PH 102 Midterm: ph102_sum08_exam1_SOLN
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Exam i solutions: an electron is released a short distance above the surface of the earth. A second electron directly below it exerts an electrostatic force on the rst electron just great enough to cancel the gravitational force on it. How far below the rst electron is the second? (cid:4) 5. 1 m (cid:3) 7. 5 m (cid:3) 10. 2 m (cid:3) 2. 5 m. What we are really asking is, at what distance below a proton would the upward force on an electron equal the electron"s weight? . To determine that, all we need to do is write down the two forces and balance them. The electron has mass me and charge e, the proton has charge e, and the distance between them we"ll call d. our origin will be at the proton"s position: = 0 meg d2 = kee ( e) d2 kee2 meg meg d =s kee2 d s26.
