Physics 302L shih test 1

# Physics 302L shih test 1 - Version 057 – Exam1 – shih...

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Unformatted text preview: Version 057 – Exam1 – shih – (56310) 1 This print-out should have 12 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Two charges q 1 and q 2 are separated by a distance d and exert a force F on each other. What is the new force F ′ , if charge 1 is increased to q ′ 1 = 5 q 1 , charge 2 is decreased to q ′ 2 = q 2 2 , and the distance is decreased to d ′ = d 2 ? 1. F ′ = 5 F 2. F ′ = 5 2 F 3. F ′ = 10 F correct 4. F ′ = 100 F 5. F ′ = 25 2 F 6. F ′ = 5 4 F 7. F ′ = 25 4 F 8. F ′ = 20 F 9. F ′ = 25 F 10. F ′ = 50 F Explanation: F ′ = k q ′ 1 q ′ 2 r ′ 2 = k (5 q 1 ) parenleftbigg q 2 2 parenrightbigg parenleftbigg d 2 parenrightbigg 2 = 10 k q 1 q 2 d 2 = 10 F . 002 10.0 points Four point charges are placed at the four cor- ners of a square. Each side of the square has a length L . − + + + D A C B L P What is the magnitude of the electric field vector E at the center of the square? 1. k q L 2 2. 4 k q 2 L 2 3. k q 2 L 2 4. 4 k q 2 L 5. 4 k q L 2 correct 6. 2 √ 2 k q L 7. 2 √ 2 k q 2 L 2 8. k q L 9. 2 √ 2 k q L 2 Explanation: The electric field is vector E = summationdisplay i vector E i = summationdisplay i k q i r 2 i ˆ r i . Consider a positive test charge placed at O . The negative charge yields an attractive force along a line through O and the positive charges repelling forces along a line through O . E C E A E B E D Version 057 – Exam1 – shih – (56310) 2 From the figure, bardbl vector E bardbl = bardbl vector E B bardbl + bardbl vector E D bardbl = 2 | E B | = 2 k q parenleftbigg L √ 2 parenrightbigg 2 = 4 k q L 2 . 003 10.0 points Consider three identical conducting spheres arranged in an equilateral triangle. 9 c m 4 C − 6 C 5 C If the spheres are connected by three thin wires, what is the final charge on the lower left-hand sphere? Assume the final charge on the wires is negligible. 1. 0.666667 2. 3.33333 3. 0.0 4. 3.66667 5. 4.0 6. 1.0 7. 1.33333 8. 2.66667 9. 1.66667 10. 2.33333 Correct answer: 1 C. Explanation: Let : a = 9 cm = 0 . 09 m , Q 1 = 4 C , Q 2 = − 6 C , and Q 3 = 5 C . a Q 1 Q 2 Q 3 Charge will flow in a conductor until the conductor is at equipotential. When the spheres are connected by a wire, the system becomes a single conductor, so the poten- tials on the three spheres must be equal; with identical spheres, their charges must also be...
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Physics 302L shih test 1 - Version 057 – Exam1 – shih...

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