19.Move it!Point chargesq1andq2are placed initially atx=aandx= 3arespectively.What is the work done by the electric field due toq2ifq1moves to pointx= 6aalong thepath shown?
A.kq1q26aB.kq1q230aC.kq1q22aD.kq1q2aSolution:
20.Electric Potential.Four identical chargesQare placed at the corners of a square of sides. What is the potential at the center of the square?
21.Calculating Electric Potential.A chargeQis uniformly distributed on a rod of length2a.What is the potential at point P as shown?

First Long Exam
Second Semester, AY 2014–2015
Physics 72
22.Potential to Add.Charge distribution A produces an electric potentialVA= sin2x+cos2y+ez. Charge distribution B produces an electric potentialVB= cos2x+ sin2y.What is the net electric field due to both charge distributions?
23.Maximum Capacity!If you can fill each of the capacitors shown with two possible dielec-tric materials of dielectric constantsκand2κ, which configuration would give the highestcapacitance?
A. A = B =κ, C =2κB. A = B =2κ, C =κC. A = B = C =κD. A = B = C =2
κ
A – 10

First Long Exam
Second Semester, AY 2014–2015
Physics 72
Solution:Ceff=1Ca+1Cb+1Cc-1
24.Nag-iba.Consider a parallel-plate capacitor with area of the platesAand separationd.What happens to its capacitance if the area of the plates is increased three times and theseparation of the plates is quartered?
25.Density.A spherical capacitor of inner radius 1 mm and outer radius 2 mm has the electricfield of magnitudeE= 2.00×106V/matr= 1.5 mmfrom the center. What is theelectric energy density in this region?

First Long Exam
Second Semester, AY 2014–2015
Physics 72
For the next two items, consider a network of four capacitorswith equal capacitance of4.00μFand a potential differenceof+25.0 Vacrossabas shown.26.Sila C1 to C4...What is the equivalent capacitance ofthis network between pointsaandb?