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2009S2UnitExam5BSolutions

# 2009S2UnitExam5BSolutions - ID B UCSD Physics 2B Answer...

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ID: B 1 UCSD Physics 2B Unit Exam 5B AC Circuits Answer Section MULTIPLE CHOICE 1. ANS: D Flux through a solenoid is the field times cross-section area times the number of turns: Φ = NBA = N μ 0 I N length Ê Ë Á Á Á Á Á Á ˆ ¯ ˜ ˜ ˜ ˜ ˜ ˜ π R 2 Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ = μ 0 N 2 π R 2 I length Self-inductance L = Φ I = μ 0 N 2 π R 2 length N = L R 2 length μ 0 π N = 24.0 H 10.0 × 10 2 m Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ 2 25.0 × 10 2 m 1.26 × 10 6 H / m Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ 3.14 ( ) = 1.23 × 10 4 Answer is rounded to 3 sig figs TOP: SELF-INDUCTANCE 2. ANS: C Calculate and compare the AC reactance for each of the three load components: X R = R = 470 Ω X C = 1 ω C = 1 57.2 × 10 5 rad / s Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ 320 × 10 12 F Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ = 546 Ω X L = ω L = 57.2 × 10 5 rad / s Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ 127 × 10 6 H Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ = 726 Ω The resistor has the lowest reactance, and therefore (by Ohm’s Law) draws the most current.

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