Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: California Science Framework



Some comments on #4...

4. "For example, students might learn about Ohm's law, one of
the guiding
principles of physics, which states that electrical current decreases
proportionately as resistance increases in an electrical
circuit operating
under a condition of constant voltage.

First of all, the example is used to illustrate the need for
"balanced instruction" between "direct instruction and
and investigative activities" (quoted from the "Introduction
to the Framework" link that Larry provided). I will comment
on this at the end.

Second, Ohm's law is presented as a relationship between I
and R (for constant V) instead of a relationship between I
and V (i.e., the ratio is constant).

In practice, the
principle accounts
for why a flash-light with corroded electrical contacts does
not give a
bright beam, even with fresh batteries.

This misrepresentation of Ohm's law is illustrated by the choice
of the example. Here we are comparing two situations. Both
flashlights have the same voltage (fresh batteries) but one has
corroded electrical contacts and so, we assume, a higher resistance.
According to the stated law, the higher resistance (at the same
voltage) leads to a lower current.

It is a simple relationship,
expressed as V=IR, and embodied in high school Physics
Standard 5.b.

I won't comment except to point out that Physics Standard 5.b
(for grades 9-12) is "students know how to solve problems
involving Ohm's law" (from
<http://www.cde.ca.gov/board/pdf/science.pdf>).

In a
laboratory exercise, however, students may obtain results that seem to
disprove the linear relationship because the resistance of a
circuit element
varies with temperature.

Apparently, they are now referring to the idea that the ratio of
V to I is independent of V (Ohm's law), not the idea expressed
earlier (that I and R are inversely proportional). After all,
I and R would still be inversely proportional (for ohmic
materials, that is) regardless of the temperature (assuming the
resistance is determined by a ratio of V/I).

The temperature of the components gradually
increases as repeated tests are performed, and the data become skewed.
In the foregoing example, it was not Ohm's law that was wrong but an
assumption about the stability of the experimental apparatus.

The "assumption" they are referring to is the assumption that
only V is changed (all other things are held constant). Changing
V might lead to a change in T which would, if not taken into account,
appear to invalidate Ohm's law.

This
assumption can be proven by additional experimentation and provides an
extraordinary opportunity for students to learn about the
scientific method.

In other words, further investigations into whether all other
variables really were held constant would reveal that was
not the case.

Had the students been left to uncover on their own the
relationship between
current and resistance, their skewed data would not have
easily led them to
discover Ohm's law.

The argument appears to be that, due to the likelihood of
these complications creeping into investigations, students
would be unable to "discover" Ohm's law on their own, i.e.,
the "discovery" method would be inappropriate for meeting
this standard.

A sensible balance of direct instruction and
investigation and a focus on demonstration of scientific
principles provide
the best science lesson."

I believe the implication is that Ohm's law must be provided
via direct instruction because students would be unable to
"discover" the relationship on their own (due to the many
complications that could arise). Investigations can then
be used to demonstrate ohm's law.

This conclusion supports the initial statement (at the beginning
of the section) which is that "some knowledge is best learned
by having students read about the subject or hear about it
from the teacher" (quote taken from Larry's link).

Regardless of whether I agree with their conclusion (I don't)
and regardless of their confusion about what Ohm's law is,
I find the use of this example as support for their conclusion
quite misguided and a bit frightening.

____________________________________________
Robert Cohen; rcohen@po-box.esu.edu; 570-422-3428; http://www.esu.edu/~bbq
Physics, East Stroudsburg Univ., E. Stroudsburg, PA 18301