University of Maryland Department of Physics Spring 2002 Laura Lising Physics 122 May 21, 2003 Final Exam Answer all questions on these sheets. Please write clearly and neatly: We can only give you credit for what is on the paper! Put your name and section number on every page. There are two kinds of problems. The first set of questions is multiple choice. This doesn't mean that words or pictures won't help you! And they will help me in figuring out what feedback to give you. I'll be dropping one multiple choice problem to compute your final grade. For the last set of problems, you need to show your reasoning, in words and/or mathematics. These problems all require explanation. Full credit is for a correct answer with a clear explanation. You’ll get partial credit if we can understand and see the physical sense of what you were thinking. But you’ll get no credit at all, regardless of the answer you give, if we can’t follow your reasoning. If you are unclear about what a question is asking, ask me! If there is something I can't tell you, I won't tell you, but it doesn't hurt to ask! __________________________________________ _______________ Name (printed) Section # At the end of the exam, rewrite and sign the pledge: I pledge on my honor that I have not given or received any unauthorized assistance on this examination. ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ ___________________________________________________________________________ __________________________________________ Signature and datePhysics 122 Final Exam – p. 2 Name_____________________ Section____________________ Multiple choice questions. Just the answer counts for these. (8 points each) 1) The picture below shows a particle (labeled B) which has a net electric charge of +1 unit. Several centimeters to the left is another particle (labeled A) which has a net charge of -2 units. Choose the pair of force vectors (the arrows) that correctly compare the electric force on A with the electric force on B. Force on A Force on B a) b) c) d) e) 2) Regarding Cases A and B, which of these statements is true regarding the force on the +q charge? a) The force on the +q charge is slightly smaller in case B. b) The force on the +q charge is much smaller in case B. c) The force on the +q charge is slightly larger in case B. d) The force on the +q charge is much larger in case B. e) The force on the +q charge is zero in case B. +q Uncharged Styrofoam block. Case A Case B -q b +q -q b -2 A +1 B Attractive, equal and opposite. Styrofoam has dipoles which polarize, but attractive force is weaker than for conductors or something with excess charge.Physics 122 Final Exam – p. 3 Name_____________________ Section____________________ 3) What is the potential difference between points A and B? The bulbs are identical. a) 0 V b) 3 V c) 6 V d) 12 V e) None of the above. 4) Compare what happens when you throw the switch in the second circuit, with two capacitors in parallel, to what happens in the first circuit. The capacitors are all uncharged at first. Suppose the current through the resistor in the first circuit, when you first close the switch, is I0, and it takes t seconds to fully charge the capacitor. When you first close the switch for the second circuit, you’d expect the current through its resistor would be a) I0/2, and it would take about 2t seconds to fully charge the capacitors. b) I0 and it would take about 2t seconds to fully charge the capacitors. c) I0 and it would take about t seconds to fully charge the capacitors. d) 2I0 and it would take about 2t seconds to fully charge the capacitors. e) 2I0 and it would take about t seconds to fully charge the capacitors. 5) Suppose you looking at a tree with a pinhole camera. If you make the pinhole aperture larger, the image of the tree on the screen will get: a) blurrier b) brighter c) bigger d) all of the above e) brighter and bigger, but not blurrier f) blurrier and brighter, but not bigger A B Switch R C Switch R C C V V 6 V Call the voltage at B "0." This means the other side of the battery must be at 6 V. We know there is no current flowing in the circuit, so there must not be a voltage difference across either bulb. Thus, point A must be a 6 V, too. 0 6 Two capacitors like this in parallel are kind of like one bigger capacitor (twice the area on both sides so twice the capacitance.) Thus, the time constant is twice as large. But the initial current is the same since the pressure forcing the current through the resistor, all from the battery, is the same. Only slightly bigger.Physics 122 Final Exam – p. 4 Name_____________________ Section____________________ 6) A converging lens is made with a material with an index of refraction that is 1.7 for violet and 1.5 for red. This implies that: a) The focal length is longer for red light than for violet. b) The focal length is shorter for red light than for violet. c) The lens cannot make a sharp image of an object that is reflecting (or producing) both red and violet light. d) The red light will be going slower than the violet light when it hits the screen. e) a and c f) b and c g) c and d 7) Two sources of waves (of wavelength 2 cm) are at points S1 and S2. The sources are in phase (synchronized.) Point A is 25 cm from S1 and 19 cm from S2. At point A, there is a) constructive interference always b) destructive interference always c) constructive interference at this instant, then destructive a half cycle later, alternating back and forth d) we can't tell unless we know the distance d between the two sources and the angle θ to point A. 8) If you can get them very cold, you can pour neutrons into a special bottle. There they will decay with their normal half-life of about 10 minutes and the electrons and protons that are produced will be absorbed into the bottle walls. If, after 1½ hours, you have 50 neutrons left, how many were initially poured into the bottle? a) about 2 x 1015 b) 51200 c) 25600 d) 900 e) 512 f) 450 S1 S2 A 25 cm 19 cm 1.5 hours = 9 half-lives. 50 = 25600 * (1/2)9Physics
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