Physics Questions

| August 14, 2017

Physics E-1b Homework Assignment #6 spring 2013Due by 6:00 pm on Thursday, March 28Assignments should be dropped into your Section TF’s locked mailbox, located on the2nd floor near room 201, and will be graded by your TF.Reading: Chapter 20 of Giambattista et al Physics, 2nd edition, pp 741-769For next lecture: Chapter 21, pp 780-799Problems: [4 pts each]1. In the August 1992 space shuttle fight, only 250 m of the conducting tether (discussedin lecture) could be let out. A 40.0 V motional emf was generated in the Earth’s 5.0×10-5T field, while moving at 7.8×103 m/s. What was the angle between the shuttle’s velocityand the Earth’s field, assuming the conductor was perpendicular to the field?2. The armature of an ac generator is a rectangular coil, 2.0 cm by 6.0 cm with 80 turns.It is immersed in a uniform magnetic field of magnitude 0.45 T. If the amplitude of theemf in the coil is 17.0 V, at what angular speed is the armature rotating?3. A long straight wire carrying a steady current is in the plane of a circular loop of wire.(a) If the loop is moved closer to the wire, what direction does the induced current flow?(b) At one instant, the induced emf in the loop is 3.5 mV. What is the rate of change ofthe magnetic flux through the loop at that instant (in webers per second)?4. A dc motor has coils with a resistance of 16 ? and is connected to an emf of 120.0 V.When the motor operates at full speed, the back emf is 72 V.(a) What is the current in the motor when it first starts up?(b) What is the current when the motor is at full speed?(c) If the current is 4.0 A with the motor operating at less than full speed, what is theback emf at that time?5. A transformer for an answering machine takes an ac voltage of amplitude 170 V as itsinput and supplies a 7.8 V amplitude voltage to the answering machine. The primary has300 turns.(a) How many turns does the secondary have?(b) When idle, the answering machine uses a maximum power of 5.0 W. What is theamplitude of the current drawn from the 170-V line?6. When the current in loop 1 increases at a steady rate of 9.0 A/s, thecurrent in loop 2 is 0.185 mA. The resistance of loop 2 is 0.66 ?.(a) What is the direction of the current in loop 2 as viewed from theright?(b) What is the mutual inductance of the two loops?7. No currents flow in the circuit (below) before the switch is closed. Consider all circuitelements to be ideal.(a) At the instant the switch is closed, what are the values of the currents I1 and I2, thepotential differences across the resistors, and the induced emf in the inductor?(b) After the switch has been closed for a long time, what are the values of the currents I1and I2, the potential differences across the resistors, and the induced emf in the inductor?8. A 0.67 mH inductor and a 130 ? resistor are placed in series with a 24 V battery.(a) How long will it take for the current to reach 67% of its maximum value?(b) What is the maximum energy stored in the inductor?(c) How long will it take for the energy stored in the inductor to reach 67% of itsmaximum value? Comment on how this compares to the answer in part (a).Multiple Choice QuestionsEach multiple choice question or incomplete statement has one best answer — determinethe best answer and explain your choice by invoking the appropriate concept, principle,and/or mathematical relation. Answers without explanation will not receive full credit,even if correct. [2 pts each]1. A horizontal rod (oriented in the east-west direction) is moved northward at constantvelocity through a magnetic field that points straight down. Which statement is true?a) The west end of the rod is at higher potential than the east end.b) The east end of the rod is at higher potential than the west end.c) The top surface of the rod is at higher potential than the bottom surface.d) The bottom surface of the rod is at higher potential than the top surface.2. A closed loop moves at a constant speed parallel toa long straight current-carrying wire, as shown.a) the induced current in the loop will progress clockwiseb) there will be no induced current in the loopc) the induced current in the loop will progress counterclockwised) the induced current in the loop will vary with the speed at which the loopmovese) none of these3. The figure shows an aluminum ring and thecurrent induced in it by the nearby magnet that isfree to move along its central axis.a) the magnet must be moving tothe rightb) the magnet must be stationaryc) the magnet must be moving to the leftd) not enough information to say anything about the magnete) none of theseI4. The coil of a generator rotates through one complete turn in a uniform magnetic field50 times every second. When this generator is connected to an external load, the currentthrough the load reverses directionsa) 25 times every second.b) 50 times every second.c) 100 times every second.d) none of the above.

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