
Chemical
Education International, Vol. 2, Issue 1, 2627, Published in August
3, 2001
A
problem in learning Bohr's theory of hydrogen atom
Dileep V. Sathe,
Dadawala
Jr. College, 1433 Kasba Peth, Pune, MH, 411011, India
dileepsathe@vsnl.net
Every
student of science has to learn Bohr's theory of hydrogen atom in
the introductory courses of physics and chemistry, particularly.
Even students interested in biology also have to learn this theory
in the core courses of science. As this theory is known for nearly
90 years, the question is: Can we take for granted that, now, there
are no problems in learning of this theory? Sadly, the observations
of educationists made in the last quarter of the 20th century
show that there are some problems in learning this theory
because there is a global problem in the learning of circular motion
itself. What is that problem? Why is it still persisting? Why is
it global in nature? In the present note, author addresses these
problems. First let me summarize that difficulty itself.
The
Problem: Educationists have observed that when students are
asked, in questionnaires, to identify the resultant force, acting
on a body performing the uniform circular motion, most of them ignore
the centripetal force and choose the tangential force as the resultant
force. John Warren reported this observation in1971 and then many
educationists from various countries observed the same. So the problem
to be addressed by teachers of physics / chemistry, particularly,
is: Why do students tend to choose the tangential force and ignore
the centripetal, while analyzing the circular motion?
About
20 years ago, I was very much surprised to know this because the
circular motion is being taught and learnt for nearly 300 years.
More over, the observation is global in nature not local or
regional. Hence I decided to restudy the relevant concepts, topics
from the logical point of view. This approach is a unique one, even
today, but it could lead me to discuss my work with Prof. Abdus
Salam in August 1991 and to dedicate a lecture to his memory in
an international symposium on experimental gravitation, in Samarkand,
Uzbekistan in August 1999.
My
hypothesis: I have shown that the said tendency among students
originates in the fact that the present treatment of uniform circular
motion logically conflicts with other basic concepts like
the concept of work. In the concept of work, we teach students to
resolve the applied force into sine and cosine components, if it
is not along the motion or displacement. More over, we insist on
taking the cosine component along the motion and then include it
also in the equation of work. In other words, teachers and educationists
have to realize that students' choice of tangential force in the
circular motion is logically consistent with our choice of cosine
component in the equation of work. As long as this conflict is there,
students will continue to ignore the centripetal force and to choose
the tangential force. Secondly, there is such conflict between the
circular motion and the law of parallelogram forces also. The latter
preaches the logic that the motion has to be in the direction of
resultant force only but in the circular this does not happen and
hence students ignore the centripetal force. My hypothesis also
easily leads one to realize why this tendency is global in nature,
as the same physics is taught all over the world the ensuing problem
is also same every where.
Another
logical factor, which motivates students to choose the tangential
force, is the fact that we cannot decide the direction of motion,
anticlockwise or clockwise, on the basis of centripetal force. This
inability has been termed as the Anticlockwise/Clockwise paradox.
It has been discussed at several places and with Prof. Abdus Salam
also, starting with the first discussion in the ICPE Tokyo Conf.
in August 1986. I suggest readers of this journal to try the following
activity in their educational institutes.
The
Activity: Choose some students of colleges, from physics, chemistry,
biology and electronics and try to have same number of students
from each department say 10 students. Divide these 40 students
into two groups, A and B, and teach them Bohr's theory of hydrogen
atom. For this, draw the anticlockwise orbit of electron on the
blackboard in the classroom of group A and draw the clockwise orbit
of electron in the other classroom of group B. Then mix these groups
and see whether they accept Bohr's first postulate or not and ask
them to write their comments. In fact, I would like to know comments
of all interested students and teachers.
References:
#1
Sathe, Dileep V. (August 1991) The Evaluation of Teaching Uniform
Circular Motion, text of my talk in the GIREP Conf., Torun, Poland,
see p. 379 of the Proceedings. It contains my original questionnaire.
#2 Sathe, Dileep V. (July 1996) The Jupiter cometcollision: some
conceptual implications, text of my talk in the IAU Collo., London,
England, see p. 150 of the Proceedings. It explains how the overwhelming
interest of the public, in the collision of ShoemakerLevy comet
with the Jupiter in July 1994, has greatly increased the necessity
of considering my hypothesis earnestly.
Last
updated
16.05.02

