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# ch31 - CHAPTER Alternating-Current Circuits 31 Note Unless...

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CHAPTER 31 Alternating-Current Circuits Note: Unless otherwise indicated, the symbols I , V , E , and P denote the rms values of I , V , and E and the average power. 1* · A 200-turn coil has an area of 4 cm 2 and rotates in a magnetic field of 0.5 T. ( a ) What frequency will generate a maximum emf of 10 V? ( b ) If the coil rotates at 60 Hz, what is the maximum emf? ( a ) E = NBA ? cos ? t (see Problem 30-8-5) ( b ) E max = NBA ? = 2 p NBAf ? = E max / NBA = 250 s –1 ; f = ? /2 p = 39.8 Hz E max = 15.1 V 2 · In what magnetic field must the coil of Problem 1 be rotating to generate a maximum emf of 10 V at 60 Hz? Use Equ. 31–4; solve for B B = 0.332 T 3 · A 2-cm by 1.5-cm rectangular coil has 300 turns and rotates in a magnetic field of 4000 G. ( a ) What is the maximum emf generated when the coil rotates at 60 Hz? ( b ) What must its frequency be to generate a maximum emf of 110 V? ( a ) Use Equ. 31-4 ( b ) Use Equ. 31-4; solve for f = ? /2 p E max = 13.6 V f = 486 Hz 4 · The coil of Problem 3 rotates at 60 Hz in a magnetic field B . What value of B will generate a maximum emf of 24 V? Use Equ. 31-4; solve for B B = 0.707 T 5* · As the frequency in the simple ac circuit in Figure 31-26 increases, the rms current through the resistor ( a ) increases. ( b ) does not change. ( c ) may increase or decrease depending on the magnitude of the original frequency. ( d ) may increase or decrease depending on the magnitude of the resistance. ( e ) decreases. ( b ) 6 · If the rms voltage in an ac circuit is doubled, the peak voltage is ( a ) increased by a factor of 2. ( b ) decreased by a factor of 2. ( c ) increased by a factor of 2 . ( d ) decreased by a factor of 2 . ( e ) not changed. ( a ) 7 · A 100-W light bulb is plugged into a standard 120-V (rms) outlet. Find ( a ) I rms , ( b ) I max , and ( c ) the maximum power. ( a ) Use Equ. 31-14 ( b ) Use Equ. 31-12 I rms = 0.833 A I max = 1.18 A

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