Physics 1252 Exam #3B

Transcription

1 Phsics 1252 Eam #3 nstructions: 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, and V. 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 2017 Universit of Georgia. Unauthorized duplication or distribution prohibited.

2 Section V (onus) Score /20 /40 /40 /10 : Multiple-Choice Questions (20 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. f a wire of some length L and with a circular cross-section of diameter D has a resistance of 360 kω, what will be the resistance of a wire made from the same material, at the same temperature, of length 10L and diameter 3D? A kω. 400 kω C. 324 kω D. 108 kω E kω 2. Three different circuits, X, Y and Z, are built with the same three resistors, R 1 > 0, R 2 > 0, and R 3 > 0, and the same batter of batter voltage E, as shown below. Compare and rank the magnitude of the voltage drops V 3 across R 3, observed in the three different circuits. ig o R 1 o R 2 o 3 R 3 E R 2 E 3 R 3 E R 1 3 R 3 R 1 R 2 (X) (Y) (Z) A. V 3 (X) > V 3 (Z) > V 3 (Y ). V 3 (Z) > V 3 (Y ) > V 3 (X) C. V 3 (X) > V 3 (Y ) > V 3 (Z) D. V 3 (Z) > V 3 (X) > V 3 (Y ) E. V 3 (Y ) > V 3 (X) > V 3 (Z) Copright c 2017 Universit of Georgia. 2

3 3. A ver thin circular ring with radius R = 4m is centered at the coordinate origin O (0, 0, 0), as shown below. The ring lies in the z-plane with current = (24/π)A flowing around the ring in the direction shown in drawing (). (A) () ig z z z --Plane View -z-plane View An infinitel long straight wire carries a current parallel to the z-ais and crosses the -ais at = +10m. The net magnetic field,, generated b and, is zero at the center of the ring. What is the magnitude and the direction of? A. (120/π)A, in z-direction;. 60A, in z-direction; C. 30A, in z-direction; D. 30A, in +z-direction; E. 60A, in +z-direction; 4. A piece of straight wire carring a current,, is subjected to a uniform magnetic field,. Which one of the following figure panels could correctl represent the wire, its current flow direction, the -field direction and direction of the magnetic force,, eerted on the wire b the magnetic ig field? (A) () (C) (D) (E) Copright c 2017 Universit of Georgia. 3

4 : Circuit Analsis (40 points) n the circuit shown below, assume that = 14A, 3 = 8A, V V = 20V, R 1 = 3Ω, R 2 = 4Ω, R 4 = 6Ω, C = 50 µ. Also assume ig that the capacitor is full charged, b the voltage drop across it, and there is no current flowing to or from either capacitor plate. R 2 a C C c R 1 p o E 4 R 4 R C 3 R 3 q b d (a) ind the total current, o, through the batter. (Use a loop to get, then a junction.) (b) ind the batter voltage E. (Use a junction to find 4, then a loop.) (c) How much heat is generated in R C during 1 hour? Copright c 2017 Universit of Georgia. 4

5 Work Space for Problem : Copright c 2017 Universit of Georgia. 5

6 : Magnetic ields and orces (35 points) Two ver long thin straight wires running parallel to the z-ais and cutting through the -ais at 1 = 4m and through the -ais at 2 = 5m, respectivel, carr currents in ig the z-direction and in the +z-direction, as shown in drawing below. is 6 times as strong as. The two currents produce magnetic field contributions 1 and 2, respectivel, at point P ( 5m, 4m, 0m) in the -plane. --Plane View P. z (a) What is the absolute value of the angle, θ, between the +-direction and the total magnetic field vector, = 1 + 2, produced b the two currents at point P? (Hint: You onl need the field strength ratio, 1 / 2, to find θ. Get 1 / 2 from the given ratio of the currents, /.) (b) f the total magnetic field strength at point P is = 3.2µT what are and? (c) Suppose an electron is shot through point P in the z-direction with a speed of m/s. ind the electron s acceleration vector, a (a, a, a z ) at P, due to the magnetic force. State all three cartesian components of a. [Hint: Use the same direction and strength of as found in (a) and given in (b), respectivel.] Copright c 2017 Universit of Georgia. 6

7 Work Space for Problem : Copright c 2017 Universit of Georgia. 7

8 V: More Magnetic orce (10 points, onus) Suppose the two current-carring wires in Problem are both 150m long, the current is 7A and the ratio of the two currents is the same as given in Problem. ind the magnetic force eerted b wire 1 on wire 2. State the strength of that force,, and the angle between and the +-direction. Copright c 2017 Universit of Georgia. 8