New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
words, in real time. Students can observe immediate changes if an ice cube is placed on the
collector plate. One of the greatest contributions to the intuitive understanding of the temperature
changes in such a system is to see, in real time, the asymptotic behaviour of the temperature
of the chamber as it approaches its maximum value, or returns to room temperature. Students
could also build more complex collectors and monitor temperatures in a number of locations.
Their basic challenge would be to modify the transducer system and write additional program
steps (software).
A second example of the value of having the patience and persistence of a microcomputerbased
laboratory instrument would be using it to monitor the period of a pendulum. In most
laboratory experiments, students are asked to determine carefully the period of a pendulum by
counting a number of swings and measuring the total time period for that number. Using the
period and the length of the pendulum, the value of g may be found. With the microcomputerbased
instrument, each period of the pendulum can be recorded over many swings, clearly showing
any decay in the period if it occurs with time. One can then answer such questions as: does
the period of the swing actually depend on the amplitude or ‘small angle’? or can air resistance
play a role if the surface character of the mass is changed? or does mass really play a role? When a
microcomputer is used as a data storage device, 256 separate periods or more may easily be
recorded and retrieved for later study. A student who is able to test intuitions about the effect
of mass, the angle of swings, and the effects of air resistance, as well as determine g, certainly
has had a thorough introduction to the pendulum.
What is required to do this kind of measurement and why would we choose a microcomputerbased
instrument? Building on the pendulum example, the first element required would be a
transducer which would be a light source (infra-red diode), a light detector and simple electronics
to produce an output voltage proportional to the light level. The second element would be an
interface between the transducer and the microcomputer so that the analog signal from the
transducer can be converted to digital information that the microcomputer can deal with. This
is called a laboratory interface board. The third element, and indeed the newest element, is the
microcomputer itself. It has the capability of following a stored set of instructions called a
program, sometimes called software, that controls the operation of the whole system. It can bring
about the analog to digital conversion, provide the timing for the system operation, provide the
time base for storing the successive individual periods of the pendulum and provide the means for
retrieving the stored data for student analysis. The final element would be an output device to
read out or display the measured periods of the pendulum. The cost for a single board microcomputer
(KIM I) [21, a Laboratory Interface Board, a power supply and a transducer and its
associated electronics would be less than $600 (1 981 price).
The power of the microcomputer-based laboratory approach lies in the fact that it will not
only monitor and record successive periods of a pendulum, or the successive temperatures of a
solar collector, but any physical process or processes amenable to general instrumentation. Thus
the same basic laboratory instrument can serve a wide variety of experiments. All that is required
is to change the transducer and the program which runs the system. In the past, it meant changing
devices entirely, which required having a variety of separate and costly instruments.
A number of other laboratory applications have been developed for the AAPT workshop.
These include the ability to move a photo-diode attached to a linear potentiometer across an
interference pattern and have the amplitude versus position graph appear on the oscilloscope.
The patterns can be quickly changed and new graphs produced. The detector could be changed
to allow studies of infra-red, ultraviolet, or ultrasonic interference patterns where the pattern
itself would not be visible. Temperature sensors may be placed along a conductor to show heat
transfer and thermal gradients, and to measure R values of insulating materials, etc. An optical
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