This is for my intro physics 2 class(adsbygoogle = window.adsbygoogle || []).push({});

1. The problem statement, all variables and given/known data

Consider the charges Q at (-a, 0), -2Q at (0, 0) and Q at (a, 0). Such a combination of charges, with zero net charge and with zero net dipole moment, is called an electric quadrupole. a. Find the electric field along the x acis, for x > a. b. Show that, for x >> a, the electric field varies by x^-4. Find the coefficient.

2. Relevant equations

[tex]\vec{E} = \frac{kQ}_{r^{2}}\hat{r}[/tex]

Here is the answer I got for part a which was correct.

[tex] \vec{E} = kQ\left[\frac{1}_{(x+a)^{2}}} + \frac{1}_{(x-a)^{2}}} - \frac{2}_{x^{2}}}\right]\hat{j}[/tex]

3. The attempt at a solution

This is what I got for part b:

[tex] \vec{E} = 2kQ\left[\frac{1}_{x^{2}}} - \frac{1}_{x^{2}}}\right]\hat{j} = 0[/tex]

I don't know how to get the book's answer of

[tex] \frac{6a^{2}}_{x^{4}}}\hat{j} [/tex]

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# Homework Help: Limit at infinity of an electric quadrupole

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