oldhomework 01-solutions

# oldhomework 01-solutions - marout (ssm979) – oldhomework...

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Unformatted text preview: marout (ssm979) – oldhomework 01 – Turner – (56725) 1 This print-out should have 10 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Two uncharged metal balls, Z and X , stand on insulating glass rods. A third ball, carrying a positive charge, is brought near the ball X as shown in the figure. A conducting wire is then run between Z and X and then removed. Finally the third ball is removed. X Z + conducting wire When all this is finished 1. balls Z and X are both positive. 2. balls Z and X are still uncharged. 3. ball Z is positive and ball X is negative. correct 4. ball Z is negative and ball X is neutral. 5. ball Z is neutral and ball X is positive. 6. balls Z and X are both negative, but ball Z carries more charge than ball X . 7. ball Z is positive and ball X is neutral. 8. ball Z is negative and ball X is positive. 9. ball Z is neutral and ball X is negative. 10. balls Z and X are both negative, but ball X carries more charge than ball Z . Explanation: When the conducting wire is run between Z and X , some positive charge flows from X to Z under the influence of the positive charge of the third ball. Therefore, after the wire is removed, Z is charged positive and X is charged negative. 002 10.0 points Three identical point charges hang from three strings, as shown. 45 ◦ 45 ◦ F g 25.0 cm 25.0 cm + + + +q +q +q 1.1 kg 1.1 kg 1.1 kg What is the value of q ? The Coulomb constant is 8 . 98755 × 10 9 N · m 2 / C 2 , and the acceleration of gravity is 9 . 81 m / s 2 . Correct answer: 5 . 47873 × 10 − 6 C. Explanation: Let : m = 1 . 1 kg , L = 25 . 0 cm , θ = 45 ◦ , and k e = 8 . 98755 × 10 9 N · m 2 / C 2 . r = 2 L sin θ = 2 L sin 45 ◦ = 2 L √ 2 2 = L √ 2 F T,x = F T sin θ F T,y = F T cos θ Each sphere is in equilibrium horizontally F electric − F T,x = 0 F electric − F T sin θ = 0 and vertically F T,y − F g = 0 F T cos θ − F g = 0 F T = F g cos θ . marout (ssm979) – oldhomework 01 – Turner – (56725) 2 From the horizontal equilibrium, F electric = parenleftbigg F g cos θ parenrightbigg sin θ F electric = F g tan θ = F g (tan45 ◦ ) = F g . For either of the end charges, F electric = k e q 2 r 2 + k e q 2 ( r 2 ) 2 = k e q 2 r 2 + 4 k e q 2 r 2 = 5 k e q 2 r 2 5 k e q 2 r 2 = mg . Thus | q | = radicalBigg r 2 mg 5 k e = radicalBigg ( L √ 2) 2 mg 5 k e = L · radicalbigg 2 mg 5 k e = (25 cm) parenleftbigg 1 m 100 cm parenrightbigg × radicalBigg 2(1 . 1 kg)(9 . 81 m / s 2 ) 5(8 . 98755 × 10 9 N · m 2 / C 2 ) = 5 . 47873 × 10 − 6 C . 003 (part 1 of 2) 10.0 points Two identical small metal spheres with q 1 > and | q 1 | > | q 2 | attract each other with a force of magnitude 83 . 7 mN, as shown in the figure below....
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## This note was uploaded on 10/24/2010 for the course PHY 56735 taught by Professor Turner,j during the Fall '10 term at University of Texas.

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oldhomework 01-solutions - marout (ssm979) – oldhomework...

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