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Unformatted text preview: Version 158/ACBDC – midterm 01 – Turner – (60230) 1 This printout should have 18 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points G . x y ++++ ++++ S . x y −−−− ++++ L . x y + + + + − − − − M . x y + + + + + + + + + + + + −−−−−− P . x y +++++ +++++ + + + + + − − − − − For which configuration(s) does the total electric field vector at the origin have non zero components in the x direction as well as the y direction ( i.e. , both x and y components are nonzero)? 1. Configurations L , G and M only 2. Configuration P only 3. Configuration S only correct 4. Configuration G only 5. Configurations L and G only 6. Configurations G , P and M only 7. Configurations L and M only 8. Configurations G and M only 9. Configuration L only 10. Configurations L , S and M only Explanation: Basic Concepts: Δ E = k Δ q r 2 ˆ r and E = summationdisplay Δ E . Symmetry of the configuration will cause some component of the electric field to be zero. Solution: Configuration L : It is anti symmetric about the yaxis (opposite sign of charges), so the electric field has no y component. x y + + + + − − − − L Configuration G : It is symmetric by a ro tation of 180 ◦ , so the electric fields generated by these two pieces have opposite directions; therefore the total field is zero. Version 158/ACBDC – midterm 01 – Turner – (60230) 2 x y ++++ ++++ G Configuration S : It is antisymmetric by rotation of a 180 ◦ , so the total field has non zero components in both x and y directions, just like the field generated by just one piece of charge. x y −−−− ++++ S Configuration P : It is symmetric about the xaxis, so the y component of the total field must vanish. x y +++++ +++++ + + + + + − − − − − P Configuration M : It is symmetric about the yaxis, so the x component of the total field must vanish. x y + + + + + + + + + + + + −−−−−− M 002 10.0 points A 10 . 7 g piece of Styrofoam carries a net charge of − . 5 μ C and floats above the center of a very large horizontal sheet of plastic that has a uniform charge density on its surface. The acceleration of gravity is 9 . 8 m / s 2 and the permittivity of free space is 8 . 85419 × 10 − 12 C 2 / N / m 2 . What is the charge per unit area on the plastic sheet? 1. 5.06743 2. 1.45775 3. 3.33201 4. 1.90896 5. 2.84609 6. 3.7138 7. 4.61622 8. 2.53371 9. 4.40797 10. 1.38834 Correct answer: − 3 . 7138 μ C / m 2 . Explanation: Let : m = 10 . 7 g , q = − . 5 μ C , g = 9 . 8 m / s 2 , and ǫ = 8 . 85419 × 10 − 12 C 2 / N / m 2 . The field due to a nonconducting infinite sheet of charge is the same as that very close to any plane uniform charge distribution. The field is E = σ 2 ǫ , where σ is the surface charge density (charge per unit area) of the plastic sheet. Call the charge on the styrofoam q , and its mass m ....
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This note was uploaded on 11/22/2010 for the course PHYS 303 taught by Professor Turner during the Spring '10 term at University of Texas at Austin.
 Spring '10
 Turner

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