This preview shows pages 1–2. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Unformatted text preview: lim (kl9356) – Homework 9 – Weathers – (22202) 1 This printout should have 14 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 (part 1 of 2) 10.0 points The quantity of charge passing through a sur face of area 1 . 87 cm 2 varies with time as q = q 1 t 3 + q 2 t + q 3 , where q 1 = 4 . 2 C / s 3 , q 2 = 4 . 6 C / s, q 3 = 3 . 6 C, and t is in seconds. What is the instantaneous current through the surface at t = 0 . 4 s? Correct answer: 6 . 616 A. Explanation: Let : q 1 = 4 . 2 C / s 3 , q 2 = 4 . 6 C / s , q 3 = 3 . 6 C , and t = 0 . 4 s . I ≡ d q dt = 3 q 1 t 2 + q 2 = 3 ( 4 . 2 C / s 3 ) (0 . 4 s) 2 + 4 . 6 C / s = 6 . 616 A . 002 (part 2 of 2) 10.0 points What is the value of the current density at t = 0 . 4 s? Correct answer: 35379 . 7 A / m 2 . Explanation: Let : a = 1 . 87 cm 2 = 0 . 000187 m 2 and t = 0 . 4 s . J ≡ I A = 6 . 616 A . 000187 m 2 = 35379 . 7 A / m 2 . 003 10.0 points A conductor with crosssectional area 9 cm 2 carries a current of 17 A. If the concentration of free electrons in the conductor is 7 × 10 28 electrons / m 3 , what is the drift velocity of the electrons? Correct answer: 0 . 00168422 mm / s. Explanation: Let : I = 17 A , n = 7 × 10 28 electrons / m 3 , q e = 1 . 60218 × 10 − 19 C , and A = 9 cm 2 = 0 . 0009 m 2 . The current in a conductor is given by I = n q v d A , where n is the number of charge carriers per unit volume, q is the charge per carrier, v d is the drift velocity of the carriers and A is the cross section of the conduction. Solving for v d , we have v d = I n q A = 17 A 7 × 10 28 electrons / m 3 × 1 1 . 60218 × 10 − 19 C × 1 . 0009 m 2 × 1000 mm 1 m = . 00168422 mm / s . 004 10.0 points The drift velocity of free electrons in a cop per wire is 7 mm / s, resistivity is 1 . 8 × 10 − 8 Ω · m, and the free electron density is 8 . 43 × 10 28 electrons / m 3 ....
View
Full
Document
This note was uploaded on 09/07/2010 for the course PHYS 2220 taught by Professor Littler during the Spring '00 term at North Texas.
 Spring '00
 Littler
 Charge, Magnetism, Work

Click to edit the document details