Labs and recitations begin this week!

1

• Labs and recitations begin this week!
• One problem from among the assigned homework each
  week will appear
• in recitation as a quiz! Your top 10 quizzes
  form your recitation grade.
• 5 web quizzes must be completed as scheduled
  online at
• http//calculus.unl.edu/edu/classes/PHYS142Fall200
  3/
• Register right away at that site. Take the
  practice quiz by this Friday.
• 2 in-class unit exams are scheduled
• Friday, Oct. 13 Chapters 19-22
• Wednesday, Nov. 5 Chapters 23-25
• One 2-hour comprehensive final Thursday, Dec 18
  1-300pm
• Grades will be determined from an 800 point
  total
• 2 unit exams (2 ? 100 points) 200

2
When two identical socks are removed from a
clothes dryer, they will usually 1)
repel each other 2) cling to each other
3) no way to predictl
3
Electron Affinity
Some atoms/molecules attract electrons
more strongly than others.
close stronger attraction
far weaker attraction
F ?
Affinity a natural liking or
sympathy
electron jumps
Electron Affinity rabbits fur glass wool silk rub
ber hard plastic
Different materials have different electrons
affinities
4
plastic rod
fur
When fur is rubbed on a plastic rod, both
acquire an equal but opposite charge. If the fur
merely rests on top of the plastic, each will
acquire equal and opposite charges. 1) True. 2)
False. Friction is necessary to provide
enough heat (energy) for electrons
to jump from the fur to the plastic.
5
True. It is not necessary to rub the fur on the
rod. If they touch with no motion between them,
electrons will still jump from the plastic to the
fur. Why is the fur rubbed? Because contact
is made by a bunch of random, jagged edges.
(Remember friction!) In the figure above,
there are only about three contact points where
the atoms are close enough for the electrons to
make the jump. If instead the fur and plastic
are rubbed, many, many more atoms come into
contact with each other and more electrons can
make the big move. The plastic and fur thus
acquire more charge, which makes it more
noticeable.
plastic
fur
Far apart electrons stay with their own atoms
? both atoms neutral Close difference in
electron affinity. Electron jumps. Move
apart electron stays with the atom it jumped
to ? both atoms charged
0
0
electron
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Note only contact between atoms is necessary.
The heat of friction plays no role.
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