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img045 - PHY2054 Spring 2008 18 A flashlight bulb...

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Unformatted text preview: PHY2054 Spring 2008 18. A flashlight bulb operating at a voltage of 4.5 V has a resistance of 8.0 Q. How many elec- trons pass through the tall) filament per second? K \ 3.5 x 10'“ (2) 1.8 x 102‘ (3) 1.1 x 1019 (4) 3.7 x 1016 (5) none of these The current is 4.5 /8.0 = 0.56 A. So in 1.0 second, the number ofelectrons is 0.56 /I.6 x 10'” = 3.5 x 10’“. 19. A large m 3 et is constructed by winding a very long aluminum wire o_cross section [GM around a cylindrical spool. If the wire carries a current of 13,000 A, what is the . ’ ' - ’ ~.. 0 I e electrons (in m/s) in the wire assuming that there is one conduction elec- tron per atom? Aluminum has a density of 2700 kg/m’, an atomic number of 13 and an ato_mic mass of 27.0. 61 MM ~ @015 (2) 0.00028 (3) 0.0042 (4) 0.038 (5) 0.097 The formula relating current to electron drift velocity is i = mAcevd, where Ac = 9 X 10'6 m2 is the cross sectional area, e is the electron charge and Va is the tin]? velocity. The number density is given by the expression 72, = p/(0.001A) = 1.0 x 10” / m3 where A = 27 is the atomic mass. Solving for vd yields var = 0.15 m/s. 20. In the Bohr model of the hydrogen atom, an electron in the 71h excited state moves at a speed of 4.47 x 10“ m/s in a circular path having a radius of 2.59 x 10‘9 m. What is the effective cur- rent (in amps) associated with this orbiting electron? ©4.4 x 10‘7 (2) 2.8 x 10‘6 (3) 7.6 x 10" (4) 5.9 x 10'3 (5) 4.9 x 10‘3 The current isgiven bye /T, where T = 2mm. Thisyields i = ev/znr = 4.4 x 104,1. ...
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