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Unformatted text preview: fullwood (tf5349) – Homework 3 DC Circuits – Catala – (12054) 1 This printout should have 24 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points An Xray tube used for cancer therapy oper ates at 4.2 MV with a beam current of 85 mA striking a metal target. Calculate the power of this beam. Correct answer: 3 . 57 × 10 5 W. Explanation: Let : Δ V = 4 . 2 MV and I = 85 mA . P = I Δ V = (0 . 085 A) (4 . 2 × 10 6 V) = 3 . 57 × 10 5 W . 002 10.0 points A wire is made of a material with a resistiv ity of 8 . 05638 × 10 8 Ω · m. It has length 4 . 39265 m and diameter 0 . 50739 mm. What is the resistance of the wire? Correct answer: 1 . 75022 Ω. Explanation: Let : ρ = 8 . 05638 × 10 8 Ω · m , ℓ = 4 . 39265 m , and r = 0 . 253695 mm = 0 . 000253695 m . By definition, the resistance of the wire is R = ρ ℓ A = ρ ℓ π r 2 = ( 8 . 05638 × 10 8 Ω · m ) (4 . 39265 m) π (0 . 000253695 m) 2 = 1 . 75022 Ω . 003 (part 1 of 3) 10.0 points A total charge of 9.8 mC passes through a crosssectional area of a nichrome wire in 3 . 3 s. a) What is the current in the wire? Correct answer: 0 . 0029697 A. Explanation: Let : Δ Q 1 = 9 . 8 mC and Δ t 1 = 3 . 3 s . I = Δ Q Δ t = . 0098 C 3 . 3 s = . 0029697 A . 004 (part 2 of 3) 10.0 points b) How many electrons pass through the crosssectional area in 49.0 s? Correct answer: 9 . 0947 × 10 17 . Explanation: Let : q e = 1 . 6 × 10 19 C and Δ t 2 = 49 . 0 s . Δ Q = N q e N = Δ Q q e = I Δ t 2 q e = (0 . 0029697 A)(49 s) 1 . 6 × 10 19 C = 9 . 0947 × 10 17 . 005 (part 3 of 3) 10.0 points c) If the number of charges that pass through the crosssectional area during the given time interval doubles, what is the resulting cur rent? Correct answer: 0 . 00593939 A. Explanation: Let : Δ Q 2 = 2 Δ Q 1 . fullwood (tf5349) – Homework 3 DC Circuits – Catala – (12054) 2 I = 2 Δ Q 1 Δ t 1 = 2 (0 . 0098 C) 3 . 3 s = . 00593939 A ....
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