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In addition to the principles given previously, here are some things that I expected you to learn from L153 and the related textbook reading. It may be helpful to compare these things to your own experiences.
- Tape is charged as it comes off the roll, and it has the same charge as top tape, which is positive.
- The aluminum foil in your experiments maintained neutrality. This doesn't mean that aluminum foil is always neutral. Foil can be charged, but in this case, by handling the foil you conducted any charge that it had to ground.
- Aluminum is a good conductor. Valence electrons move through it freely.
- The positive charge of a solid material, whether conductor or insulator, resides in the nuclei of the atoms, which are fixed in place.
- Both top and bottom tape are attracted to the foil due to the ability of electrons in the atoms of the foil to shift either toward or away from the tape. This is called polarization. The shift is always such as to result in attraction.
- In order to definitely determine the charge of an object, a check for repulsion from a test strip is required. (A test for attraction has two possible interpretations.)
- Electronegativity (or electropositivity) is an intrinsic property of a material and is not affected by how an object is charged. The charge on an object is an extrinsic property, and may be modified.
- The correct electronegativity series is, from most to least electronegative: balloon, Styrofoam, Plexiglas.
About L153B, #3: Some people noticed that after the paper bits attracted to the ruler, some of them jumped off. They actually jumped rather than simply falling off. The bits are attracted in the first place, because the charged ruler polarizes charge in the neutral bits. The bits are attracted through induction. Once they touch, though, they may acquire a net charge the same as the ruler's charge through conduction. Thus, the paper bits are repelled.
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