UnitExam3Bsolutions

# UnitExam3Bsolutions - ID B UCSD Physics 2B Answer Section...

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ID: B 5 UCSD Physics 2B Unit Exam 3B Current, Resistance, Circuits Answer Section MULTIPLE CHOICE 1. ANS: A when all resistors are identical R SER = NR = 3 × 15 Ω= 45 Ω TOP: SERIES 2. ANS: B R = ρ L A = s s 2 = Rs = 1320 Ω () 17.2 × 10 2 m Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ = 227 Ω• m TOP: RESISITIVITY 3. ANS: C The middle lamp is "shorted out" and therefore has no current through it since the potential difference across it is zero. At the same time, the total resistance of the circuit is lowered so more current flows through the remaining bulbs. You can also see this by recognizing the circuit as a voltage divider. When the middle bulb is shorted, the battery voltage is divided between only two remaining bulbs, so they are brighter than when the voltage was divided by 3. TOP: DEMO 4. ANS: A Each differential area element dA of a circle (cross section) is a ring of thickness dr and length (circumference) 2 π r . The area of this ring is the product, dA = 2 rdr I = J = Jr 2 0 R = 2 Kr 3 dr 0 R = 2 K R 4 4 = 1 2 3.14 12.2 A / m 4 Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ 16.6 × 2 m Ê Ë Á Á Á ˆ ¯ ˜ ˜ ˜ 4 = 14.5 mA TOP: CURRENT DENSITY + INTEGRATION 5. ANS: B Resistance is directly proportional to length.

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UnitExam3Bsolutions - ID B UCSD Physics 2B Answer Section...

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