# hw8 - Physics 21 Fall 2004 Solution to HW-8 26 P 31 The...

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Physics 21 Fall, 2004 Solution to HW-8 26 P25 Determine the magnitudes and directions of the currents through R 1 and R 2 in Fig. 26-39. 9.0 V 6.0 V 15 22 I 1 I 3 I 2 1 2 We can choose loop 1 so it involves only one unknown current. The equations are current: I 1 = I 2 + I 3 loop 1: 6 + 9 = 22 I 1 loop 2: 9 = 15 I 2 +22 I 1 Solve loop 1 to get I 1 =15 / 22 = 0 . 682 A. Substitute I 1 into loop 2 and ±nd I 2 = 6 / 15 = 0 . 4 A. Rounding o², we get I 1 =0 . 68 A leftward through 22 Ω I 2 =0 . 40 A leftward through 15 Ω 26 P27 Determine the magnitudes and directions of the currents through each resistor shown in ±g. 26-40. The bat- teries have emfs of E 1 =9 . 0V and E 2 =12 . 0 V, and the resistors have values of R 1 =15Ω , R 2 =20Ω, R 3 =40Ω . 15 20 40 9V 12 V 1 2 I 3 I 1 I 2 The equations are current: I 1 + I 3 = I 2 loop 1: 9 = 15 I 1 +20 I 2 loop 2: 12 = 40 I 3 +20 I 2 We can solve these by eliminating I 3
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