Diagonal Wheatstone-bridge (not balanced!)

Given that r = 2 Ω r= 2 \, \Omega and R = 6 Ω , R = 6 \, \Omega, find the equivalent resistance (in ohms) across A B . AB.


The answer is 1.25.

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2 solutions

Steven Chase
Feb 2, 2018

I can think of two ways to do this one:

1) Apply a fictitious voltage source and use linear algebra to solve for the node voltages, source current, and equivalent resistance
2) Apply successive delta-wye transforms and simplify. I have outlined this approach below. Pardon the eccentric color schemes.

Great solution sir!

Tapas Mazumdar - 3 years, 4 months ago

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Thanks.........nice problem too

Steven Chase - 3 years, 4 months ago

edges = { 1 2 , 1 3 , 1 4 , 2 3 , 3 4 , 2 5 , 3 5 , 4 5 } \text{edges}=\{1\leftrightarrow 2,1\leftrightarrow 3,1\leftrightarrow 4,2\leftrightarrow 3,3\leftrightarrow 4,2\leftrightarrow 5,3\leftrightarrow 5,4\leftrightarrow 5\}

weights = { 2 , 1 , 6 , 1 , 1 , 6 , 1 , 2 } \text{weights}=\{2,1,6,1,1,6,1,2\}

resistance = With [ { Γ = PseudoInverse [ With [ { wam = WeightedAdjacencyMatrix [ $#$1 ] } , DiagonalMatrix [ Tr/@ wam T ] wam ] ] } , Outer [ Plus , Diagonal [ Γ ] , Diagonal [ Γ ] ] Γ Γ T ] & ; \text{resistance}=\text{With}\left[\left\{\Gamma =\text{PseudoInverse}\left[\text{With}\left[\{\text{wam}=\text{WeightedAdjacencyMatrix}[\text{\$\#\$1}]\},\text{DiagonalMatrix}\left[\text{Tr}\text{/@}\text{wam}^T\right]-\text{wam}\right]\right]\right\}, \\ \text{Outer}[\text{Plus},\text{Diagonal}[\Gamma ],\text{Diagonal}[\Gamma ]]-\Gamma -\Gamma ^T\right]\&;

g = PlanarGraph [ edges , EdgeWeight Table [ 1 r , { r , weights } ] , VertexLabels Automatic , EdgeLabels EdgeWeight ] g=\text{PlanarGraph}\left[\text{edges},\text{EdgeWeight}\to \text{Table}\left[\frac{1}{r},\{r,\text{weights}\}\right],\text{VertexLabels}\to \text{Automatic},\text{EdgeLabels}\to \text{EdgeWeight}\right]

( 0 13 14 75 112 33 28 5 4 13 14 0 75 112 5 4 33 28 75 112 75 112 0 75 112 75 112 33 28 5 4 75 112 0 13 14 5 4 33 28 75 112 13 14 0 ) \left( \begin{array}{ccccc} 0 & \frac{13}{14} & \frac{75}{112} & \frac{33}{28} & \frac{5}{4} \\ \frac{13}{14} & 0 & \frac{75}{112} & \frac{5}{4} & \frac{33}{28} \\ \frac{75}{112} & \frac{75}{112} & 0 & \frac{75}{112} & \frac{75}{112} \\ \frac{33}{28} & \frac{5}{4} & \frac{75}{112} & 0 & \frac{13}{14} \\ \frac{5}{4} & \frac{33}{28} & \frac{75}{112} & \frac{13}{14} & 0 \\ \end{array} \right)

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