I have found each of these misconceptions in science
text books, and other presumably
authoritative sources. It is extremely difficult
to “unteach” ideas which have been
learned and reinforced in earlier years.
These myths originate from failure to understand the
concepts behind factoids that have
been memorized from time to time, and lack of effort to
attempt to connect the pieces
together to form a unified, overall scheme. Of
course, for beginning students, science
concepts need to be simplified. Unfortunately,
this often leads to oversimplification to
the point of falsehood. While simplifying, the
teacher needs to keep in mind the “big
picture”, and insure that the simplified beginning can
be easily built upon later to develop
the full complexity of the concept without requiring
“unlearning”. Students need to be
made aware that they are learning a simple framework
into which the details can be fit
later. Exposing students to a level a bit beyond
that which they are expected to master, is
not a bad idea.
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Being accustomed to memorizing
isolated factoids, few noticed that this assertion implies
a knowledge of every snow flake
that ever was and ever will be. Nor did many think of
the statistical improbabilities
involved. I have heard those who should certainly know
better try to salvage this myth.
“If you consider their molecular structure, then no two snow flakes are the same shape.”
In that case, no two ping -pong
balls are the same shape either. Nor ball bearings . Can
you live with that? If so, what’s the point in singling out snow
flakes? Why not grains of sand, drops of water, or whatever?
This myth requires a combination
of prescientific notions, as well as reversal of
cause and effect relationships.
First we need to know what the words, “warm” and “cool”
mean. If you ask your
students, you will find that most of them think of them as specific,
rather than comparative,
temperatures. By probing you can
establish that, to most students,
warm is a temperature higher than
the observer’s skin temperature, and cool is lower than
the observer’s skin
temperature. And of course skin temperature changes.
From this you can reinforce
that “warm” is comparative by pointing out that during a
Minnesota winter, the warm air
mass may have a temperature of -10o Fahrenheit,
because the cold air mass has a
temperature of -15o F. In a Death Valley summer,
the cold air may have a
temperature of 115o F, because the warm air is at 120
o F.
In any case, air, no matter
what its temperature, always sets firmly on the support
underneath it! In a sealed
container, no matter how hot the air, the force exerted by the air
will always be greater
against the bottom of the container than against the top, the fluid
pressure (equal in all
directions) plus the weight of the contained air (on the bottom only).
Students generally accept,
without having thought about it, the naive pre-Newtonian
principle of levity
(antigravity). Those substances with levity naturally fall
upward, away
form the center of the universe,
those with gravity fall downward, toward the center of the
universe. Air is assumed to
be nothing. Hot air rises because it IS heat, and heat has
levity.
In fact, for warm air to move
upward
it must be pushed, lifted, against its weight
(gravitational attraction for
Earth), by something else. To create a conceptual
understanding of the cause and
effect relationships involved in the process called
convection requires understanding
several component concepts. Generally text books and
teachers “simplify” the
explanation in not more than a paragraph, to save time and space.
This is then reviewed in single
sentence form, which is what the student memorizes as the
complete explanation. The
highly abbreviated version leaves the student with the
impression that warm air somehow
pulls itself away from Earth leaving a vacuum.
Water evaporates from the
ocean. The water vapor rises in the air. The wind
blows the water vapor over the
land. When the water vapor is cooled below the dew point,
it condenses to form
clouds. When the clouds become heavy enough, it precipitates as
snow or rain to the ground.
Then it flows back to the ocean, where it evaporates again.
(Though often more complete than
this, it ends with an “Oh, WOW” factoid.) “Since the
same water evaporates and
condenses over and over, we have the same water on Earth
now that has always been
here. So, the water you drink is the same water that the
dinosaurs drank!”
These people have failed to
connect the various factoids they learned in school
together. Specifically, the
chemistry of a candle flame, respiration, and photosynthesis,
with
the water cycle.
We’ve known for 200
years that the net chemical equation for photosynthesis is:
6H2 O + 6CO2 >
C 6
H 12 O
6 + 6O 2 Since the 1950’s we have known that the oxygen
released by photosynthesis is
split from the water molecules. Notice that both water and
carbon dioxide molecules have
been destroyed in this reaction. Each molecule of O 2 in the
air, or dissolved in water,
represents two molecules of water destroyed.
In chemistry we learned
that
most fuels, such as natural gas contain both carbon
and hydrogen. When burned,
combined with oxygen from air, the products are carbon
dioxide and water. New
molecules of water and new molecules of carbon dioxide have
been created.
If the rates of creation and destruction of
water molecules were exactly balanced,
there would be little or no O2 in the atmosphere, and
no fossil fuels. Using some
simplifying assumptions, I have estimated (a very rough
ball-park figure) the half life (the
time it takes to destroy by photosynthesis, half of all
the water present at a given time) of
water molecules on Earth to be about 2.5 million years.
(Can you provide a more
authoritative estimate?) In that case, of the
water you drink, one molecule in 224
( 17,000,000) will have an age of 60 million years or
more. So I guess its fair to
say, “Some of the water you drink may have been drunk
by a dinosaur.
This myth resulted from
abbreviating a description of the oxygen cycle, and then
disconnecting a few factoids
which are then misinterpreted. When carrying on
photosynthesis, plants remove
carbon dioxide from air (or water), and release oxygen into
air (or water). In
respiration, organisms remove oxygen from air (or water), and release
carbon dioxide into air (or
water). These statements are true, individually. But when
taken as a complete treatment of
a cycle, it is totally false. They are fragments of the
oxygen cycle, and the carbon
cycle. Carbon dioxide does not have a cycle, it is created
and destroyed in parts of the
other cycles.
Since the short versions
of the oxygen cycle, and the carbon cycle overlap, it is
convenient to describe them
together. In photosynthesis, a plant takes water from the soil,
carbon dioxide from the air, and
energy from light. The light energy is used to separate the
hydrogen from the water
molecules. The oxygen is released into the air, the hydrogen is
combined with the carbon dioxide
to form food and some new water molecules. The
energy that was light is now
chemical energy in the food and oxygen (the oxygen is not a
waste product, it is part of the
energy storage system). In respiration, an organism takes in
oxygen, combines it with food to
make new water and new carbon dioxide thereby releasing
their stored energy.
In one half of the cycle
oxygen is free in air and some in food molecules, in the other half the
oxygen is in water and in carbon
dioxide. In one half of the cycle carbon is in food, in the
other half carbon is in carbon
dioxide.
Indirectly, we might be
able to claim that Earth receives heat from Sun, but not as
students understand the
statement. Radiant heat is a 19th century concept. Remember,
“Heat is the energy of the random
motion of the particles in matter.” Where there are no
particles of matter, there can
not be any heat. “Radiant heat” is an obsolete name for
infrared radiation, a kind of
electromagnetic energy. Like all other electromagnetic
radiation from Sun (or any other
source), radio, microwave, infrared, light, ultraviolet,
x-ray, cosmic ray, infrared
radiation absorbed by matter (Earth included) is changed into
heat energy in the matter.
At the same time, heat in matter is being converted to
electromagnetic energy, and
radiated away. Which is why the ground cools at night.
The idea that heat rises
results from abbreviating the naive identification of hot air
as heat, with the misconception
that warm air rises. Since heat is the kinetic energy
(energy of movement, momentum) of
the random (in any direction and changing) motion of
molecules, and is transferred
from molecule to molecule by direct interaction, heat is
conducted equally well in every
direction. The net movement of heat will always be from
a place of higher temperature
(higher concentration) to a place of lower temperature
(lower concentration), regardless
of direction. In a convection current, it is the matter
with the higher temperature (and
therefor lower density), that floats (is pushed)
upward,
carrying its heat with it.
A statement frequently
found in science books is, “Earth rotates once each day”.
While you could make a case that
the statement is "legally correct", it is misleading, students
understand it to mean that
Earth’s period of rotation is exactly 24 hours. This
misconception is reinforced by
diagrams which attempt to explain the phenomenon of day
and night while ignoring Earth’s
orbital motion. Because Earth has moved along its orbit,
from noon one day till noon the
next, Earth must rotate an average of approximately 361 o
each day. [ diagram 1 ]
The mean (average) solar day in the year 1900 was exactly 24 hours ( by
definition ). Earth’s period of rotation is
approximatly 23 hours 56 minutes 4 seconds.
That is why the stars rise an
average of about 4 minutes earlier each night.
This conclusion came out
the confusion of an Earth day equaling one Earth
rotation. It was assumed
that a day on any other planet would exactly equal its period of
rotation. It can easily be
demonstrated the number of days (noon to noon) per year (once
around its orbit) is equal to the
number of rotations per revolution minus one, negative
numbers represent retrograde
(backward) motions. [diagram ]
The law of planetary days and years.
Mercury has a day equal
to
176 Earth days, twice the length of its year.[
diagram ] Venus,
with retrograde rotation, has a
day of about 117 Earth days, -1.9 days per year, with Sun
rising in the west. [ diagram ]
The Bernoulli effect is,
the greater the velocity of a fluid, the less its pressure on a
surface parallel to the
velocity. Accompanied by a diagram of the ideal airfoil as adapted
to aircraft in the 1920’s, it is
explained that because the air moving over the top of the
wing must travel farther than
that moving under the wing, it must move faster across the
top, and therefore produces an
area of lowered pressure above the wing. So far, so good.
But then it is implied, if not
stated, that the pressure difference caused by the Bernoulli
effect provides the total lift
for the aircraft. Having seen planes flying upside down, I
ask,
“Please turn your wing diagram
over, and explain how the wing provides lift in that
position!”
So, how does a plane fly?
1. The lower surface of the wing is never
parallel to the direction of flight, as shown in
most diagrams. It is slanted upward slightly
(angle of attack), so the lower surface
becomes the front side, and the upper surface becomes
the back side. As the wing is
pushed into the still air ahead of it, it shoves the
air out of the way. Air has inertia (It
takes time for the molecules to change speed and
direction.) so it gets crowded together
(compressed), producing an area of high pressure along
the under surface of the wing. At
the same time the back side, upper surface is being
pulled away from the air behind it. It
takes time for the air above to expand downward to fill
in behind, leaving a low pressure
area (partial vacuum) along the upper surface of the
wing. The difference between the
ram air pressure below and the partial vacuum above
produces lift on the wing.
2. As the slanted lower surface of the wing is
driven into the air ahead of it, The air
molecules are batted downward. According to
Newton’s third law of motion, the downward
acceleration of the air is accompanied by an upward
reaction force on the wing, more lift.
3. Most, but not all, wings are designed to
take advantage of the Bernoulli effect for
more lift, especially during takeoff and landing.
Actually, the ideal airfoil, the wing usually
shown,
was designed to get maximum
additional lift from the Bernoulli effect for what we
now consider slow flight. As air
speed increases, the drag created by a thick wing soon
costs more than the benefit of
Bernoulli effect lift. Thinner wing less drag,
less Bernoulli effect, greater speed. Compare
the wing thickness of the “Spruce Goose” (a 1940’s
plane) with the C-17.
I expect that most of us
are familiar with story of Columbus’s arguing with learned
professors about the shape of
Earth. He saying it is round and he could sail west to Japan,
and they saying if he sailed far
enough he would fall off the edge of the world. This myth
apparently started with a
biography of Columbus which became popular in the early 19th
century.
In fact, about 1500
years before Columbus was born, the diameter of Earth was
accurately measured by
Eratosthenes of Alexandria (c. 276 B.C. - c. 194 B.C.). For
centuries before Columbus, every
school boy knew that “Earth is a sphere set at the center
of the universe.” The
problem was that Columbus had gotten hold of a mistranslation of
Eratosthenes’ work, and believed
Earth to be much smaller than it is. So he insisted he
could sail west to Japan.
The learned professors said his crew would starve because the
ships couldn’t carry enough food
for so long a voyage. The professors were right.
However, because of the
unexpected intervention of the Americas less than half way to
Japan, Columbus became a
hero. And to his dying day he was convinced that he had
visited the islands of Japan.
There are too many versions of “The Causes of
Seasons” to refute each
separately. Most commonly heard from eighth
graders is the naive theory, “We are
closer Sun in summer than in winter.” Below are
some more creative ones.
“Twice each year Earth’s north pole is
parallel with Sun’s north pole.” (Winds of
Change, a CD-ROM by the Educational Affairs Office at
JPL)
(We will disregard the confusion between axes and
poles.)
Most likely, a teacher said, “In summer Earth’s north
pole tilts toward Sun, and in winter
Earth’s north pole tilts away from Sun.” Perhaps
the teacher understood that Earth’s axis
is always (virtually for our lifetimes) pointing the
same direction while we revolve
around Sun. In the hearers mind, Earth’s axis
flip-flops between seasons.
“In addition, the Earth’s rotation affects the
rays of sunlight beating upon the
earth, causing them to bend. This motion and the
bending of sunlight causes seasons.
“(Interdisciplinary Lessons For The Middle School
Curriculum, Judith D. Kalish, Ed.D. ,
Lorraine P. Marshall, M.S. , Automated Weather
Source) “I guess, in winter sunlight is
reflected off the moon and the planets or something."
(Anonymous).. "When the sun
shines upon Earth it sends out two kinds of rays,
direct and indirect.” (tutorial on an
astronomy web site)
These confusions undoubtedly started with the
statement,
“In summer, the Sun’s
rays strike the earth more directly.” In the
speaker’s mind “directly” means, “The Sun’s
rays hit the ground at close to a 90 degree angle,
perpendicularly.” In the mind of the
hearer “directly” means, “In a straight line”.
Therefore, “less directly” means
“indirectly”, that is, not in a straight line.
Now, in the position of teachers, these hearers
feel compelled to create theories to explain how
Sun’s light can reach Earth by an
indirect path , that is, not traveling in a straight
line from Sun to Earth, during winter.
(the real cause of seasons)
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