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# Assign7 - PHY 54 Assignment 7 Summer 2011 Reading AC...

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Reading : AC Circuits Key concepts: Reactance; impedance; phase difference; rms values; series resonance. 1. Questions about voltages and power in a series LCR circuit. a. In a DC circuit, energy conservation requires that the sum of all the potential changes in a circuit loop be zero. An AC voltmeter measures rms voltages. Do these add to zero for an AC circuit loop? Explain. b. Do the instantaneous AC voltages add to zero for a loop? c. The instantaneous power supplied to the various elements in an LRC circuit add up to the power supplied by the generator. Is that true of the average power? d. Where does the average power supplied by the generator ultimately go? What is the role of the capacitor and inductor in this? 2. More questions about voltages and power. a. Is the instantaneous power supplied by the generator ever negative? If so, under what circumstances? b. Is the instantaneous power supplied to the resistor ever negative.? c. Can the instantaneous voltage across the capacitor or inductor exceed the maximum voltage of the generator? If so, under what circumstances? d. Can the rms voltages across the capacitor or inductor exceed that of the generator? Is so, under what circumstances? 3*. A circuit consists of the usual 120 V, 60 Hz source as emf connected to a 60 W light bulb and an inductor with L = 2/ π H. a. What are the resistance of the bulb, the reactance of the inductor, and the impedance of the circuit? Ans : R = 240 Ω , X L = 240 Ω , Z = 240 2 Ω . b. What is the rms current? Ans : I = 2 /4 A. c. What is the phase difference φ ? Ans : φ = π /4 . d. What is the average power consumed by the bulb? Compare it to the average power supplied by the source. Ans : P (to bulb) = 30 W; same. PHY 54 Assignment 7 Summer 2011

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4. In a series LCR circuit the rms generator voltage is 100 V, the resistor is 1 Ω , and the rms current is 0.1 A. a. What average power is supplied by the generator? Ans : 0.01 W.
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