren't there better
ways to demonstrate that? (Carts on an air track come to mind.)


How do I get a tangible result? From a student's point of view, a pasco cart loses
speed after each bounce, but nothing is noticeably warmer.


If the objective is to demonstrate energy dissipation, aren't there better
ways to demonstrate that? (A nice steady rubbing motion comes to mind.)


OK, but I still need to convince HS students of where the GPE ends up. (By the
way one student countered that rubbing the hands together was simply concentrating
his body heat, no work was done; amazing what they can come up with )



Pete Lohstreter <plohstreter@MAIL.HOCKADAY.ORG> wrote:


Doesn't the impact damage the micro-encapsulated liquid crystal?


You bet. This demo eventually destroys small sections of the card.


Are you
sure you are looking at a temperature change and not a physical damage
artifact?


No not at all, which is why I brought it up.


brian whatcott <inet@INTELLISYS.NET> wrote:

I suspect that this interesting demonstration may possibly be
sailing under false colors. I understand that temperature
indicating cards feature a thin film of temperature sensitive
liquid crystal. If you apply enough pressure, I fear that the
liquid will be squeezed out with a conseqential color change
no matter what the temperature.


A (2-3 mm dia) gray spot appears at the point of impact. A blue ring appears
around this and disappears as the card cools. The gray spot will no longer change
color. The area around the spot will still change color. Something must of made
the card warm enough to cause the color change in the outer ring.


As a check, using a press to slowly apply pressure from a cooled
ball onto such a card might demonstrate some interesting color
change also.



I have tried manually to make the color change by pressing against a hard surface
( no change) , but I do not have access to a serious press. I would like to see
the result. Would the color change even be for the same reason?


Thanks for the input,

Scott



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*****************************
S.Goelzer
Physics Teacher
Coe - Brown Northwood Academy
Northwood NH 03261
603-942-5531 ext43
sgoelzer@coebrownacademy.com
*****************************