4kq 2. D) south A) F B) 2F C) 4F D) 8F E) 16F

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1 efore you begin: Use black pencil. Wrie and bubble your SU ID Number a boom lef. Fill bubbles fully and erase cleanly if you wish o change! 20 Quesions, each quesion is 10 poins. Each quesion has a mos one correc answer. Marking 2 or more bubbles will auomaically give you 0 poins. 1. My exam version is: A C D E 2. Find he magniude and direcion of he ne elecrosaic force on charge +q. Assume he disance from +q o any of he surrounding poin charges is R, and a posiive value represens he posiive y- axis while a negaive value represens he negaive y-axis. 4q +3q +q +3q A) kq 2 kq 2 4kq 2 4kq 2 ) C) D) R 2 norh R 2 souh R 2 norh R 2 souh E) 6kq 2 R 2 norh The horizonal componens of he forces cancel, herefore, he ne force is direced norhward owards he negaive charge. Wih Coulomb s law applied, he magniude F e is 4kq 2 /R Two small spheres have equal charges q and are separaed by a disance r. The force exered on each sphere by he oher has a magniude F. if he charge on each sphere is doubled and r is halved, he force on each sphere has a new magniude of: A) F ) 2F C) 4F D) 8F E) 16F Using Coulomb s law, he ne force is direcly proporional o he magniude of he produc of he charges; in case boh charges double, he ne force increases by a facor of four. If he disance is halved, he ne force furher increases by a facor of four, since he force is inversely proporional o he square of he disance beween charges. Therefore, he oal increase is by a facor of 16. 1

2 4. Charges +Q and -4Q are siuaed as shown below. The ne elecric field is zero closes o which poin? A C D E A poin A, he field due o +Q is o he lef, and he field due o -4Q is o he righ. Since -4Q is larger in magniude by a facor of four, is disance from poin A mus be double ha of +Q for he ne elecric field o be zero; his is indeed he case in he figure. 5. Which of he following capaciors, each of which has plaes of area A, would sore he mos charge for a given poenial difference V? A C D E A smaller disance beween he plaes resuls in a greaer capaciance, and he presence of a dielecric beween he plaes furher increases he capaciance. 6. The figure below shows some equipoenial surfaces in a region of space. An elecron placed a posiion D would mos likely approach which poin afer being released from res? A) poin A ) poin C) poin C D) no move E) poin A or C An elecron will move/accelerae owards a region of higher poenial, since he force on he elecron opposes he direcion of he elecric field. The region of higher poenial is for an elecron a region of lower poenial energy! 2

3 7. In he diagram below, wha is he elecric poenial a poin P? A) kq 2d ) kq 2d C) kq d D) kq d E) zero V kq d kq d kq d kq 2d kq 2d 8. The equivalen capaciance of he nework conneced o he baery is: A) 10 7 F ) 3 2 F C) 7 F D) 7F E) 14F 3 C eq 5 ( ) 1 7F 9. The maximum charge sored in he 5 F capacior in problem #8 is mos nearly: A) 360 C ) 500 C C) 710 C D) 1100 C E) 1800 C The poenial difference across he capacior is 100 V, same as he baery. Therefore, Q=CV=500 C. 3

4 10. Poins R and S are he same disance from wo unequal charges, +Q and +2Q, as shown below. The work required o move a charge Q from poin R o poin S is: A) dependen on he pah aken from R o S. ) direcly proporional o he disance beween R and S. C) posiive. D) zero. E) negaive. Poins R and S have he same poenial. Therefore, moving he charge from R o S (saring from res and ending a res, and assuming absence of fricion) requires no ne energy. 11. A single elecron is acceleraed from res by an elecric field. Is pah is parallel o he elecric field. Which of he following saemens is correc? A) The charge is moving owards lower poenial. ) The direcion of force on he charge and he direcion of he elecric field are he same. C) The charge is gaining poenial energy. D) The charge is gaining kineic energy. E) None of he above As he charge is acceleraed by he field, is poenial energy decreases and kineic energy increases. 12. A wire made of brass and a wire made of silver have he same lengh, bu he diameer of he brass wire is 4 imes he diameer of he silver wire. The resisiviy of brass is 5 imes greaer han he resisiviy of silver. If R denoes he resisance of he brass wire and R S denoes he resisance of he silver wire, which of he following is rue? R A) R 5 16 R S ) R 4 5 R S C) R 5 4 R S D) R 5 2 R S E) R 16 5 R S l l 5 l 5 R A r (4 r ) 16 S 2 2 S S 4

5 13. How many elecrons are moving hrough a curren of 2A for 2 seconds? A) 3.2 x ) 6.3 x C) 4 D) 2.5 x E) 1 Q Q I 2A2C/s 2s 1e Q4C C 19 Q4C e An immersion heaer of resisance R convers elecrical energy ino hermal energy ha is ransferred o he liquid in which he heaer is immersed. If he curren in he heaer is I, he hermal energy ransferred o he liquid in ime is: A) IR ) I 2 R C) IR 2 D) IR 2 E) IR E 2 2 PVI I R E I R 15. The volage drop across he 6 resisor is mos nearly: R I A) 16 V ) 11 V C) 8 V D) 32 V E) 24 V eq 12 (6 12 ) 16 T V R eq 2A If he oal curren is 2A, he volage drop across he 12 resisor is 24 V. Therefore, he volage drop across he 6 resisor is 32 V 24 V = 8 V, and he curren hrough he 6 resisor is 1.3 A. 5

6 16. The circui shown in he figure below carries 1A and 2A in wo branches. The value of R is mos nearly: A) 10 ) 20 C) 30 D) 40 E) 50 Using Kirchhoff s Rules, he volage drop in he loop mus be equal o zero, and wih he loop rule he curren hrough R mus be 3 A. V 50 3R20 0 R A 20- resisor has 10 A of curren in i. The power generaed is: A) 2000 W ) 2W C) 200 W D) 4000 W E) 20 W P 2 2 I R (10) (20) 2000 W 18. Assuming he capacior in he RC circui below is iniially uncharged, which of he following graphs represens he curren vs. ime in he circui afer he swich is closed? A) ) C) D) E) The curren sars a is maximum value and diminishes exponenially wih ime. The same is rue for a discharging capacior. 6

7 19. The capacior in he circui in problem #18 is charged o 90% of is maximum value in 2.5 ms. Wha is he value of he capacior if he resisor R 1 is 100? A) F ) 1.1 x 10-5 F C) F D) 1.1 x 10-8 F E) 8.1 x 10-3 F RC RC Q ( ) Q(1 e ) 0.90 Q e 0.10 ln(0.10) RC ln(0.10) C F (100) C 20. The resisance of an axon beween successive nodes of Ranvier is 25 M. The capaciance of each secion beween nodes is 1.6 pf. If he nodes are spaced approximaely 1 mm apar, calculae he speed a which he nerve impulse ravels down he axon. A) m/s ) 16 m/s C) 25 m/s D) m/s E) 25,000 m/s l l.001 m v 25 m/s 6 12 RC (2510 Vs/C)( C/V) 7