Tech Directions - October 2007 - (Page 15)

onance (Figs. 3 and 4). An associated lab activity can have students measure for maximum voltage drop across the voice coil terminals as evidence of maximum impedance at the corresponding resonant frequency, commonly called free-air resonance and denoted as Fs (Fig. 5). This exercise reflects characteristics of the parallel RLC circuit being used as a bandstop filter as it exhibits maximum impedance, minimum current, and maximum voltage drop at resonance. Remind students that RLC circuit fundamentals are key to almost every type of electrical application, from wireless communications systems to power delivery and distribution systems, to wired computer networks. Lab Exercises Time, energy, and frequency elements interact with this type of transducer in the physical world, rounding out the electrical technology student perspective. Students can measure and record voltage and current magnitudes all along the audio frequency continuum between 20 Hz and 20 KHz, building small data bases for future x-y graph construction, further helping to clarify each transducer’s dynamics. Since voltage is proportional to impedance, changes in voltage at the voice coil with changing frequencies will reveal the inductive effect, opening the door to lab experiments in reactance, calculating inductive reactance, and proving its significance in determining impedance. Valuable lessons in mathematics can follow to pinpoint voltage and current phase angles in the reactive Fig. 6—Fun formulas Capacitive reactance Xc = 1/2πfC Inductive reactance XL = 2πfL LC filter quality factor Q = XL/R LC filter bandwidth BW = Fr/Q subwoofer by calculating impedance. Riding on the coattail of impedance so to speak, the electrical instructor can open the floor for discussions around power factor (Fig. 6), since every new electrical technology student knows that power is a by-product of voltage and current. Instructors can also go back to those elements of electricity Georg Simon Ohm (1789-1854) felt so strongly about: voltage, current, and www.techdirections.com ELECTRICITY/ELECTRONICS 15 http://www.elexp.com http://www.graymarkint.com http://www.graymarkint.com http://www.elexp.com http://www.techdirections.com

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Tech Directions - October 2007 Technically Speaking Contents Direct from Washington The News Report Technology Today Technology’s Past Mastering Computers Rock Your Classroom!—Use Subwoofers to Teach Electricity and Science Sure, They Can Build It, But. . . . Manufacturing Students Need Process Planning Skills Teach Graphic Design Basics with PowerPoint Free Teacher Resources Product Central More than Fun

Tech Directions - October 2007

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