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TEXTBOOK PHYSICS MISCONCEPTIONS

from PHYS-L physics teaching discussion


Updated 2/12/98. 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 (kowalskiL@alpha.montclair.edu) IN PRIVATE if you want to
remove some of YOUR items or reedit them. Or send additional items to me
ALSO IN PRIVATE. Let us follow the suggestion that POSITIVE is used
instead of NEGATIVE; I changed my item 4 to accomplish this. Also COMMON
misconceptions found in many TEXTBOOKS, not student misconceptions. I do
not think that we should compete with The Physics Teacher column of Mario
Iona (who goes after individual authors, documenting everything, etc.) If
you feel that a reference is important for an item send it to me and I
will append it at the list of references. 

                     Ludwik Kowalski, (kowalskiL@alpha.montclair.edu)

                  A LIST OF TEXTBOOK MISCONCEPTIONS
                *************************************
 
  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 are not equal in the
     dc circuits.
 
  4) Melting of ice under skating blades is not due to high pressure.
 
  5) Coriolis effect does not make the toilet water go round and round.

  6) 

  7) Batteries are not sources of charge.

  8) 

  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.
     ( see 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 laser cavity: by the great distance 
     that the wavefronts travel as they reflect back and forth.

 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 few 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) The Physics of Flight: It's air-deflection, not wing shape
 
 22) The green house effect: suppression of convection is the main factor.
 
 23) Capacitors in series: the equilibrated charges are not equal in dc 
    the circuits.
 
 24) Melting of ice under skating blades is due to high pressure.

 25) Light rays entering the eye change direction only at the lens, 
     not the cornea. (more common in Biology texts).

 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 superposed 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 dryer
     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 just 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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