
#1
Aug404, 05:13 AM

P: 31

Twelve ideal Rohm resistors are connected in a cubic configuration (i.e., each one forms an edge of a cube), as shown here. What is the total resistance R_{AB} between the opposite vertices A and B?




#2
Aug404, 05:28 AM

Sci Advisor
HW Helper
P: 2,004

There are 6 ways for the current to get from A to B. All 6 ways have resistance 3R.
So I guess [itex]R_{AB}=\frac{R}{2}[/itex]. 



#3
Aug404, 05:40 AM

Emeritus
Sci Advisor
PF Gold
P: 11,154

My guess is 5R/6 {or R*(1/3 + 1/6 + 1/3)}




#4
Aug404, 05:42 AM

Emeritus
Sci Advisor
PF Gold
P: 11,154

ThreeDimensional Resistance
It would be harder to find the effective resistance between any other pair of points (for example, 2 neighbouring points) ...




#5
Aug604, 01:16 AM

P: 4

Current passing from b to each three immediate resistors are equal.
Let I10, I11, I12 be currents passing through R10, R11, R12, the last three resistors of the cube. Current passing at A is given by IA= IB = I10 + I11 + I12 = V10/R10 + V11/R11 + V12/R12 and since the voltages and resistances are equal IA = It = 3*V10/R10 Vt = It*RAB = 3V10*Rt/R10 R10*Vt /3*V10 = Rt For parallel connection V is constant Vt = V10 thus Rt = R10/3 or Rt = RAB = R/3 



#6
Aug604, 10:21 AM

P: 403

The current flows equally through 3 resistors, then 6 resistors and then through the last 3 resistors. So, the total resistance is
R/3+R/6+R/3 = 5R/6 . 



#7
Aug804, 03:03 PM

P: 31

I see some correct answers here; for the others, here’s how it goes:
Imagine a current I entering one of the vertices (say, node “A”). Because of symmetry (i.e., each of the three paths leaving node A “looks” the same to the current I), the current I divides evenly into three equal currents I_{0} (1) I_{0} = (1/3)I among the three braches A1, A2, and A3, as shown here. As each I_{0} enters nodes 1, 2, and 3, it too—again, because of symmetry (i.e., each of the two paths leaving each of the nodes 1, 2, and 3 “looks” the same)—divides evenly into two equal currents I_{00} (2) I_{00} = (1/2)I_{0} at the branch pairs 15 and 16, 24 and 26, and 34 and 35. The currents then recombine at nodes 6, 5, and 4 into three equal currents I_{0}, which, in turn, recombine into I at node B. The cube is simply a bunch of flexible wires connecting resistances. Let’s simplify our visualization by stretching out the cube and laying it flat like this (note that the wires are actually connected only at the points indicated by the solid dots “•”). Now, calculate the voltage between node A and B along any path: (3) V_{AB} = I_{0}R + I_{00}R + I_{0}R. Substituting (1) and (2) into (3), we get (4) V_{AB} = (1/3)IR + (1/6)IR + (1/3)IR = (5/6)IR. The total resistance between nodes A and B is, therefore (5) R_{AB} = V_{AB}/I = (5/6)R ohms. Here’s another way of looking at it: Since the voltage drop from A to 1 is the same as the voltage drop from A to 2 which is the same as the voltage drop from A to 3, which is the same as the voltage drops from 6 to B, 5 to B, and 4 to B (because each equals (1/3)IR), then nodes 1, 2, and 3 are all at the same potential, and nodes 4, 5, and 6 are at the same potential. In other words, V_{12} = V_{13} = V_{23} = 0 and V_{65} = V_{64} = V_{54} = 0. We can therefore connect each of these two sets of three nodes together with a short circuit and not change anything. If we do that, the circuit would appear like this. It should be obvious that this configuration is just a series connection of three parallel connections of 3, 6, and 3 Rohm resistances, which works out to be (5/6)R ohms. 


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