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Unformatted text preview: 79. (a) If S1 is closed, and S2 and S3 are open, then ia = ε/2R1 = 120 V/40.0 Ω = 3.00 A.
(b) If S3 is open while S1 and S2 remain closed, then
Req = R1 + R1 (R1 + R2) /(2R1 + R2) = 20.0 Ω + (20.0 Ω) × (30.0 Ω)/(50.0 Ω) = 32.0 Ω,
so ia = ε/Req = 120 V/32.0 Ω = 3.75 A.
(c) If all three switches S1, S2 and S3 are closed, then Req = R1 + R1 R'/(R1 + R') where
R' = R2 + R1 (R1 + R2)/(2R1 + R2) = 22.0 Ω,
i.e.,
Req = 20.0 Ω + (20.0 Ω) (22.0 Ω)/(20.0 Ω + 22.0 Ω) = 30.5 Ω,
so ia = ε/Req = 120 V/30.5 Ω = 3.94 A. ...
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 Spring '08
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 Physics

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