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Didaktikogenic Misconceptions in Physics: An Example
Dhrubajyoti Chattopadhyay
Educational Officer
North Bengal Science Centre
This article deals with situations where textbooks actually
promote misconceptions in physics students. The particular
case of weighing a filled balloon is taken up. An experimental
approach to exploring this topic is described, bringing home
the fact that because of Archimedes’ principle, one cannot
weigh air in a container surrounded by air at the same
pressure. This example is used to make some general points
about addressing misconceptions.
P.O. Matigara, Siliguri Dist.,
Darjeeling 734 010, India.
Email: [email protected]
Introduction
In medical science, there is a term ‘iatrogenic disease’, which
means an ailment caused by the doctor. Similarly, in schools and
colleges, ‘teacher- and textbook-caused misconceptions’are called
‘didaktikogenic misconceptions’. The term‘didaktikogenic’, or
‘didaskalogenic’, is a new term in the field of science and is
derived from the Greek: dáskalos or did-as’-Kal-os means teacher,
and ‘genic’ means induced by. Such misconceptions can arise at
any level of formal education in any subject. They are more
pervasive than we realize. Science subjects are particularly more
vulnerable in this matter as they require more analysis and deduction rather than mere information gathering. In order to eradicate
these errors in science, science teachers along with non-formal
institutes such as the Science Centres must play a vital role.
Where Do These Misconceptions Come From?
Richard Feynman, in his famous book, Surely You’re Joking, Mr.
Feynman, wrote, “That’s the way all the books were: They said
things that were useless, mixed-up, ambiguous, confusing, and
partially incorrect. How anybody can learn science from these
books, I don’t know, because it’s not science”. There are indeed
many textbooks that are mainly responsible for scientific
misconceptions, which often arise through the use of inappropriate
analogies in these textbooks as well as during the course of
instruction.
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Keywords
Weight, weightlessness.
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As long as a
misconception
‘seems to work’ or
at least ‘doesn’t
fail’, it will persist.
Examples of such misconceptions include the idea that breaking
of a bond can release energy, when, in fact, all bonds require
energy to break; the depiction of chemical reactions using nonrandom molecular motions; the concept of electron orbitals, which
we know from quantum mechanics is incorrect; and the molecular
level effects of mutations on organismic phenotypes.
Sometimes, misconception comes from slogans, such as ‘action
equals reaction’, ‘every effect has a cause’ and ‘nature abhors a
vacuum’. These encour age super ficial thinking. Many
misconceptions arise from superficial ‘explanations’ that don’t
examine details.
Why do these persist in formal education? The principle of
positive reinforcement is at work here. The student’s misconception
is never challenged by an examination, experiment, or homework
problem. As long as a misconception ‘seems to work’ or at least
‘doesn’t fail’, it will persist. Misconceptions that are never put to
the test will persist. Misconceptions that are rewarded will also
persist. Many exam questions allow a student to use misconceptions
to get the ‘right’ answer. It almost seems as if those who set the
question papers share the same misconceptions that the students
have, or at least are blind to them. Many misconceptions arise
from superficial ‘explanations’ that don’t examine details.
How Can These Misconceptions Be Eradicated?
Teachers can play a vital role to eradicate the very root of
misconceptions from the minds of students. Instead of the ‘chalk
and talk’ method of instruction, if they arranged experiment-based
teaching methods for science subjects, it will help create rational
minds. Institutes such as the Science Centres, impart science
education mainly through hands-on activities, most of which are
experiment based. So the student has the opportunity to ask why
and how at each and every step of this learning process. Moreover,
there is no exam fear as it is non-formal and they are free to ask
questions. The National Council of Science Museums has
developed innovation hubs where this type of opportunity will be
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more widely available to students. Teachers may arrange regular
visits to these science centres.
Weight of Air
Here I am going to discuss in detail one such didaktikogenic
misconception in physics and how to overcome it. This is the
most common misconception that is introduced to students at an
early age. Let us follow few steps to find out the nature of
misconception and how to get the solution.
Step 1: Let us take a small balloon and a rubber band. Now with
the help of a chemical balance, their weights are measured and
noted. Let the total weight be x mg. Then, the balloon is filled
with air and the opening is tied up with the rubber band. After
that, the weight is measured again. Now, the weight has increased
to, say, y mg (y>x). (See Figure 1.) The question asked to the
students is, ‘Why does the weight increase?’ Almost all of them
will answer,‘It is due to the weight of the air’. To the student it
appears that the weight of air has been measured. They will not
question whether we can measure the weight of air in this way or
not; in fact, it is not possible to get the weight of the air in this way
as the buoyant force of air on the air-filled balloon acts upwards,
cancelling the weight of the air in the filled balloon. However,
from textbooks and this kind of experiments, students have
gathered this type of misconception.
Figure 1. Many textbooks
use this type of picture to
show that air has weight.
Step 2: Now, to show students that
this is a misconception, the same
experiment is repeated with a plastic
carry bag, instead of a balloon. The
plastic carry bag is bigger than the
balloon. The students will be surprised
when they observe that in both cases
(the empty plastic bag and air-filled
plastic bag) the weights are the same.
The students will not be able to give
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the reasons behind this. As the plastic bag is bigger than the
balloon, it is expected to contain more air and so weigh more. But
the practical experiment shows that there is no change in weight.
Step 3: Now, in this step, the same experiment is done under
water. First, the weight of a plastic bag and rubber band inside
water (we have to add an extra weight to the plastic bag so it
remains under water and does not float) is taken. The bag is then
filled with water and its weight is measured. There will be no
change in weight. If we ask the students the question why we have
failed to measure the weight of water in water, a few of them will
be able to explain why the weight did not change. Those who
know Archimedes’ principle will say that it is not the real weight,
but it is the apparent weight. As we know, the apparent weight
equals the actual weight minus the weight of the displaced water
of the same volume. Here, the actual weight and the weight of
water displaced is the same; so it cannot be measured by the
common balance or chemical balance. Hence, it will be clear to
the students that the weight of air taken in the air is not its actual
weight; rather it is its apparent weight in air and that it is not
possible to take the weight of air in the air in the conventional way
as we do in our day-to-day practice. The way many textbooks
show this experiment to prove that the air has weight is wrong.
Figure 2. Pressure acts
equally in all directions
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Step 4: Now, after the experiment, the question that comes to
mind is, ‘Why is the balloon filled with air heavier than the empty
balloon?’ It is due to the elastic property of balloon – the balloon
will try to regain its original shape, which creates pressure on the
air inside it (Figure 2). As a result, the balloon contracts to some
extent and thus, air inside the balloon will be compressed to some
extent and its density will increase. Air inside the balloon is
denser, and it displaces an equal volume of less dense air, and
hence buoyancy is less than the weight, and the apparent weight
seems to be more than before. The case of the air-filled plastic
bag is different because the bag is not elastic, and hence there is
no apparent weight gain.
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More on Weight and Weightlessness
Much confusion arises from the way some textbooks define
‘weight’ inconsistently in different situations. For example, in the
chapter on Unit and Dimension, most of the books define weight
as ‘product of mass and acceleration due to gravity’, i.e., W = mg
at the surface of the earth.
Again, in the chapter on fluids, when discussing Archimedes’
principle, students are taught that the ‘weight of a body in air’ and
the ‘weight of the same body immersed in a liquid’ are not the
same. The difference between these two, called the ‘loss of
weight’, is used in the calculation of the density of the body. But
the gravitational force on the body in the lab is the same whether
it is immersed in air, or in any liquid. It does not ‘lose weight’ in
the ‘W = mg’ sense in these experiments.
Then, in the chapter on Gravitation, an astronaut in a space shuttle
orbiting a few hundred miles above the earth’s surface is said to
be experiencing ‘weightlessness’. A simple calculation shows
that the gravitational force at that height (200 miles) is only about
6% less than at the earth’s surface; so by the above definition,
weights of objects in the shuttle are also only 6% less than on the
surface of the earth. To explain this weightlessness we consider
the satellite as a free-falling body, i.e., it is falling with an
acceleration of g, and hence its effective weight w = m (g – f)
becomes 0 as f = g in this case. But we never consider that if a
satellite is a free-falling body with respect to earth, then the earth
Figure 3. W eight in air and
weight in water – both are
apparent.
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too is a free-falling body with respect to the sun and so on. In fact,
in that sense the entire universe is a free-falling body.
To avoid this confusion, we may say that weight is not a
fundamental property of a substance. It may be defined as ‘the
amount of force required to support a body in equilibrium in its
rest frame’.
Suggested Reading
[1]
Clifford E Swartz and Thomas Miner, Teaching Introductory Physics, a
Sourcebook, American Institute of Physics, 1998.
[2]
J W Warren, The Teaching of Physics, Butterworths, 1965.
[3]
https://www.lhup.edu/~dsimanek/scenario/miscon.htm.
[4]
Richard Feynman, Surely You’re Joking, Mr. Feynman, W W Norton &
Co., USA, 1997.
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