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# 14 - 14.EXP.23(Grading key 25 pts Number of excess...

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14.EXP.23 (Grading key: 25 pts) Number of excess electrons on a tape (a, 1 pt) 1 e (smallest amount of charge there is) (b, 1 pt) If every atom in the tape gained an electron, that would be an extreme upper bound. Mass of 0.16 grams, mainly carbon (organic polymers), so about in the tape. A less extreme upper bound is to assume that every molecule on the surface gains an electron: on the tape. (c, 5 pts) diagram—should be clear and clearly labeled (d, 10 pts) physically correct analysis, Q in electron charges and coulombs Typical values for tape 20 cm by 1.2 cm: Q = 10 –8 C, 6 × 10 10 e –4 if wrong method of analysis –2 if do not explicitly state simplifying assumption of point charges –1 if do not explicitly state simplifying assumption that Q 1 = Q 2 or equivalently the magni- tudes are equal if start from zero net charge and make oppositely charged U and L tapes (e, 2 pts) argument for under- or over-estimate (f, 4 pts) Using calculation in part (b): or about 1 in 500,000 –1 for simple arithmetic error (g, 2 pts) Within about an order of magnitude of the critical electric field magnitude: 0.16 grams 12 grams/mole ---------------------------------------- 6 23 × 10 atoms/mole ( ) 8 21 × 10 electronic charges = 0.012 m ( ) 0.2 m ( ) 3 10 × 10 m ( ) 2 per molecule --------------------------------------------------------------------- 3 16 × 10 electronic charges = 6 10 × 10 electronic charges 3 16 × 10 surface sites ------------------------------------------------------------------ 2 6 × 10 = E 1 2 9 12 × 10 C 2 /N·m 2 ( ) ----------------------------------------------------- 10 8 C 0.012 m ( ) 0.2 m ( ) --------------------------------------------- 2 5 × 10 N/C = =

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14.RQ.34 The positive charge also repels the positive side of the molecules, though with a smaller force, so that the net force is an attraction.
14.RQ.38 Repulsion is a better test for the sign of an unknown charge because attraction can occur even if one of the objects is neutral, but the only way repulsion can occur is if the two objects have the same sign of charge.

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14.RQ.39 Plastic Metal
14.RQ.41 A + + + + + + + - + - - - - - - - E + E - E 1 E 2 Net electric field must be zero inside the metal in static equilibrium; otherwise the electron sea would shift some more. Note: It is NOT generally true that the field made by the “+” polarization charges cancels E , nor that the field made by the “–” polarization charges cancels E + . All one can say is that the field made by all the polarization charges cancels the field due to the external charges.

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14.RQ.42 The original charge is shared between the piece of metal and your body (it spreads over both objects). So there might be a little bit of charge left on the piece of metal (it might be too little to have a noticeable effect on anything).
14.RQ.44 There aren't any mobile charges on the surface of the tape (the tape is an insulator), so what is likely to happen is that the charged U tape will attract ions from your finger. For example, if the U tape is negative, positive sodium ions will be deposited onto the surface of the tape (if the tape is positive, negative chloride ions will be deposited). That makes the net charge of the U tape much smaller (maybe almost zero), so now it will have little effect on other objects.

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