Physics 1252 Exam #3C

Transcription

1 Phsics 1252 Eam #3C Instructions: This is a closed-book, closed-notes eam. You are allowed to use a clean print-out of our formula sheet, an scientific calculator, and a ruler. Do not write on our formula sheet, ecept for our name: it must be handed in, signed but clean, with our eam. There is space after each question to show our work; if ou need more space, ou ma use the back of the page, or request more paper. Please clearl indicate where our work for each problem is. Underline or draw a bo around our final answer. The eam consists of four sections. Read all the questions at the start so that ou can allocate our time wisel. Do eas ones first! You ma not share our calculator. The use of cell phones or an other electronic devices (besides calculators) is prohibited. All such gadgets must be turned off and put awa throughout the eam. Do not open the eam until told to begin. You have the one entire class period to finish the eam. Put our last name on ever page of the eam and on the formula sheet. You must provide eplanations and/or show work legibl to receive full credit for Sections II and III. Make sure that our answers include appropriate units and significant digits. (Note: or intermediate steps in our calculation, it s best to carr more significant digits.) undamental constants and unit prefies are on the ormula Sheet, last page. signing below, ou indicate that ou understand the instructions for this eam and agree to abide b them. You also certif that ou will personall uphold the universit s standards of academic honest for this eam, and will not tolerate an violations of these standards b others. Unsigned eams will not be graded. Signature: UGACard #: Copright c 2015 Universit of Georgia. Unauthoried duplication or distribution prohibited.

2 Section I II III Score /30 /35 /35 I: Multiple-Choice Questions (30 points) or each question below, choose the single best response and write the corresponding capital letter in the bo provided. There is no penalt for guessing the wrong answer. 1. In the circuit fragment shown below, given currents are = +7A and = +3A; and resistances are R b = 6 Ω and R c = 12 Ω. What is the electrical potential difference V bc V b V c between points b and c in the circuit? a R c c ig R b b Arrows indicate positive current directions: I>0 if I flowing into arrow direction I<0 if I flowing against arrow direction A. +60V. 96V C. +24V D. 24V E. +96V Copright c 2015 Universit of Georgia. 2

3 2. A small particle, carring an electrical charge, q, is traveling in a uniform magnetic field,. Which the following figure panels could correctl represent the -field direction; the magnetic force,, eerted on the particle b the magnetic field; and either the particle s velocit vector, ig v, or its q v-vector, whichever is indicated in the panel. v (1) (2) v qv (3) (4) v (5) qv A. Panel (2), but onl if q is positive.. Panel (1), regardless of the sign of q. C. Panel (1), but onl if q is positive. D. Panel (5), but onl if q is positive. E. oth Panels (3) and (4), but onl if q is negative. 3. An anti-proton is a sub-atomic particle with the same mass, but the opposite charge as a proton. If an anti-proton is initiall placed at a finite distince d from a positive point charge, Q, the anti-proton must be given a certain minimum initial speed, v min, in order to be able to travel awa, to a ver large (infinite) distance from the point charge Q. What would be the value of this minimum required speed, v min, if instead the anti-proton were placed at a distance at a distance d = 5d from another positive point charge, Q = 16Q? A v min 256. v 25 min 5 C. v 4 min 4 D. 5 v min E v min Copright c 2015 Universit of Georgia. 3

4 4. A wire of some length L and with a circular cross-section of diameter D, made out of a material with an electrical resistivit ρ, has a resistance of 350 kω. What will be the resistance of a wire of length 16L and diameter 4D, made from a different material, with a resistivit 1 7 ρ? A. 200 kω kω C kω D kω E. 50 kω 5. The three different circuits, X, Y and Z, shown below, are built with the same three resistors, R 1 > 0, R 2 > 0, and R 3 > 0, but each with a different batter, chosen so that the voltage drop V 3, across R 3, is the same in all three different circuits. Compare and rank the batter voltages, E, in the three different circuits. ig I o R 1 I o R 2 I o R 3 E R 2 E R 3 E R 1 R 3 R 1 R 2 (X) (Y) (Z) A. E(Y ) > E(X) > E(Z). E(Z) > E(X) > E(Y ) C. E(X) > E(Z) > E(Y ) D. E(X) > E(Y ) > E(Z) E. E(Y ) > E(Z) > E(X) Copright c 2015 Universit of Georgia. 4

5 6. Two ver thin circular rings, labeled 1 and 2 in the following, with radii R 1 (unknown) and R 2 = 48cm, respectivel, and R 1 > R 2, are both centered at the coordinate origin O (0, 0, 0), as shown below. Ring 1 lies in the -plane with a current flowing around the ring in the direction indicated below in panel (). Ring 2 lies in the -plane with a current flowing around the ring in a direction that ig. is 3.59 not indicated below in panel (C). (A) () (C) Plane View --Plane View --Plane View Assume = 4 5 and the net magnetic field produced b both rings at the origin O, shown in panel (A), lies in the --plane and makes an angle of eactl 45 o with the +-direction. ind the radius R 1 and the direction of the current. A. R 1 = 60cm; -direction is counter-clockwise in panel (C).. R 1 = 75cm; -direction is clockwise in panel (C). C. R 1 = 32cm; -direction is counter-clockwise in panel (C). D. R 1 = 60cm; -direction is clockwise in panel (C). E. R 1 = 75cm; -direction is counter-clockwise in panel (C). Copright c 2015 Universit of Georgia. 5

6 II: Proton Motion in Point Charge ields (35 points) Two big point charges, Q 1 = 8nC and Q 2 = 8nC, are affied to the -ais, at 1 = 2cm and at 2 = +2cm, resp. A proton is shot towards Q 1 and Q 2, starting from an initial position on the -ais, at some i < 0, ver, ver far awa from Q 1, meaning: i =! The proton s initial speed at i is v i = 700km/s and it is traveling initiall in +-direction. (a) ind the net electric potential, V f, generated b Q 1 and Q 2 on the -ais at f = 5cm. (Assume V = 0 at infinite distance from Q 1 and Q 2.) (b) If the proton reaches f, what is the proton s speed, v f, at f = 5cm? Else, where does the proton come to a stop, before reaching f? (c) The charge Q 2 is now removed, the sign of Q 1 is reversed, and the eperiment is then repeated with all other parameters the same as before. Let C ( c, c ) denote the proton s point of closest approach to the charge Q 1. How far from Q 1 is C? ind c! Hint: At C, the proton s speed is v c = 363.6km/s, and c = cm and c < 0. (You don t have to show this.) Copright c 2015 Universit of Georgia. 6

7 Work and Drawing Space for Problem II: Copright c 2015 Universit of Georgia. 7

8 III: Circuit Analsis (35 points) In the circuit shown below, assume that =14A, = 8A, V V = +6V, R 1 = 4Ω, R 2 = 3Ω, R 4 = 6Ω, C = 75 µ. Also assume that the capacitor is full charged, b the voltage drop across it, and there is no current flowing to or from either capacitor plate. ig p I o a R 1 R 2 E I C C R 3 R C I 4 R 4 q b (a) ind the total current, I o, flowing through the batter. (b) ind the batter voltage E. (c) How much heat is generated in R C during 1 hour? Copright c 2015 Universit of Georgia. 8

9 Work Space for Problem III: Copright c 2015 Universit of Georgia. 9