Physics 121 Smple Common Exm 1 NOTE: ANSWERS ARE ON PAGE 8 Nme (Print): 4 Digit ID: Section: Instructions: Answer ll questions. uestions 1 through 16 re multiple choice questions worth 5 points ech. You my need to do some clcultion. Answer ech of these on the Scntron sheet using #2 pencil. You my use bcks of the question ppers for extr spce. uestions 17 nd 18 re extr credit work-out problems worth 10 points ech. Show ll work nd nswer them on the exm booklet. Be sure your nme nd section number re on both the Scntron form nd the exm booklet. Also write your nme, id, nd section t the top of pges 5 nd 6, which my be seprted for grding. You my use one side of n 8.5 x 11 sheet or two sides of 5x8 crd s formul sheet. A defult formul sheet is lso provided (see the lst pge). Mke sure to bring your own clcultor Plese turn off ny cell phones, pgers, communiction or entertinment devices, etc..shring of clcultors is not permitted. Students re not to communicte with ech other once the test hs strted. If you hve questions or need something cll your proctor or instructor. 1. The electrosttic force between two negtively chrged cubes, 10 cm. on ech side, with 1 = -7 µc nd 2 = -4 µc is: A) Attrctive. The force on cube 1 is twice s lrge s the force on cube 2. B) Attrctive. The force on cube 1 is equl to the force on cube 2. C) Zero. D) Repulsive. The force on cube 1 is twice s lrge s the force on cube 2. E) Repulsive. The force on cube 1 is equl to the force on cube 2. 2. The force cting on n electron 100 cm from positive.1 µc point chrge in vcuum is: A) 2.8 x 10 +10 N. Repulsive B) 1.0 x 10-19 N. Attrctive C) 1.4 x 10-12 N. Repulsive D) 1.4 x 10-16 N. Attrctive E) 1.6 x 10-16 N. Attrctive 3. A positively chrged prticle with q= 2 µc is plced between two other positively chrged prticles with q 1 = 9 x 10-3 C nd q 2 = 3 x 10-3 C s shown on the figure. The distnce between q 1 nd q is 6 cm nd the distnce from q to q 2 is 12 cm. Wht re the mgnitude nd direction of the totl force cting on the middle prticle? A) 4.1 x 10 +4 N. right B) 2.8 x 10 +9 N, right C) 4.9 x 10 +4 N, left D) 4.9 x 10 +4 N, right E) 9.0 x 10 +9 N, left q1 q q2 + + + 6 cm 12 cm Pge 1 of 7
4. The mgnitude of the chrge on ech of the four blls is 12 µc distributed uniformly on the surfce of ech one. Two re positive nd two re negtive s shown in the sketch. They re plced t the corners of the dimond s shown in the digrm. Ech of the sides hs length = 3 cm. Wht re the mgnitude nd direction of the electric field t the center of the dimond shpe? A) 24.1 x 10 13 N/C. Down B) 1.6 x 10 12 N/C. Up C) 7.9 x 10 19 N/C. Left D) 1.4 x 10 10 N/C. Right E) Zero -. E - 5. For n isolted sphere of chrge, which of the following sttements bout the electric field outside it is correct? A) It is directly proportionl to the distnce from the sphere. B) It is inversely proportionl to the squre of the distnce from the sphere. C) It is directly proportionl to the squre of the distnce from the sphere. D) It is inversely proportionl to the distnce from the sphere. E) None of the bove. 6. Which of the following is NOT true of the electric field ner n infinitely lrge conducting plne? A) It is inversely proportionl to the squre of the distnce from the plne. B) It is directly proportionl to the surfce chrge density. C) It is constnt t ll distnces from the chrged plne. D) The field lines re ll prllel. E) All of the bove. 7. For n infinite line of chrge, which of the following is true of the electric field? A) It is directly proportionl to the distnce from the chrged line. B) It is constnt for ll distnces from the chrged line. C) It is directly proportionl to the squre of the distnce from the chrged line. D) It is inversely proportionl to the squre of the distnce from the chrged line. E) None of the bove. 8. The field lines for uniform electric field point south s shown. If proton is relesed somewhere in the field, which wy will it move? A) West B) Est C) South D) North E) Closer to the proton W N E S Pge 2 of 7
9. A negtively chrged prticle is plced in uniform electric field directed West. Wht re the mgnitude nd direction of the prticle's ccelertion if its chrge is q= -6 µc, its mss m = 2 grms, nd the vlue of electric field E is 6 x10-6 N/C? Select the closest nswer: N A) West. = 1.8 x10-8 m/s 2 B) Est. = 1.8 x10-3 m/s 2 C) West. = 1.8 x10 +3 m/s 2 D) Est. = 1.8 x10-8 m/s 2 E) Est. = 1.8 x10-5 m/s 2 W E S 10. An electron with mss m = 9.11 x 10 31 kg is relesed t rest ner very lrge positively chrged non-conducting chrged sheet lying horizontlly s shown. Wht should the surfce chrge density σ on this sheet be to keep the electron blnced t rest bove the ground? Is it bove or below the sheet? A) 8.55 x 10-1 C/m 2 B) 2.35 x 10-6 C/m 2 C) 1.8 x 10-18 C/m 2 D) 9.88 x 10-22 C/m 2 E) 1.84 C/m 2 ++++++++++++++++++++++++++++++++++++++ - g 11. Find the electric flux through the surfce of rectngulr Gussin surfce with chrge of 3.1 C. plced t it's center. The ends of the box re squres whose sides re 4.0 cm. The length is 8.0 cm. A) 4.5 x 10 +11 N m 2 /C B) 3.5 x 10 +11 N m 2 /C C) 0.1 x 10-16 N m 2 /C D) 4.4 x 10-10 N m 2 /C E) Insufficient informtion + 12. A chrge is plced on n isolted, hollow metl rectngulr solid with closed ends s shown. How will this chrge be distributed fter while? A) The chrge will be distributed uniformly over the volume of the solid. B) The chrge will continuously move long the surfce, creting constnt current. C) The chrge will be concentrted s point chrge t the center of the solid. D) The chrge will be distributed over the outer surfce of the solid. E) The chrge will be distributed over the outer nd inner surfces of the solid. Pge 3 of 7
13. A negtive chrge = - 2 µc is plced inside neutrl cubicl conducting box with cvity in it. The length of the side of the inner cvity is nd the outer box's side length is b. Wht chrges will be induced t the inner nd outer surfces of the box? A) Inner chrge = 2 µc. Outer chrge = +2 µc B) Inner chrge = +2 µc. Outer chrge = -2 µc C) Inner chrge = 0 µc. Outer chrge = -2 µc D) Inner chrge = +2 µc. Outer chrge = 0 µc E) Inner chrge = 0 µc. Outer chrge = 0 µc 14. The sme negtive chrge = - 2 µc is plced inside the sme box in the problem bove, but now in ddition, n extr negtive chrge of ex = - 4 µc. is initilly plced on the the inner surfce of the cvity. Wht will the chrges be on the inner nd outer surfces of the box fter some time hs pssed? A) Inner chrge = 0 µc. Outer chrge = -6 mc B) Inner chrge = + 2 µc. Outer chrge = -6 µc C) Inner chrge = -6 µc, Outer chrge = -2 µc D) Inner chrge = 4 µc. Outer chrge = +2 µc E) Inner chrge = +2 µc, Outer chrge = -2 µc Extr chrge ex 15. A solid, conducting sphere hs net positive chrge of = 4.5 C on it. It's rdius R is 3.0 m. Wht is the mgitude of the electric field t distnce D of 9.0 m. from the center of the sphere. A) 9 x 10 9 N/C B) 0.0 N/C C) 1.0 x 10 9 N/C D) 5 x 10 8 N/C E) 1.0 x 10 8 N/C D R 16. For sme solid, chrged conducting sphere of the problem bove, find the mgnitude of the electric field t distnce D = 0.5 m. from the center of the sphere. A) 9.0 x 10 9 N/C B) 0.0 N/C C) 1.0 x 10 9 N/C D) 0.5 x 10 9 N/C E) 2.25 x 10 9 N/C Pge 4 of 7
Nme (Print) 4 Digit ID Section Show ll work for the following problem 17. Four prticles re plced t the corners of squre whose sides = 100 mm. The two right prticles hve positive equl chrges of = 6 C nd the left two prticles hve negtive equl chrges of - = 6 C. Find the mgnitude nd direction of the net electric field E exctly t the center of the squre. Drw cler vector digrm nd present ll your clcultions; Write your finl nswer in the spce provided below: Mgnitude of E = Direction of E = y Show the equtions you pln to solve: - Are there ny symmetries you cn use? o - x Solution: Pge 5 of 7
Nme (Print) 4 Digit ID Section Show ll work for the following problem: 18. A smll, nonconducting bll of mss m = 0.1 gm. nd chrge q = 5.0 x 10-6 C. (distributed uniformly through its volume) hngs from n insulting thred tht mkes n ngle θ = 20 with verticl, uniformly chrged nonconducting sheet (shown in cross section). Considering the grvittionl force on the bll nd ssuming tht the sheet extends fr verticlly nd into nd out of the pge, clculte the surfce chrge density σ of the sheet. ) Drw cler free body digrm which shows the blnce between the tension, electricl, nd grvittionl forces. b) Clculte σ, the chrge on ech bll. Write your finl nswer here: σ = Pge 6 of 7
Physics 121 Common Exm 1 Formuls 1/4πε o = 9 x 10 9 N-m 2 /C 2 ε o = 8.85 x 10-12 C 2 /N-m 2 e = 1.60 x 10-19 C m e = 9.11 x 10-31 kg 1 q q F = 1 2 rˆ 1 µc = 10-6 C 1 nc = 1 nno-coulomb = 10-9 C 4 πε 2 r 0 1 E = rˆ (point chrge) F = qe τ 4 πε 2 dipole = pxe U dipole = -p.e p = qd r 0 Φ = electric flux = q enc /ε o = E. d2 A over Gussin surfce E = σ/ε o (conducting sheet of chrge) σ = surfce chrge density E = σ/2ε o (non-conducting sheet of chrge) E = (1/2πε o )λ/r (line of chrge) v = v o + t x x o = v o t + ½t 2 v 2 = v 2 o + 2(x x o ) x x o = ½(v + v o )t F net = m W pp = K f K i +U f U i U o = q o V V = (1/4πε o )/r V V o = -E(x x o ) Are of circle = πr 2 Circumference of circle = 2πr 1mm = 10-3 meters 1cm = 10-2 meters Pge 7 of 7
Answers to Physics 121 Smple Common Exm 1 1. E 2. D 3. A 4. E 5. B 6. A (lso E by logic) 7. E 8. C 9. D 10. D 11. B 12. D 13. B 14. B 15. D 16. B 17. E =3.05 x 10 13 N/C. Direction = to the left 18. σ = 1.26 x 10-9 C/m 2 Pge 8 of 7