University of the PhilippinesCollege of SciencePhysics 72Set AFirst Long ExamSecond Semester, AY 2016–2017Name:Instructor:Section/Class Schedule:Student Number:Course:College:

First Long ExamSecond Semester, AY 2016–2017Physics 72INSTRUCTIONS:Choose thebestanswer and shade the corresponding circle on your answersheet. To change your answer, cross-out and sign your original answer and then shade yournew answer. No computational devices allowed (e.g. calculators, mobile phones). Any formof cheating in examinations or any act of dishonesty in relation to studies, such as plagiarism,shall be subject to disciplinary action.Following instructions is part of the exam.Useful formulas:AreaVolumeSphere (radius=r)4πr243πr3Cylinder (radius=r, height=h)2πr2+ 2πrhπr2hsin(π/3) = cos(π/6) =√3/2cos(π/3) = sin(π/6) = 1/2sin(π/4) = cos(π/4) =√2/2Useful constants:e-1.60×10-19Cme9.1×10-31kg08.854×10-12C2/Nm2k8.988×109Nm2/C2
1.The Ring.Charge is uniformly distributedon a ring of radiusain a coordinate systemas shown.What is the magnitude of they-component of the electric field due to an in-finitesimal chargedqat pointPon the ring axis,a distancexfrom center of the ring?
A.x(x2+a2)3/2k dqB.a(x2+a2)3/2k dqC.x(x2+a2)1/2k dqD.a(x2+a2)1/2k dqSolution:The electric field of an infinitesimal section isdE=kdqr2The distance from the source point to field point is√x2+a2. SodE=kdqx2+a2They-component isdEy=kdqx2+a2sinα=kdqx2+a2ar=kdqx2+a2a√x2+a2=ka(x2+a2)3/2dqA – 2

First Long ExamSecond Semester, AY 2016–2017Physics 72
2.Spears.Three spheresX,Y, andZof the same size have initial charges+8Q,-13Q, and-13Q, respectively. SpheresXandZare conductors, andYis an insulator. First, spheresXandZare brought in contact with each other and separated. Then, spheresYandZarebrought in contact with each other and separated. What is the final charge of sphereZ?
qz,new=qx,old+qz,new2For the next two items, consider five identical charges withQ= +1μCdistributed equidistantly on a circle of radiusR= 2 cmas shown. PointPis the center of the circle.

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- Spring '11
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- Physics, Electric charge, Ceq, qenc, College of Science