PHYS 272 Lecture Notes - Lecture 1: Time Constant, Farad
Document Summary
Practice question: two 3. 8 micro farad capacitors, two 2. 2 k ohm resistors and 12 v source is in series. Answer (a) equivalent resistance = (cid:894)2. 2 + 2. 2(cid:895) k (in series) (b) equivalent capacity = c(cid:3117)c(cid:3118) = 1. 9 uf [ans] (c) time constant t = rc 4. 4 x 103 * 1. 9 x 10-6. Or 2. 7 ma [ans] (e) current given as 1. 5 (d) current i = (cid:2902) e-t/rc. Initial t = 0, i = (cid:2902) e0 (cid:2902) (cid:2869)(cid:2870) (cid:2872). (cid:2872) x (cid:2869)(cid:2868)(cid:3119) e-t/0. 0083 (f) u = (cid:2869)(cid:2870) qv where q = c 1. 9 uf * 12 = 22. 8. Hence 1. 5 x 10-3 = e-t/0. 0083 = 0. 55 (cid:2869)(cid:2870) (cid:2872). (cid:2872) x (cid:2869)(cid:2868)(cid:3119) 0. 0027 a hence t = 0. 005 sec [ans] hence u = (cid:2869)(cid:2870) * 22. 8 x 10-6 * 12 136. 8 uj. 75% of u = 102. 6 uj this is (cid:2869)(cid:2870)(cid:2901)(cid:3118)c. 102. 6 x 10-6 = (cid:2869)(cid:2870)(cid:2901)(cid:3118)c or q = 1. 97 x 10-5.
