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Transcript

College Physics, PHYS 104, Behavioral Objectives, Unit III (b) on Electromagnetic Induction C A. a. b. c. d. e. f. g. In relation to electromagnetic (E-M) induction, be able to: identify how physical factors influence it, i.e. material, speed of motion, frequency, and their effects. apply physical principles in a practical example, e.g. magnet in a coil. apply the right-hand rule (positive charge flow) to find the direction of the magnetic field, motion of charges, and/or moving wire in terms of the other two variables. identify physical devices which apply these principles, e.g. generator, tape recorder, and the basic principles of their operation such as direction of rotation using the right-hand rule. identify the basic ways a voltage can be induced in a wire and match each with an illustration. identify magnetic flux situations and their outcome. apply Lenz’s Law to explain the outcome of practical situations. O B. a. b. Solve for the: magnetic flux thru a coil of known magnetic field and area, potential difference for a coil (wire): (i) or one of the other variables for a given no. of turns, cross-sectional area, and a magnetic field changing at a given rate, and vice versa (ii) given speed and length in a given magnetic field, (iii) and current induced (including direction) in a wire loop for a magnet moving in a given direction, (iv) for no. of turns, area, or magnetic field change in a given time in terms of other variables. C C. a. In relation to motors and generators, be able to: apply physical principles to how they operate, i.e. why are armature and field windings of an electric motor usually wound on an iron core, distinguish between the two noting their different characteristics and the type of current a generator generally produces, solve for the time interval when a rotating coil of known frequency has a maximum (and zero) emf, explain a DC generator’s operation and role. b. c. d. C. O D. a. b. c. d. e. In relation to a self-induced emf and inductance, be able to: identify the principles of physics behind its operation (including direction of induced emf) and the effects it produces for practical situations, practical example of inductance, the factors that influence inductance and apply to explain practical egs. solve for the inductance of a coil and solenoid of given length, radius or turns of wire in terms of the other variables, solve for the inductance of given current drop, time of drop, or average emf in terms of the other variables. 2 O. E. For the growth and decay of current in a circuit containing inductance (or capacitance), and resistance, be able to: a. sketch the current vs time curve for an inductor; charge vs time for a capacitor, b. identify what is meant by the time constant and relate it to the curves and practical situations, c. solve for the inductance (or capacitance), resistance, or time constant in terms of the two other variables. Learning Activities for Electromagnetic Induction: Read: Textbook, College Physics, 9th ed. Serway, Chapt. 18, pps. 629-633; Chapter 20, (omit pps 704-705). College Physics (Schaum’s Outline), pps 354-367, 372-378, 383 and 387-388 in llth ed. (pps. 314-325, 331-336, 341, and 345-346 in 10th ed). Handout: Lenz’s Law to Predict Current Direction of Changing B. Optional: Conceptual Physics, pps. 477-490 in 10th ed (pps. 478-491 in 9th ed) on reserve in the Library, www.micro.magnet.fsu.edu/electromag/java/lenzlaw www.walter-fendt.de/ph14e/index.html -check Electrodynamics Section Homework: Chapter 20; MC Q’s 5, 11; Concept Q’s 4, 7 & 10; Electromagnetic (E-M) Interactions Exercises Chapt. 20 Problems 2, 11, 15, 21, 25, 30, 34, 39, 41, 48*, 50 and 58*; plus 1. The plane of a rectangular coil, 5.0 cm by 8.0 cm is perpendicular to the direction of a magnetic field B. If the coil has 75 turns and a total resistance of 8.0 ohms, at what rate must the magnitude of B change to induce a current of 0.10 A in the coil windings? 2. A battery is connected in series with a 3.0 ohm resistor and a 12 mH inductor. The maximum current in the circuit is 150 mA. Find:(a) the time constant of this circuit and (b) emf of battery. Chapt. 18- Problems 31 and 34. Electromagnetic Interactions (handout) Questions Answer to Selected Even-Numbered Problems: 2(a)1.00 x 10-7 Tm2 (b)8.66 x 10-8 Tm2 30 1.00 m/s 34(a)7.5 x 103V 50(a) 4.44 mH (b)555 mJ Extra 1 2.7 T/s 2(a) 4.2 ms (b) 0.45 V Chapter 18 34. 1.3 x 10-4 C Laboratory: Induced Electromotive Force, The Generator, The Direct-Current Motor, Making A Capacitor Laboratory Objectives, Week II, Induced Electromotive Force, The Generator, and The Direct Current Motor A. Using a bar magnet and a coil hooked up to a galvanometer, be able to: (a) state the effect of motion (speed) of the magnet, motion of the coil, number of turns of the coil, and motion of both on the induced current and name the poles of the coil, (b) predict the current’s direction and poles of the coil when the magnet is inside the coil and moved outside the coil in either direction, (c) describe how the magnetic field has changed in (b) and the effect on the induced emf and current. B. In regard to an electromagnet’s effect on a nearby coil hooked up to a galvanometer (secondary), be able to use a DC power supply hooked up to another coil (primary) to note the: (a) effect of opening, (b) effect of closing, (c) keeping a steady direct current thru the circuit (both with and without the insertion of a soft iron core in the coil) to PHYS 104 Objectives Unit III Sp 13 3 describe physically what happens to the galvanometer readings as well as the resulting emf, number of magnetic field lines (flux). etc. PHYS 104 Objectives Unit III Sp 13 C. Using a simple generator, be able to identify: (a) the armature, field magnets, commutator, slip rings, and brushes and their physical role, (b) with the slip rings armature, the current direction for a specific armature rotation, type of emf produced, effect on emf of turning rate of armature, effect on emf with magnet’s pole distance, and magnitude of the emf for various given armature positions, (c) with the commutator, the type of emf produced (also explain physical principles) and how the direction the armature is turned affects the emf direction. D. Using a simple DC motor, be able to identify: (a) the field magnet, armature, commutator, electromagnet, and brushes and their physical roles, (b) with the split ring commutator, the polarity of each end when the armature is turned, position of the armature when polarity change occurs, role of armature with the field magnet pole given, role of split ring commutator, effect from reversing the armature current, and effect on rotation speed by input voltage and field magnet distance as well as explain the physical principles in a practical application of each, (c) effect on rotation of a series and shunt-wound motor, (d) properly hook up any of the motor connections done in lab. PHYS 104 Exercises 1. 2. 3. 4. 5. 6. 7. 8. 9. Exercises on Electromagnetic (E-M) Interactions Why does an iron core increase the field strength of a coil of wire? Two separate but similar coils of wire are mounted a few meters apart, as in figure 24.11. The first coil is connected to a battery and has a direct current flowing through it. The second coil is connected to a galvanometer. What do the galvanometer readings show when the current in the first coil is increasing, decreasing, or remaining steady? What is the difference between an electric motor and an electric generator? How is the current induced in a coil affected by a change in the speed or frequency of the changing magnetic field? Does the voltage output increase when a generator is made to spin faster? Explain. What is the principal advantage of ac over dc? Why would long-distance transmission of electric power, if done at low voltages, incur heavy losses? Why is it important that the core of a transformer pass through both coils? How does the current in the secondary compare to the current in the primary when the secondary voltage is doubled?