Items (6) and (8) were added; it deals with cetrifugal forces. I hope that
item (6) will not trogger another long debate.
DO NOT REPLY TO THIS MESSAGE. USE THE THREAD "ABOUT THE LIST" IF YOU WANT
TO POST A COMMENT. OR START A THREAD DEVOTED TO AN ITEM.
Contact me IN PRIVATE if you want to remove some of YOUR items or re-edit
them. Or send additional items to me ALSO IN PRIVATE. Let us follow the
suggestion that both POSITIVE and NEGATIVE are used expose misconceptions.
Also COMMON misconceptions found in many TEXTBOOKS, not student
misconceptions.
Ludwik Kowalski, (kowalskiL@alpha.montclair.edu)
A LIST OF TEXTBOOK MISCONCEPTIONS
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1) The Physics of Flight: It's air-deflection, not wing shape
2) Suppression of convection is the main factor responsible for higher
temperatures in green houses and closed cars. The misconception is
that absorption by the glass in greenhouses is the main factor
responsible for higher temperatures inside.
3) Capacitors in series: the equilibrated charges, Q1 and Q2, are not
equal in the dc circuits. The values of Q1 and Q2 are determined by
the way in which the applied difference of potential, V, is
distributed. This in turn is determined by the ratio of leakage
resistances, R1 and R2. In some cases R1 and R2 are equal and this
corresponds to equal charges.
4) Ice does not melt under skating blades "due to high pressure".
The falacy of this myth can be demonstrated numerically on the
basis of the "tripple point curve" for water. The falacy can also
be demonstrated experimentally; a loaded wire does not cut through
a block of ice when the air temperature is below freezing.
5) Coriolis effect does not make the toilet water go round and round.
6) The use of the term "centrifugal force" does not automatically imply
bad physics. Some phenomena, such as what happens inside a cetrifuge
tube, or what students feel on amusement park wheels, can be very
effectively explained in rotating frames of reference. Centrifugal
forces, fictitious in non-accelerating frames, are real in other
frames. The choice of the frame of reference is usually dictated by
pedagogical considerations.
7) Batteries are sources of charge.
8) It is not true that the magnetic field of the Sun can be measured as
far away from it as Pluto as stated in some astronomy textbooks. The
diameter of the Sun is about 0.1 AU and its magnetic field is probably
immeasurably small at distances greater than 1 AU. What we do measure
at long distances from our Sun is the magnetic field of solar wind
particles which are no longer part of the Sun.
9) Filled and empty balloons on a balance beam cannot directly
demonstrate air's weight.
10) Mechanical friction is not caused by surface roughness or
"interlocking asperites."
11) Infrared light is not "heat". ANY frequency of light will heat up an
object which absorbs it, and hot objects can emit MANY frequencies
both above and below the IR band.
12) Ben Franklin's kite was never struck by lightning. His conductive
kite string intercepted a thunderstorm's vertical leakage current.
( 1986 World Book Encyclopedia "Electricity" entry )
13) Clouds, fog, teakettle-steam, etc., are not made of water vapor,
they are made of small liquid droplets.
14) The Earth's "N" pole actually resides in the southern hemisphere
15) Light from the sun is not parallel. The sun may be very distant,
but the sun also is very large.
16) Spatial coherence of laser light is not caused by atoms' in-phase
emission. It is caused by the great distance that the wavefronts
travel as they reflect within the laser cavity.
17) Solids and liquids are not far better than gas at conducting sound.
However, solid rods can act as transmission lines, and liquids can
provide an impedance match which lets sound escape from submerged,
vibrating objects.
18) For every motion there is not equal and opposite motion. Newton's
3rd law is about force pairs, and no "action" such as throwing
bricks from a skateboard or a canoe is required.
19) Salt water contains no molecules of NaCl, it contains Na and Cl ions
20) Macroscopic light waves do not continue to propagate at c when
they enter a transparant medium
21)
22)
23)
24)
25) Light rays entering the eye are principally refracted at the
cornea, not the lens (as this is where the biggest difference in
refractive indices exists). I am yet to see a biology text that has
this correct. And I have seen a few physics texts that also have
the rays changing direction at the lens only.
26) If, in a given Lorentz frame, a closed circuit of conductors is
stationary and there is no Q or partial dB/dt source producing an E
field acting on the conductors' carriers, there can be no induced
current in the circuit. The "cutting of magnetic field lines" or the
"change in enclosed magnetic flux" or "the motion of a conductor
relative to magnetic field lines" can be confused and not always
useful criteria for the existence (or not) of an induced current in
a circuit.
27) Reentering spacecraft are heated by gas compression, not just by
air friction.
28) Laser light is not a special form of "in-phase" light, ideally it is
just plane-wave or sphere-wave light, or call it "point-source
light". Plane/sphere waves are single patterns, it takes at least
TWO patterns before a phase difference between them has meaning.
29) Laser light is not inherently parallel. Actually, most lasers
produce spreading light, and a lens is added to parallelize the beam.
30) Gravity fields in space are not necessarily zero or very small, (and
NASA's 'microgravity' can be a misleading term )
31) "Electricity" is not a form of energy. Electrical energy is EM, it
is photons. "Electricity" is not a spot on the EM spectrum.
32) Electrical energy does not pass lengthwise through light bulb
filaments and then return to the power supply.
33) "Static" and "current" are not two types of electrical energy
34) Triboelectricity is not caused by friction. It is caused by
intimate contact between dissimilar insulating surfaces.
35) Batteries are good conductors. The path for an electric current is
normally into, THROUGH, then back out of the battery.
36) 2-plate capacitors are charged with energy, not with electrical
charge. A charged capacitor contains just as many + and - charged
particles as a discharged capacitor.
37) The "electricity" which travels around a circuit is not provided by
the power supply, it is provided by the conductors.
38) "Static charge" is not electric charge which remains static.
"Static charge" retains all of its familiar characteristics even
when it flows. Better to call it "imbalanced charge"?
39) The electrons drift slowly during an electric current in a metal,
they do not move at or near the speed of light.
40) Electric current is not always and forever a flow of negative charge.
When an electric current passes through the earth, through water, or
through people, positive and negative charges flow, but not electrons.
41) "Static electricity" applies to all circuits, not just to drier
cling and fur-rubbed plastic. Surface charges on the wires are the
cause of electric currents in circuits.
42) "Current" is not a substance which flows within conductors. Charge
or charges can flow, but electric current simply "is".
43) A "conductor" is not a material which can pass charges through
itself. After all, a vacuum is a good insulator. Conductors are
materials which contain movable charges.
44) While "electric current" is a sensible term, "current electricity"
is obscure in meaning and misleading when used.
45) Electric companies do not sell "electricity." The coulombs of
electricity within power lines sit in place and vibrate back and
forth, so no "electricity" is delivered to households.
46) Electromagnetic energy propagates across electric circuits. It does
not vanish into one end of a circuit, only to magically reappear at a
load.
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