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2008 2220 Exam 3b Key

2008 2220 Exam 3b Key - Physics 2220 Spring 2008...

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Unformatted text preview: Physics 2220 Spring 2008 Examination 3 Name: Key 2 - . Student ID: Instructor (circle one): Littler ‘Matteson , Weathers This test consists of 10 multiple-choice questions and 6 free—response problems, for a total of 110 points (so that 10 points of extra credit are possible). To receive credit for the free— response problems, you must show all of your work on the pages provided.~ Don’t hand in any extra sheets or other paper. You may also earn partial credit for the some of the multiple choice problems if you show your work. ' Suggested procedure for solving the problems: 1. Read each problem carefully and make sure you know what is being asked before starting the problem. Draw a figure for the problem. List the parameters given. Write down the equations to be used. Solve for the answer symbolically. Substitute numbers into you final equation and circle your answer. F‘P‘P‘PP WORK THE EASY PROBLEMS FIRST!! See km, 1 £V' explamotms NOTE: The value of each question is given next to the question number. Please circle the correct answer. ' ' Section 1 : Questions 1. (5) If a planar coil of wire is placed in a constant magnetic field, an emfcan be induced in the coil ' ‘ @if the coil is rotated about an axis perpendicular to the magnetic field direction and parallel to the plane of the coil. (b) if the coil is rotated about an axis perpendicular to the magnetic field direction and the plane of the coil. (0) both (a) and (b) (d) An emf will not be induced; the magnetic field is constant. 2. (5) An electromagnetic wave is made up of oscillating electric and magnetic fields. (b) oscillating magnetic fields. (0) oscillating electric fields. (d) oscillating electrons. 3. (5) Which of the following electromagnetic waveshas the lowest frequency? (a) Gamma rays (b) Ultraviolet light @ Microwaves (d) Infrared radiation (e) Visible light . 4. (5) A transformer has 100 turns in its primary and 200 turns in its secondary. If the primary is connected to a 200V DC source, What is the voltage across the secondary? (a) 400 V (b) 200V (0) 100 V (d) 50 V .ov 5. (5) Two identical magnets are dropped into two long hollow tubes. One of the magnets moves slowly down its tube, while the other undergoes essentially free fall. Which of the following could explain this behavi01? * (a) One of the magnets is dropped with its north pole entering the tube first, while the other 18 dropped south pole first. . .Only one of the tubes 13- made of conducting material _ , (C) One of the tubes has horizontal slots cut through its wall almost all the way arOund its circumference at seveial places along its length. ._ ‘ - ((1) There is a large electromagnet at the end of one tube, attracting that tube’s magnet. 6. (5) In an LC circuit, when the current is at its minimum, . (a) the magnetic field in the inductor is at a maximum. (b) the charge on the capacitor is at its minimum. (c the angular frequency of the ciicuit reaches its minimum. ‘the charge on the capacitor is at its maximum.‘ _ _ I or E f _ ,, , Charity. 'HIIWSDMO ‘0le Capo-(Flor as [Mg (15 flu W 15 "WM. In iii/us Circuit, winch oscillates, flu Current will “also? th9‘ and 7%qu in “townie abred‘rmy. 'flu mammalian? will hon flu 04104le atléz WWW: WWIM [1'1 Mlisndm‘nml. 7. (5) A displacement current can exist only when a time-varying electric field is present. (b) electrical charges are in motion. (0) a time—varying magnetic field is present. (d) the line integral of the magnetic field around a closed path vanishes. (e) the line integral of the electric field around a closed path does not vanish. 8. (5) The net magnetic flux through a closed surface will be nonzero under which of the following circumstances? (Circle all that apply.) (a) A loop of wire is placed inside the surface, and current is passed through the wire. (b) A bar magnet is placed inside the surface. (0) A bar magnet is cut in half, and one of the halves is placed inside the surface. ((1) A bar magnet is placed half in and half out of the surface, so that the surface passes between its poles. (e) A charged, insulating sphere is spun around inside the surface. (L5) None of the above. 9. (5) Radio waves are propagating in the —i direction At a particular instant, the electric field is observe to be in the +k direction. What 13 the direction of the magnetic field at this instant? , , (a) +l A A .s ' ' A 4- <b>—i S=—LEKB I=> ' DJ @+j #0 (d)—j :2 > ~ - (6) +16 A f ' “ ' (t) —k tiff-2 """ :. . --v J x s 10. (5) A series RLC circuit consists of a 60 Q resistor, a 20 {JP capacitOr, and a’ 60 mHinductor. ' What 13 the impedance of this circuit when driven at its resonant frequency? ' :2:-;(a)117Q ~ * ' - * '” *' ~~ -’ p. 60 9 1(0) 831 9 __ . A . ‘ _ (e) O Q L Section II—Problems ' Show all work on each problem. Credit will not be given unless work 15 shown. Remember that fields and forces are vectors” .direction 18 part of the answer. ' 1. (10) A 7000 kg spacecraft (capsule and sail) employs a 1.00 X 106 in perfectly reflecting surface to provide propulsion for travelling through the inner Solar System 1f- the average solar intensity is constant at 1370 W/rn2 for a ti 1p starting from Earth and assuming ideal conditions (no gravitational effects constant acceleration, sail facing the Sun, etc.) find: (a) the force exerted on the sail, (b) the acceleration of the spacecraft, .3» F_ 6'4“ .. “'3’“ z “a- M'BMO /s WMSM (c) the time re uired to ass the Moon, which is 3.84 x 108 in from the Earth. Cl P , S7 2. (10) Consider the circuit shown, where E— “ 15 V, L— “ IOmH, and R: 25 Q. a , 1 (a) What IS the inductive time constant of the circuit? (b) Calculate the current in the circuit 300 us after the switch is closed. 1= $04.71)“? = ’5" 531—0 63%) =31va (c) How long does it take the current to reach 40% of its maximum value? 1-1110! ) 400/“ ‘M’ M 32.41: 3. (10) A circular coilof 15 turns and radius of 20 cm is wrapped around a l m solenoid of radius 10 cm and 400 turns/meter. The currentin the solenoid varies according to the expression I(t) = (20A) cos[(100 34):]. - _ - (a) Find the induced emf in the 15~turn coil as a function of time. Iii—turn coil 7. E = NCO) 1W Smwtpm err rm“ m--- : 15-10% ~20A- 4mlo‘ll‘m -400/g'fi@:lm)19uwt z 0.473v WWW] (b) If the 15 turn coil were tilted at an angle of 30° with respect to the vertical, would the induced emf change? Why or why not? No, Wilma” Wm 1mm WWMM. 4. (10) A generator produces an rms current of 50 A at 600 V.- If the voltage is stepped up to 14,000 V by an ideal transformer andtransmitted a long distance through a power line of total resistance of 15 ohms, determine the percentage of power lost in transmission to a resistive load. (hint: % power lost is (PM / P) x 100). L ' 5/1.. ‘Rm- 5% M tin, = 2.30Kl0-3= 0.230% R“ ' (Al/z)? , 04.000” 5. (10) A thick glass plate (llglass = 1.62) lies at the bottom of a tank'of water (nwater = 1.33). A light ray enters the water from air at an angle of 60° with respect to the normal to the surface. What angle does the ray make with the normal When it is in the glass? Assume that the top face of the glass plate is horizontal. I ' i I asiWMfi m8) ill/«ISM (H52 SMCD 0) 1,332.},0 ,, _- 6. (10) If an R: 2. 00- k0 resistor, a C— — l OO~pF capacitor, and an L= 0. ZOO-H inductor are connected 1n series with a V= (150 V) sin [(377 s )z‘] source find ' (a) the phase angle between the current and the voltage in the source, and, I I ._ 37 . “‘5" x - 00° ...___. (b) the maximum current delivered by the source. r50 : VW : M : 46-0MA Rzi'édL-flz ”(20003)”? (3’77/5.0.2H' 37%1‘10’?) 1M ...
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