Physics 1252 Exam #3E (Make-Up)

Transcription

1 Physics 1252 Exam #3E (Make-Up) Instructions: This is a closed-book, closed-notes exam. You are allowed to use a clean print-out of your formula sheet, any scientific calculator, and a ruler. Do not write on your formula sheet, except for your name: it must be handed in, signed but clean, with your exam. There is space after each question to show your work; if you need more space, you may use the back of the page, or request more paper. Please clearly indicate where your work for each problem is. Underline or draw a box around your final answer. The exam consists of four sections. Read all the questions at the start so that you can allocate your time wisely. Do easy ones first! You may not share your calculator. The use of cell phones or any other electronic devices (besides calculators) is prohibited. All such gadgets must be turned off and put away throughout the exam. Do not open the exam until told to begin. You have the one entire class period to finish the exam. Put your last name on every page of the exam and on the formula sheet. You must provide explanations and/or show work legibly to receive full credit for Sections II and III. Make sure that your answers include appropriate units and significant digits. (Note: For intermediate steps in your calculation, it s best to carry more significant digits.) Fundamental constants and unit prefixes are on the Formula Sheet, last page. By signing below, you indicate that you understand the instructions for this exam and agree to abide by them. You also certify that you will personally uphold the university s standards of academic honesty for this exam, and will not tolerate any violations of these standards by others. Unsigned exams will not be graded. Signature: UGACard #: Copyright c 2015 University of Georgia. Unauthorized duplication or distribution prohibited.

2 Section II III Score / /35 /35 Copyright c 2015 University of Georgia. 2

3 II: Electron Between Charged Surfaces (35 points) Two large, planar, parallel, square surfaces, S 1 and S 2, each of area A = 4.0m 2, and unknown spacing d (with d A) carry unknown charges Q s and Q s, respectively, uniformly spread out over each surface. Their electric potential difference is V 2 V 1 = 6000V, where V 1 and V 2 are the electric potentials on the top and bottom surface, S 1 and S 2, respectively. An electron is shot through a small hole in S 1, into the space between S 1 and S 2. Reminder: Between S 1 and S 2, the electric potential, V s (y), due to the surface charges, Q s and Q s, varies linearly with y: V s (y) = V 2 E y y, where E y is a constant. y S 1 Fig S 2 x (a) What is the minimum speed, v min, the electron must have, as it passes through S 1, in order to be able to reach the surface S 2? (b) If the electron is shot through the hole at some angle from the y-axis, its point of closest approach to S 2 is at a distance of y c =(0.6) d from S 2, and its speed is v c =36500km/s at that point. What was the electron s speed, v 1, as it passed through the hole at S 1? (c) Assuming Q s =+1.77µC on S 1, what is the spacing, d, between the two surfaces? (d) A point charge, Q p, is now added at a distance of y p = 3.0cm from S 2, on the y-axis. Assume the same spacing d as in (c) and the same two surface charges, ±Q s, as before, remaining uniformly spread out over S 1 and S 2, respectively. If the electron is shot through the hole, at some angle from the y-axis, with initial speed v 1 = 74000km/s at S 1, it will impact on S 2 with a speed v 2 =85000km/s at x 2 = 2.5cm. Find Q p! Copyright c 2015 University of Georgia. 3

4 Work and Drawing Space for Problem II: Copyright c 2015 University of Georgia. 4

5 III: Circuit Analysis (35 points) In the circuit shown below, assume that I 4 =45A, I C =75A, R o =2Ω, R 1 =5Ω, R 2 =2.5Ω, C =50µF, and the electric field energy, U E, stored in the capacitor is U E = +360mJ. Also assume that the capacitor is fully charged, by the voltage drop across it, and there is no current flowing to or from either capacitor plate. Fig a I o I 1 R 1 I 2 R 2 x I C C y R o I 3 R 3 E I 4 R 4 b (a) Find the battery voltage E. (b) Find the current I 1. Hint: Use a junction (node) to find I 2, then a loop to find I 1. (c) Find the resistance R 4. Hint: Use a junction to find I o, then a loop to find R 4. (d) Find the resistance R 3. Copyright c 2015 University of Georgia. 5

6 Work Space for Problem III: Copyright c 2015 University of Georgia. 6