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Unformatted text preview: lew (dl9564) – hw 1 – Opyrchal – (121006) 1 This printout should have 10 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A charge of +1 coulomb is place at the 0 cm mark of a meter stick. A charge of +4 coulombs is placed at the 100cm mark of the same meter stick. Where should a proton be placed on the meter stick so that the net force on it due to the two charges is 0? Correct answer: 33 . 3333 cm. Explanation: The forces are both repulsive and point in opposite directions. The force due to the +1 charge is F 1 = k (+1) (+1) r 2 1 = k r 2 1 and due to the +4 charge, F 4 = k (+4) (+1) r 2 4 = 4 k r 2 4 . In order for these forces to be equal, k r 2 1 = 4 k r 2 4 r 2 4 = 4 r 2 1 = (2 r 1 ) 2 r 4 = 2 r 1 The ratio of the two distances is 1:2, so r 1 = 1 3 (100 cm) = 33 . 3333 cm . 002 (part 1 of 2) 10.0 points Three charges are arranged in the ( x, y ) plane (as shown in the figure below, where the scale is in meters). 6 nC 8 nC − 7 nC y ( m ) 0 1 2 3 4 5 6 7 8 9 10 x ( m ) 1 2 3 4 5 6 7 8 9 10 What is the magnitude of the resulting force on the 6 nC charge at the origin? Correct answer: 9 . 57923 nN. Explanation: Let : q o = 6 × 10 − 9 C , ( x o , y o ) = (0 m , 0 m) , q a = 8 × 10 − 9 C , ( x a , y a ) = (7 m , 0 m) , q b = − 7 × 10 − 9 C , and ( x b , y b ) = (0 m , 10 m) . Coulomb’s Law for q o and q a is bardbl vector F oa bardbl = k e q o q a radicalbig ( x a − x o ) 2 + ( y a − y o ) 2 = k e q o q a radicalbig x 2 oa + y 2 oa = 8 . 98755 × 10 9 N C 2 / m 2 × (6 × 10 − 9 C) (8 × 10 − 9 C) radicalbig (7 m) 2 + (0 m) 2 = 8 . 80413 × 10 − 9 N . F oa x = − k e q o q a r 2 oa cos θ oa = − k e q o q a r 2 oa x oa r oa = − (8 . 98755 × 10 9 N C 2 / m 2 ) × (6 × 10 − 9 C) (8 × 10 − 9 C) (7 m) 2 × (7 m) (7 m) = − 8 . 80413 × 10 − 9 N . lew (dl9564) – hw 1 – Opyrchal – (121006) 2 F oa y = − k e q o q a r 2 oa sin θ oa = − k e q o q oa r 2 oa y oa r oa = − (8 . 98755 × 10 9 N C 2 / m 2 ) × (6 × 10 − 9 C) (8 × 10 − 9 C) (7 m) 2 × (0 m) (7 m) = 0 N ....
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This note was uploaded on 03/22/2009 for the course PHYS 121 taught by Professor Opyrchal during the Fall '08 term at NJIT.
 Fall '08
 Opyrchal
 Charge

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