PHY9Cans1

# PHY9Cans1 - Answer Set 1 Physics 9C section B, winter 2008...

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Answer Set 1 Physics 9C section B, winter 2008 Q21.4 Clothing becomes charged as pieces rub against each other, and oppositely charged portions attract. If all the clothes were the same material, there would be less charge transfer (since all pieces would attract electrons with the same strength) and less cling. Q21.8 Electrons transport both heat (energy) and charge. In an insulator, where (at least in our picture) the electrons don’t move, they can’t carry either heat or charge through the material. In a metal, where the electrons do travel easily, they move both heat and charge through the material. Q21.15 The two charges feel only the forces from each other. By Newton’s third law, the forces on the charges are equal in magnitude but opposite in direction. (You can also get this from Coulomb’s law.) Since the charges feel the same strength force and have the same mass, the initial acceleration of the second charge has magnitude a 0 . Q21.19 Since Feld goes as distance-squared, E o is a factor of 3 smaller at a distance 3 R . P21.12 a) The force is attractive, so the second charge must be positive. Get its magnitude Q from 0 . 200 = 1 4 πε 0 0 . 550 × 10 - 6 Q (0 . 300) 2 , or Q = 3 . 64 × 10 - 6 C. b) ±orce is 0.200 N downward, by Newton’s third law or by using Coulomb’s law. Q21.28 a) Since the electric Feld E is constant, the force on the electron and hence its acceleration a are also constant. Use s = 1 2 at 2 to Fnd the acceleration: 4 . 50 = 1 2 a (3 . 00 × 10 - 6 ) 2 , so a = 10 12 m/s 2 . The force giving this acceleration is F = ma = (9 . 11 × 10 - 31 )(10 12 ) = 9 . 11 × 10 - 19 N. (Get electron mass from inside back cover of textbook.) Then E = F/q = (9 . 11 × 10 - 19 ) / (1 . 60 × 10 - 19 ) N/C. Since the electron accelerates upward, the electric Feld is directed downward . (The Feld points in the direction a positive charge would accelerate.) b) The gravitational force on an electron is mg = (9 . 11 × 10 - 31 )(9 . 8) 10 - 29 N. This is about 11 orders of magnitude smaller than the electric force in this problem, so it gives a negligibly small correction.

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## This note was uploaded on 05/13/2008 for the course PHY 9C taught by Professor Zieve during the Spring '08 term at UC Davis.

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PHY9Cans1 - Answer Set 1 Physics 9C section B, winter 2008...

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