# Nerd sniping

Staff Emeritus
From "xkcd.com"[/URL] :

[ATTACH=full]120592[/ATTACH]

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Gokul43201
Staff Emeritus
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Is it 0.916333... ohms?

Damn, I missed dinner!

siddharth
Homework Helper
Gold Member
Is it 0.916333... ohms?

Damn, I missed dinner!

How did you get that? I think that to solve such problems (where a simple superposition doesn't give you the answer), a discrete fourier transform is required, and then the final integral which gives the resistance looks very tricky.

D H: 4 points (or is it 2 and 1, for the engineer? :tongue:)

Staff Emeritus
At least three points, since I made Gokul miss dinner.

This http://arxiv.org/abs/cond-mat/9909120" [Broken], question 10) uses a different expression and that enables Mathematica to yield $(8-\pi)/(2\pi)\approx0.773$ ohms.

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siddharth
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Gold Member
This http://arxiv.org/abs/cond-mat/9909120" [Broken] (see appendix A) gives an absolute mess.

The article uses the same principle of taking the fourier transform and applying ohm's law. The integral one gets after the fourier transform is (eqn 25 in the article)

$$\frac{1}{4 \pi^2} \int_{-\pi}^{\pi} \int_{-\pi}^{\pi} \left( \frac{1-\cos(2x+y)}{2-\cos{x} - \cos{y}} \right) dx dy$$.

I'm at home and don't have access to an integrator. Does anyone have access to a math package that evaluates this integral, or a way to find the value? Does it come out as $(8-\pi)/(2\pi)\approx0.773$?

Mathworld seems to have used some substitution in its expression.

EDIT: Nevermind. They've apparently used a contour integral and then a substitution. It's there in appendix A, which I missed the first time.

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Staff Emeritus
The difference between a normal person and a scientist:

Evo
Mentor
The difference between a normal person and a scientist:

:rofl:

That's actually a normal person and Homer Simpson => Homer = Scientist?? :surprised

Gokul43201
Staff Emeritus
Gold Member
How did you get that? I think that to solve such problems (where a simple superposition doesn't give you the answer), a discrete fourier transform is required, and then the final integral which gives the resistance looks very tricky.
I actually did use a superposition - I don't know why it fails! =(

I haven't yet looked at the article posted by D H.

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It's funny how this thread turned into a discussion about the problem in the comic strip rather than the humour about it.

ranger
Gold Member
It's funny how this thread turned into a discussion about the problem in the comic strip rather than the humour about it.

It just proves the first sketch of the comic strip. Nerd sniping in action.

mheslep
Gold Member
From "xkcd.com"[/URL] :

[/QUOTE]
A friend of mine finished school some years ago with a BS in physics and immediately chucked it all to become a lawyer. After seeing this Nerd Sniping XKCD strip I sent it to him on a workday to see if he was truly a lawyer or if he would be sniped, along with with full disclosure of my intentions. Couple days later I checked back, sure enough, four billable hours went down the drain. :devil: Good thing he wasn't standing in the road.

BTW, engineer's solution: assume the infinite grid looks like a short to any one node except for the four resistors directly attached into the node. The parallel resistance into the node then is 1/4 ohm and therefore the resistance between any two, non-adjacent nodes is 1/2 ohm. With actual (cheap) 20% resistors thats close enough to the actual (.7xx is it?)

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Staff Emeritus
The resistance betweeb any two horizontally or vertically adjacent nodes is exactly 1/2 ohms. The resistance between nodes further apart than that is not. That 20% fudge on top of 0.5 ohms yields 0.6 ohms, not 0.773 ohms. Its even worse for nodes further apart than that.

Why don't you try working out the answer for nodes separated by two diagonal hops?

mheslep
Gold Member
The resistance betweeb any two horizontally or vertically adjacent nodes is exactly 1/2 ohms. The resistance between nodes further apart than that is not. That 20% fudge on top of 0.5 ohms yields 0.6 ohms, not 0.773 ohms.
Yes, ok, I'll have to make clear I'm using really cheap resistors when I go for the Google interview.

Its even worse for nodes further apart than that.

Why don't you try working out the answer for nodes separated by two diagonal hops?
Think I'll code up a quick sim for fun.