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/PHYS142Fall2003/ 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 1-3:00pm Grades will be determined from an 800 point total 2 unit exams (2  100 points)200 Comprehensive Final Exam200 Recitation Quizzes100 5 Web Quizzes (4  50 points) 200 Laboratory100800 points possible

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. Moving charge by friction Some atoms/molecules attract electrons more strongly than others. Affinity: a natural liking or sympathy far: weaker attraction close: stronger attraction electron jumps Electron Affinity rabbit’s fur glass wool silk rubber hard plastic F  1r21r2 Different materials have different electrons affinities

4. fur plastic rod 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 electron Note: only contact between atoms is necessary. The heat of friction plays no role.