PSSC: Instant Credibility
for a Beginning High School Physics Teacher
by John W. Layman[*]
I began my high school teaching in 1958 just at the time
that PSSC was emerging. Even though I had not encountered
the materials prior to my teaching, I was immediately struck
with the laboratory program, the complete outlines of
proposed course scheduling, recommendations for limited
coverage, and the availability of the carefully integrated
ancillary materials. There was also a statement suggesting
that “the most common use of the laboratory experiments is
to introduce a topic or contribute to the early stages of
its development”. My students found that they could read and
understand the textbook after they completed their lab work.
The Laboratory
Program
I was so struck with the ripple tank that even though I had
no budget for materials I scrounged old window sash from my
uncle’s lumber yard and began using these after repairs for
leakage.
The major feature of the labs was the informality of the
text aimed at the users. There was no evidence of a formal
laboratory procedure, just suggestions and questions with
enough guidance to allow students a good start. The formal
terminology of physics was used, but never in a tone of
“this needs to be memorized.” The “To Students” section
began with “This Guide is designed to help you with your
laboratory work.” Other elements in the “To Students”
section were the need for good working habits, keeping clear
records, working with partners, and recognizing the need for
additional experimentation, with the caveat of “It is up to
you to decide what to do in each case.”
The use of Slinkys (referred to as coiled springs in the lab
guide) enabled us to move into the hallways to accomplish
our physics as well as advertise PSSC. I had participated in
the design of the physics space in the new high school, and
we had a steel beam above the lecture table and one at right
angles to its direction at the back of the classroom. We
could string a cable between the two, with a number of
Slinkys tied together to constitute a master Slinky to be
used during class to resolve Slinky questions during class
discussions and laboratory work. It also provided
opportunities for genuflections as students entered the
classroom, to recognize a small legacy of earlier pagan
activities.
“Large Distances” allowed us to go to the athletic field, in
clear view of students in other classes, to measure the
distance from our hilltop to a radio station antenna in
downtown Kansas City.
“Molecular Layers” allowed me to visit chemistry and other
classes to advertise physics, guaranteeing students the
opportunity to measure the height of a molecule with a meter
stick, measure the mass of an electron, and guarantee them a
C for just coming through the classroom door, and assuring
them of wonderful physics activities that would enable them
to then earn an A, B, a D or an F depending on their
willingness to work. I had scaled the grading scheme in such
a way that students doing the labs would achieve enough
points to guarantee a C in the course.
Skate Wheel
Carts
The skate wheeled carts with spring enclosed pushing rods
were wonderful for calculating the energy stored in a spring
by upending the cart, loading masses on the rod, and of
course monitoring the resulting displacements due to Hooke
and his law. We devised our own experiment to compare the
potential energy stored in the spring with the kinetic
energy acquired by the cart when the tube with the spring
was forced against a stopping block C-clamped to the end of
the table, released from a specific compression mark, and
tracked with a ticker-tape timer. We stressed the fact that
these were totally independent measurements of energy – the
potential energy stored in the spring and the ultimate
kinetic energy of the cart the moment the spring became
uncompressed. Some noticed that the push rods moved beyond
their compression points. What to do about that? Other
characteristics within the experiments allowed students to
create explanations that would help account for the observed
potential and kinetic energy differences.
Successive
Experiments
The laboratory program also provided wonderful sets of
successive experiments ultimately enabling students to carry
out a final fundamental measurement of something of great
sophistication and significance using relative simple
apparatus. One of the prime examples culminated in students
being able to measure the mass of the electron. The series
began with “The Magnetic Field of a Current”, enabling
students to measure the influence of the number of turns of
wire surrounding a compass, with a constant current in the
wire. The second experiment “The Measurement of a Magnetic
Field in Fundamental Units”, introduced the students to a
coil of wire (now recognized as a solenoid) and a current
balance to measure the magnetic field in the center of the
coil. The culminating experiment involved using the solenoid
to produce a uniform magnetic field in which was placed an
electron tube or tuning eye, something our younger readers
will not know about. In the absence of any magnetic field
electrons emitted from a central cathode pass by deflecting
electrodes that form straight-line shadows on the
fluorescent surface of the anode where the electrons are
collected at the end of their paths. Because of the anode’s
curved surface, it intercepts on its lower edge the
electrons that were closest to the cathode, while it
intercepts on its upper curved surface the other electrons
that traveled further from the cathode. With no magnetic
field in the coil, the shadows produced by the deflecting
electrodes are straight lines. When a uniform magnetic field
is applied, the uppermost-emitted electrons, which traveled
further to reach the surface, are deflected further around
the circle of the curved path of the electrons in the
magnetic field. The edge of the shadow is now curved.
Students knew the strength of the uniform magnetic field
B as a function of the solenoid current and the strength
of the electron accelerating voltage V; they used a
transparent disk with lines of various curvatures to measure
the radius R of the circular path of the accelerated
and deflected electrons. Taking the charge of the electron
q as a known quantity, they could calculate the mass
of an electron: m=B2qR2/2V
Total Internal
Reflection: Water and Microwaves
My last example of a major contribution of one of the
extraordinary PSSC laboratory experiment series harkens back
to a contribution derived from the already mentioned ripple
tank series. It is a contribution that strengthened an
understanding of both the teacher (me) and my students, and
is tied in with the Feynman Lectures on Physics.
In our ripple tank work we could support a rectangular glass
plate just beneath the water surface to create a shallow
region surrounded by a deeper region. Plane waves
approaching from the deep to shallow area normal to the
shallow region, clearly passed on through, but more slowly
with reduced wavelength, regaining their speed and
wavelength as they passed back into deeper water beyond the
opposite face. With a triangular plate, the plane waves
coming into the side of the prism would change wavelength
and arrive at the back surface at an angle of incidence
greater than the critical angle, thus total internal
reflection was clearly visible with the waves emerging from
the opposite face, with none emerging beyond the longest
side of the prism.
I had purchased a microwave source and had a number of diode
detectors. We could do excellent single, double slit
experiments as well as interferometer experiments. We had
also studied both optical and ripple tank observations of
total internal reflection but it was always a conundrum as
to how a wave knew when to totally internally reflect
(remain inside the denser medium), when there had been only
a change in the angle of the incidence of the approaching
wave. We surmised that somehow the wave had to sample the
second medium to know that it should totally internally
reflect.
In The Feynman Lectures on Physics I read that
the wave had indeed to sample the second medium and in fact
should be found in the second medium. It was pointed out
that the wave intensity in the second medium would tail off
within a single wavelength distance. I was reading this at
home one evening and realized that we had used paraffin as a
good medium for microwaves and were also dealing with
microwave radiation with significant wavelength. When I got
to school the next morning I immediately got some of my
student assistants to fashion two right angle prisms made by
stacking together some of the our ripple tank paraffin
blocks. Two things happened when we sent microwaves in
normal to one of the smaller sides of the prism. First, it
was easy to detect the total internal reflection of the
microwaves by placing a detector in front of the opposite
small side. However, when we moved another detecting diode
near the face of the prism where the total internal
reflection was occurring, a signal could be detected close
to this surface. The waves (microwaves in this case) were
indeed sampling the medium outside the paraffin surface
where the total internal reflection was occurring.
This was further evidenced when we moved a second right
angle prism with its long face parallel to the long face of
the first prism, closer to the first prism. When the
separation of the second prism face became less than one
microwave wavelength the total internal reflection within
the first prism disappeared, and in fact the microwaves
could now be detected coming through both prisms, without
any total internal reflection. There were two detectors, one
beyond the opposite face of the first prism providing
evidence of microwaves emerging via total internal
reflection, and beyond the second prism where no microwaves
were detected. As the second prism was moved closer to the
first, students could observe a gradual reduction in the
total internally reflected waves and a strengthened emerging
of waves from the opposite side of what became a cube of
paraffin.
We had two sets of observations to support our supposition
that the microwaves did indeed spend time sampling the air,
so they knew it was all right to totally internally reflect
from the first prism’s longest face. This experiment
remained in the repertoire of Oak Park High School for the
rest of my high school teaching career, and I brought it to
the University of Maryland when I changed levels of
teaching.
The Science
Study Series
The Science Study series played a major role in a strange
required-but-not-graded physics assignment. During each
semester I required one outside reading report per month,
one of which had to be a book. We had many of the Science
Study Series books as resources. The report consisted of one
3 by 5 card on which the student was to provide
bibliographic information, a few sentences about the theme
of the article or book, and finally a brief judgment of how
and why the student liked or did not like it. I merely
collected these and checked them as done. Lest you think
that was a fairly thin expectation, I found that on the free
response portions of the examinations students would often
make reference to an article and its relevance to the topics
that we were doing in the course. This increased as the year
progressed and provided the students with personal evidence
that our physics studies enabled them to increase their
understanding of the books or articles. Some students read
more than one book per semester, some not within the Science
Study series. Scientific American and The Physics Teacher
were also major resources.
Closing
I am struck with the philosophy and learning conditions and
expectations visible within the PSSC program that mirror our
present expectations bolstered by many additional years of
research into learning and teaching. Phrases such as: “The
student is expected to be an active participant in this
course”; “Hence, the most common use of laboratory
experiments is to introduce a topic or to contribute to the
early stages of its development”; “To this end, the
Physical Science Study Committee judged it wise to shift the
emphasis in secondary-school physics away from technology
toward a deeper exploration of the basic ideas of physics
and the nature of inquiries that can lead to these ideas”
and “Achieving these aims in a one-year course meant that
coverage of the field of physics had to be sharply
restricted in favor of a deeper development of ideas that
are central to a comprehension of the fundamentals of
contemporary physical thought.”
PSSC was the single factor that got me off to a fine start
in my early days of high school teaching.
[*]John Layman, East
High School, Kansas City, MO; North Kansas City
High School; Oak Park High School, 1968. Now
Professor Emeritus, University of Maryland.
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