I wrote about Alice a couple of weeks ago, and
reported a couple of the things I found out about
her thinking when she talked to me. Another point
that really interested me concerned an example of
what I have referred to as a 'fragmentation'
learning impediment - in effect not making the
link that the teacher might expect or wish.

Alice explained the 'party trick' of sicking a
balloon to a wall in terms of the relative charge
of a neutral object in relation to a charged one.
(This is not valid, of course, but presumably
derived by analogy form the relative way we talk
about potential). So a balloon can be attached to
a neutral wall because it was relatively positive
(or negative).
Alice proposed that "because you've got opposite
charges, you've got the say negatively charged
balloon, and then your positively charged wall".
Although the wall "hasn't had anything done to it
as such", Alice suggested that "maybe in
comparison to your very negatively charged
balloon, it's still likely to attract." Alice
agreed that she was suggesting that "it's
relative", that because the neutral object is
positive by comparison with the negative object,
they're effectively both charged.

It is not so surprising that Alice could not
explain why the balloon stick to the neutral wall
- until we consider her explanation of
intermolecular forces: she explained the
integrity of a solid in terms of "strong enough Š
intermolecular forces holding things together".
She suggested that these forces might be "van der
Waals' forces" which were where "you've got if
you like an electron cloud between, surrounding Š
each molecule, and as these clouds don't stay in
one fixed place, there's always going to be sort
of momentary areas of dipole. And that's where
you get your positive and negatives attracting
each other again."

So in the context of intermolecular bonding,
Alice discussed how neutral species could be
attracted due to induced dipoles. However, she
did not consider a possibility along these lines
to explain how the charged balloon could somehow
have an attraction with a neutral wall. Here the
potential linkage was missed.

This is quite an abstract example, but how often
do students fail to make the links (a
fragmentation learning impediment -
http://people.pwf.cam.ac.uk/kst24/Learning%20doctors%20guidebook%202006.pdf)
that we may readily assume are so obvious when
teaching?

Keith



--
Dr. Keith S. Taber
http://people.pwf.cam.ac.uk/kst24/
University of Cambridge Faculty of Education
http://www.educ.cam.ac.uk/staff/taber.html

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