Well this will be tricky

Each edge of an Icosidodecahedron is a 2 Ω 2\Omega resistor. Then find the effective resistance between any 2 adjacent vertices.

Round off your answer up to 2 decimal places.


The answer is 0.97.

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2 solutions

Shivam Mishra
Apr 5, 2016

R = 2 ( v 1 ) r / v . n R=2(v-1)r/v.n where v v is the number of edges of the figure, n n is the number of edges meeting at point and finally r r is the resistance of each edge. This formula works for any polyhedron.

Substituting these values we get R = ( 29 30 R=(\frac{29}{30} ) Ω \Omega = 0.97 ( Ω \boxed{0.97}(\Omega )

I got the problem wrong because of a really stupid mistake :( However I have the way to prove that formula using the principle of superposition: let A and B be two adiacent points. We will consider two configurations: 1) a current i=1A flows trough A: B will receive 1/4 A because there are 5 adjacent vertices to A. There are 29 vertices from which i will flow out: trough each of them flows 1/29A 2) a current i is "sucked" from B: there is still 1/4A which goes from A to B; from every vertex there is 1/29 A entering current.

Superimposing those two, the potential difference between A and B is 1/2 R i, while in the present configuration we have a 0 current in each vertex different from A and B, where flows i+i/29=30/29 i current through a resistance r, which corresponds to a potential V=R eff 30/29 i Equating the two potentials gives Reff=R*29/60. In this case R=2 ohm

Gabriele Manganelli - 3 years, 10 months ago

The formula makes it a piece of cake . however i wasn't aware of it so i used kirchoff's laws .

Aditya Kumar - 5 years, 1 month ago

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Perhaps it takes a lot of time, doesn't it?

Swapnil Das - 5 years, 1 month ago

Great!!! I just found this formula somewhere ,i tried to prove it but was unable to do so,but this result is pretty useful where you have to solve a lot of questions in a limited amount of time.

shivam mishra - 5 years, 1 month ago

hey, tried it by using symmetry and still i'm getting wrong !, i'm getting 11/15 . help if u can !

A Former Brilliant Member - 4 years, 7 months ago

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never mind, it's an Icosidodecahedron, I misread this.

Michael Mendrin - 3 years ago

Describe the icosidodecahedron. The 1 2 \frac12 edgeweights are conductances and are the reciprocal of the 2 Ω 2\Omega resistances.

nvertices = PolyhedronData [ Icosidodecahedron , VertexCount ] ; \text{nvertices}=\text{PolyhedronData}[\text{Icosidodecahedron},\text{VertexCount}];

edges = Table [ e [ [ 1 ] ] e [ [ 2 ] ] , { e , PolyhedronData [ Icosidodecahedron , Edges ] } ] ; \text{edges}=\text{Table}[e[[1]]\leftrightarrow e[[2]],\{e,\text{PolyhedronData}[\text{Icosidodecahedron},\text{Edges}]\}];

g = PlanarGraph [ Range [ nvertices ] , edges , VertexLabels Name , EdgeWeight ConstantArray [ 1 2 , Length [ edges ] ] ] ; g=\text{PlanarGraph}\left[\text{Range}[\text{nvertices}],\text{edges}, \\ \text{VertexLabels}\to \text{Name},\text{EdgeWeight}\to \text{ConstantArray}\left[\frac{1}{2},\text{Length}[\text{edges}]\right]\right];

The Wolfram Mathematica Mathematica algorithm that does the mesh analysis to compute all vertex to vertex effective resistances.

The PseudoInverse is the Moore-Penrose Inverse .

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]\&;

The invocation of the algorithm above.

r = resistance ( g ) ; r=\text{resistance}(g);

The extraction of the effective resistance across all edges and a tally of the values present (the algorithm outputs resistance for conductance and vis-a-vis).

result = Tally [ Table [ r [ [ e [ [ 1 ] ] , e [ [ 2 ] ] ] ] , { e , edges } ] ] ( 29 30 60 ) \text{result}=\text{Tally}[\text{Table}[r[[e[[1]],e[[2]]]],\{e,\text{edges}\}]] \Longrightarrow \left( \begin{array}{cc} \frac{29}{30} & 60 \\ \end{array} \right)

This result shows that, in fact, all the effective resistances across each edge are the same. Below are the numeric version of the values.

N [ result ] ( 0.966667 60. ) N[\text{result}] \Longrightarrow \left( \begin{array}{cc} 0.966667 & 60. \\ \end{array} \right)

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