Wires With Will

A useless wire having a total resistance of 48 Ω 48 \space \Omega is cut into 48 equal pieces. Then, a regular Deltoidal Icositetrahedron as shown below.

If the equivalent resistance between two opposite points, where four edges meet together is R Ω R \space \Omega , then enter your answer as the value of 100 R 100R .


This question is part of the set Platonic Electricity .
Image credit: Wikipedia


The answer is 80.

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2 solutions

Abhay Tiwari
Jun 9, 2016

The total resistance was 48 Ω 48 \space \Omega . And it was divide into 48 equal parts. So, resistance of each piece will be 1 Ω 1 \space \Omega .

Node A A and node Z Z (at the bottom, not shown), are two opposite nodes and this will be followed further in the solution.

Now, the current I I travels from node A A to node Z Z (not shown in the figure). At node A A the current divide it selves into four parts into the branch ( A C ) , ( A E ) , ( A G ) , ( A I ) {(AC), (AE), (AG), (AI)}

Since the resistances are equal in the above branches, the current also divides equally i.e. I 4 \frac{I}{4} current from each branch. Now, these branches will be at same potential and because of that the branches in between the nodes B , C , D , E , F , G , H , I B, C, D, E, F, G, H, I becomes dead and current will not pass through them.

The current now passes through the branches ( C L ) , ( E N ) (CL), (EN) and other two branches(connected by nodes G G and I I ) not shown in the figure the current is still I 4 \frac{I}{4} .

Again by method of equipotential surface, the branches in between nodes K , L , M , N , O , P , Q , R K,L,M,N,O,P,Q,R are dead.

The solid is symmetrical about the set of points K , L , M , N , O , P , Q , R K,L,M,N,O,P,Q,R , so same procedure has to be followed. And we will see the current never divides itself again and continues to flow as I 4 \frac{I}{4} untill it reaches node Z Z , where all the four branches carrying the current meet.

Now, while doing the traversing we encountered 16 16 resistance arranged in group of four which are parallel to each other and those are connected by set of nodes ( A , C , L , V , Z ) , ( A , E , N , X , Z ) (A,C,L,V,Z),(A,E,N,X,Z) and two set of other five nodes not shown in the figure.

So, R = 4 4 4 p a r a l l e l r e s i s t a n c e = 1 R=\large \underbrace{\frac{4}{4}}_{4 \space parallel \space resistance}=1

So, 100 R = 100 100R=\color{#3D99F6}{\boxed{100}}

@Abhay Tiwari Nice problem, but I have a couple of questions...

Doesn't it depend on which opposite vertices you use? i.e. The ones you used here were A and Z (where 4 edges meet together). However, there are other opposite vertices to consider, since this doesn't have the symmetry of a Platonic solid, where all vertices are equivalent. What happens, for example, if you consider D and its opposite vertex, where three edges meet? Is it the same answer? And if so, you should probably mention this in the solution...

Also, for your example, how do you know that D and E have the same potential?

Also, current almost certainly passes from D to M, no?

Geoff Pilling - 4 years, 12 months ago

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Sir, Sorry for the trouble caused. And point C, E, G, I will be at same potential as when current enters from point A it divides into the four wires. Since the four wires are having equal resistance, the current will divide equally and which in turn makes the voltage in the branches AC, AE, AG, AI equal. Since the points C, E, G, I will be at same potential current will not pass in between them. I hope this will help?

And sir I have edited the problem. Thanks for the oversight!

Abhay Tiwari - 4 years, 12 months ago

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Your assumption implies that by Kirchoff's laws, that there would be no current flowing from D to M, since there would be no current in DE or DC. However, current almost certainly does flow through DM. Therefore, I question the result of 1 ohm for the final resistance. I would think it would likely be a bit less than 1 ohm.

Geoff Pilling - 4 years, 12 months ago

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@Geoff Pilling Sir, can you explain how current is flowing from D to M? Maybe, I did something wrong :-\

Abhay Tiwari - 4 years, 12 months ago

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@Abhay Tiwari Yeah, something definitely seems fishy... So, lets see, we know that E to N has a potential drop since, as you say, current is flowing through that resistor. However, if no current flows through the path ED, DM, MN, then this would imply that they are all at the same potential which would be a contradiction.

Geoff Pilling - 4 years, 12 months ago

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@Geoff Pilling I think that the assumption that the resistors connecting BCDEFGHI are dead resistors isn't quite correct. I think that CEGI have the same potential, and BDFH have the same potential, (but different from CEGI). Therefore the resistors connecting them wouldn't be dead.

Geoff Pilling - 4 years, 12 months ago

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@Geoff Pilling Sir, I applied Kirchhoff law, and You are right, though very small but some amount of current flows into the branch CD, DE, LM, MN. And so the resistance value will be slightly less than one. I am sorry for the inconvenience. I have done some modification in the question. I hope now it will not create problems for others.

Abhay Tiwari - 4 years, 12 months ago

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@Abhay Tiwari Lemme try to calculate the answer exactly... I think its about .8 ohms, so the actual answer would be 80. This is where I wish we could see the answers that come in, so if we saw lots of 80's we could get the answer adjusted....

Geoff Pilling - 4 years, 12 months ago

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@Geoff Pilling Okay, Sir.

Abhay Tiwari - 4 years, 12 months ago

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@Abhay Tiwari OK, here goes... See if you agree with my logic...

I think the answer should be 80 (0.8 ohms)

If you pump 1 Amp into A, then 1/4 Amp goes through to C, E, G and I.

From each of those nodes, say for example C, 3/20th of an amp goes to L, and 1/20th goes to each of B and D, for a total of 5/20th or 1/4th. Finally, from B to K is 1/10th of an Amp. I believe this satisfies Kirchoff's laws everywhere. By symmetry, currents are similar all the way around the Deltoidal Icositetrahedron.

Now, by symmetry, the "middle band" vertices (K,L,M,N,O,P,Q, and R) are at the same potential, so this is half of the potential at A and the currents on the "bottom half" do exactly the same thing as the top half.

Therefore the total resistance is the sum of all these currents, so:

  • R t o t = 1 / 4 + 3 / 20 + 3 / 20 + 1 / 4 = 0.8 R_{tot} = 1/4 + 3/20 + 3/20 + 1/4 = 0.8 Ohms

So, 100 R = 80 100R = \boxed{80}

Make sense?

If you agree, we should go ahead and ask Calvin if he can update the solution to 80, so that others wont get frustrated if they get the answer wrong but were actually right.

Otherwise, lemme know if I made a mistake somewhere.

Geoff Pilling - 4 years, 12 months ago

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@Geoff Pilling yes, you are correct. @Calvin Lin Sir, can you help updating the answer to 80 .

Abhay Tiwari - 4 years, 12 months ago

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@Abhay Tiwari Thanks. I have updated​ the answer to 80.

Calvin Lin Staff - 4 years, 11 months ago

@Abhay Tiwari @Abhay Tiwari You might want to check your cuboctahedron one as well, "What do we have here!"... I think the answer should be 16 ohms, no?

Here is my thinking... Consider the following figure:

Now, lets consider the opposite vertices A (joining edges 1, 4, 5 and 24), and B (joining edges 14, 15, 20 and 21).

So, we inject 1 Amp into A. By symmetry, the current through 1, 4, 5 and 24 is 1/4 Amp.

To satisfy Kirchoff's laws at each node you get:

  • The current through 7, 10, 12 and 17 is 1/6 Amp (Same with 14, 15, 20 and 21 at the bottom)
  • The current through 6, 11, 13 and 16 is 0 (These are "dead" by symmetry)
  • The current through 2, 3, 8, 9, 18, 19, 22 and 24 is 1/12 Amp

So the effective resistance from A to B is 1 / 4 + 1 / 6 + 1 / 4 = 2 / 3 1/4 + 1/6 + 1/4 = 2/3 (of one resistor)

So, R e f f = 2 / 3 ( 24 o h m s ) = 16 R_{eff} = 2/3*(24 ohms) = \boxed{16} ohms

Make sense?

Geoff Pilling - 4 years, 11 months ago

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@Geoff Pilling Sir, Even I calculated 16 Ohms. I did the same. You will find it weird but, while calculating I did some silly mistake. I will post the question again.

Abhay Tiwari - 4 years, 11 months ago

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@Abhay Tiwari OK sounds good!

Geoff Pilling - 4 years, 11 months ago

@Abhay Tiwari Or we could just ask Calvin to change the answer again?

Geoff Pilling - 4 years, 11 months ago

@Abhay Tiwari Hi @Abhay Tiwari ... I hate to bother you again, but I think you may need to look again at the The Rhombic Triacontahedron answer. I don't want to go over my solution here since I don't want to give away the answer to people who I'm sure would like to try it out first. (In fact, should I remove the solution above as well?) But, if we can find a private chat somewhere I'd be happy to go over my logic.

I suspect that the difference might again be in the assumption of which resistors are "dead" and carry no current.

Also, a side note: Like the Deltoidal Icositetrahedron you may want to specify that the opposite vertices should be opposite vertices connecting four edges.

Looking forward to seeing your updated cuboctahedron problem! :-)

Geoff Pilling - 4 years, 11 months ago

@Geoff Pilling I don't understand how the sum of only those four current is overall resistance.can you help me here sir?

pranav patil - 4 years, 1 month ago

I did similar calculations but got it 75.From 1 vertex 4 parallel, then 8 parallel, again 8 parallel, and gain 4 parallel at the other vertex(neglecting the wires connecting the points with equal potentials.Thus the total resistance R =3/4(which i got)

Manurag M - 3 years ago

@Abhay Tiwari why then is the answer 80ohms?

Ishan Pednekar - 7 months, 2 weeks ago

By symmetry the above circuit can be simplified as:-

This can be simplified further using Wheatstone bridge.

The equivalent resistance comes out to be 4 5 Ω \frac{4}{5}\Omega . So the answer is 80 \boxed {80}

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