Phys_240_W08_Exam_1_Sols - gm Form 1 Physics 240 First Exam...

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Unformatted text preview: gm Form 1 Physics 240 First Exam Winter 2008 1. Last Name: MA First Name: 2. Please circle your discussiofrilrstructor and section below: Instructor Time Section Instructor Time Section Vallery ‘ 8—9 007 Krisch 12-1 013 Vallery T 9—10 008 Krisch 1—2 "614 Vallery 10-11 009 Krisch 4 2—3 015 Kurdak 10—11 010 T * Kurdak 1 1-12 4 01 1 Campbell Noon—l 260 Kurdak 12—1 012 T 3. Fill in your name, 8 digit student ID number, discussion section number, and the exam FORM number on the scantron sheet 4. There are 20 multiple choice problems worth 5 points each for a total of 100 points. Choose the one best answer and transfer it to the scantron sheet. You must mark the correct answer on the scantron sheet to get credit for the problem. There is no penalty for guessing, so be sure to enter an answer for every question even if it is just a best guess. 5. At the end of the exam hand in both your scantron and exam booklet in separate piles set up by the proctors. 6. This is a closed book, 90 minute exam. Turn off and put away all electronic devices. You may use a calculator and one 3”x5” note card or equivalent. k = “471280 =9x109 Nmz/CZ so: 8.85x10‘12 CZ/Nm2 e : 1.6x10'19 C 1 av: 1.6x10'19J u=micro =10”6 g=9.8 m/s2 3,9; as? EMz/Vt FzDRll/l ,l. W cs 1. We would like to use the induction method to negatively charge one of the isolated copper spheres used in the lecture demonstrations. Consider the following procedural steps: (1) attach a ground wire to the sphere (2) remove the ground Wire from the sphere (3) touch a negatively charged rod to the sphere (4) bring a + charged rod near, but not touching, the sphere (5) remove the charged rod To charge the copper sphere by induction, one can use the sequence of steps: A) 3, 5 B) l, 4, 5, C) 4 1 ,5 VNN ' E) " 31,2, 5‘ 2. The diagrams below, depict four different charge distributions. The charged particles are all the same distance from the origin. Rank the distributions by the electric potential V at the origin, front lowest to highest potential. (Take V=O at infinity in each case) 3. A point charge of 8 {JG is placed on the X axis at x=2.0 meters. Where on the X axis in meters should we place a point charge of ~2.0 uC so that the total force on a third point charge at the origin (x=0) is zero? W “E? ai Version l Page 1 4. A dipole is released from rest near an isolated point charge. Describe its subsequent motion. Pomt . _* / charge P «PC 6Ai it heginsjgroflt’atefland moves away from the point charge fl) It begins to rotate and moves toward the point charge C) It does not rotate and it does not translate D) It does not rotate but it does move away from the point charge E) It does not rotate but it does move toward the point charge 5. A charged oil drop with a mass of 2 >< 10‘"4 kg is held suspended by a downward electric 2; field of 300 N/C. The charge on the drop is: ‘ («9m +6.5x10“6c <33 ~6.5><f10*§@ C) —1.5><10‘6c D) o E) +1.5x10‘6c 6. An insulator in the shape of a sphere with radius R has positive charge uniformly distributed throughout its volume. If the zero ofeleetric potential is taken to be at infinite radius where do the electric potential V and the electric field magnitude E reach their respective maximum values? ‘ xv)? (A): V and E are max at R B) V and E are max at the center of the sphere C E is max at the center and V is max at R Cb V max the 1.r s max at R E) V is max at infnity and E is max at the center Version I Page2 QQntain hing two Very small ph ere then we can claim: 7. The diagram shows the electric field lines in a region or space charged spheies (Y and Z). Neglecting he espace in nside each s 1‘ ‘l _ '1 1"; 1‘ I” in: H:}L.fl.._ fo—uyfl ’ —-I:—_1; ——-—~——12 1—H a, .1 J «1.», r-In (V 1;“ ‘Fflfg/ fish; I" 12“.“- '1‘," the electric field 15 not zero anywhere (except infinitely far from the spheres) _ C fl B) the electric field is strongest midway between Y and Z E M lat W ? C) All of the equipotential surfaces are spherical W I; W M D) Y and Z repel and hence must have the same sign of charge m M W E) the magnitude of the electric field is the same everywhere W 8. An electron is released from rest a distance d away from an infinite plane sheet with uniform positive charge density (5 (in C/mz). What is the electron‘s speed just before it srtkSe teh sheet? E f B) theelectron is repelled from the sheet W, W J W ’ C) O'd 14180 D) 260d meo E) 6061’ 277450 9. A charged point particle is placed in a region of space where there is an existing electric potential scalar field, V. No force is exerted on this point charge: A) if the particle is movmg along an equ1potent1al surface f 1 m ..... VV/ g E E at locations where V is zero :2 gi “’“ ct locinsto herethe ‘ r1e1toiV iszero if the particle is moving along an electric field line B) if the particle rs moving perpendicular to a field line Version 1 Page 3 ‘1 10. A long line of charge with charge her unit lingt h M runs along the axis ofa conducting cylindl ical tube which carries a charge per ni length of kg The charge per unit length on the inner and outer surfaces of thec ’ - tube respectively are: A) Oandks-rh Wétjififl WW B) —7\.L and Ag— XL sz C) 0 and is )NNER’ SORF/tazs __ _;\, and 7t . . . —1Landis+ ‘5 ' ,1 wmsuxr M ALMS . 5,5 M wit/WW l 1. How much charge IS contained within a cubical volume 2 meters on a si e if the electric field is given (in N/C) by E(x,y, z) — 2i +1.51" + 322k . The cube is located with one corner at the origin and with edges along the he +y, and +2 directions. Y (0,0,0) Z \ A) 7.1x1010C B) 2.1x10_9 (3 E) 0c M 12. A hollow conductor is uncharged. A small negatively charged metal ball is lowered by a silk thread through a small opening in the top of the conductor and allowed to touch its inner surface. After the ball is removed, the hollow conductor “will have: A) a charge whose sign depends on what part of the inner surface it touched B) no appreciable charge a charge whose Sign depends on where the small hole is located in the conductor ,5 a negative charge h E) a positive charge Version l Page 4 13. An insulating rod is bent into a semicircle of radius R. The top half has charge +Q uniformly distributed along it while the lower half has charge —Q distributed uniformly along it. The magnitude and direction of the electric field vector at the center is: 14. An electric dipole is held at rest so its dipole moment of 0.03 Our makes an angle of 115 degrees with a uniform electric field of 250 N/C. If we release the dipole so it is free to turn with its moment of inertia of 1 = 1.5 kg—m2 what is the dipole's maximum angular velocity, in radians/ second? Version 1 Page 5 15 An Darin nuclear model Of the atom had a positivel v point— —charged nucleus of charge +Ze at the center of a uniformly negatively charged (- Ze) distribution with a Spherical radius of R. (The electronic charge is uniformly distributed throughout the volume of the spherical atom.) What is the magnitude of the electric field for any point at radius r inside the atom (KR)? l r kZé ;2_+F B I , ) kZe -1§—+—I—3— r R / C) 1 rd [C28 7—? / ) kZe i] 16. Imagine you are assembling four point charges (initially at infinite separation) at the corners of a square as shown What should you choose for charge q so that the work required to assemble all four chai ges is zeio? Q Q $7 dej—LTQ/ o Versio 1 17. Two conducting, horizontal metal plates have large area and small separation. The plates are neutral—~they have no net charge. Midway between the conducting plates we insert a thin insulating sheet containing 28 micro—Coulombs of charge uniformly distributed over the same large area. What are the charges on the top surface of the upper plate (ch) and the lower surface of the bottom plate (q4)? A q; is ~14 uC and Q4 is ~14 uC‘ B) q; is O and (14 is O “‘90 q} is --28 MC and q4 is +28 uC D) q1 is —28 C and “3+8 i —l4 uC and (14 is +14 uC 18. An isolated conducting sphere with radius r = 0.05 m is charged so its electric potential is+lOll V2 ,, potential very far away. The charge density on its surface is: +1.8 WC/mz 10“7 C/m2 C) +2.2 >< 10-7 C/m2 D) _2.2 x 10—7 C/m2 E) #35 >< 10“7 C/m2 Version 1 Page 7 l9. An infinitely wide and long horizontal slab of thickness 2a has constant volume charge density o. For this planar slab of charge, which graph best represents the magnitude of the electric field E as a function of y? "VG 20. Two very large parallel conducting plates are 1.0 meter apart and are maintained at an electric potential difference of 10 Volts. An uncharged conducting plate of the same large area and 0.5 meter thick is inserted between these plates, filling half the volume. What is the electric field vector in the lower gap between the plates at point P? The units for the field magnitude are N/C = Volts/m. E5353 ‘ , T 0 volts /C)20T mo K E)10T Version 1 Page 8 ...
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