Problem 5. Electric quadrupole Four charged particles are located in the x-y plane each a distance d from the origin. The two charge particles on the x axis have charge q. The two on the y axis have charge -q. :Y a. Determine the electric field at (x,y,z) = (0,0,0). b. Determine the electric field at any point on the z axis, i.e. at (0,0, z). c. Find an exact expression for the electric field along the x axis, Ē(x,0,0), for x > d, -d < x < d and x < -d. d. Find an approximate expression for your result in part c in the limit x >> d. You will need to evaluate all four terms to the same order in d/x. That means that for two of the terms, you will have to carry out the binomial approximation to second order, namely a(a – 1), (1+ 8)ª - 1+ ad + 2 You will obtain a quadrupole field that falls off with distance as 1/distance". Hopefully you are, by now, seeing pattern. In your next E&M class you will explore what is referred to as a "multipole" expansion of electric fields which involves a sum of terms that fall off with distance, r, as 1/r2+n and that involve multipole moments that reflect the geometry of the charge distribution. The monopole moment is the total charge. The dipole moment you explored in problem 2. The quadrupole moment and higher moments are more complicated. But observe that you can write the quadrupole field in terms of a product of q and an area. Also look back at the correction term in part d of problem 4. It is also a quadrupole term.

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Problem 5. Electric quadrupole
Four charged particles are located in the x-y plane each a distance d from the origin. The two charge
particles on the x axis have charge q. The two on the y axis have charge -q.
:Y
b.
a. Determine the electric field at (x,y,z) = (0,0,0).
b. Determine the electric field at any point on the z axis, i.e. at (0,0, z).
c. Find an exact expression for the electric field along the x axis, E(x,0,0), for x > d, -d < < d and
x < -d.
d. Find an approximate expression for your result in part c in the limit x >> d. You will need to
evaluate all four terms to the same order in d/x. That means that for two of the terms, you will
have to carry out the binomial approximation to second order, namely
а(а —
1)
(1+ 8)ª
2 1+ að +
You will obtain a quadrupole field that falls off with distance as 1/distance“. Hopefully you are, by
now, seeing pattern. In your next E&M class you will explore what is referred to as a "multipole"
expansion of electric fields which involves a sum of terms that fall off with distance, r, as 1/r2+n
and that involve multipole moments that reflect the geometry of the charge distribution. The
monopole moment is the total charge. The dipole moment you explored in problem 2. The
quadrupole moment and higher moments are more complicated. But observe that you can write the
quadrupole field in terms of a product of q and an area. Also look back at the correction term in
part d of problem 4. It is also a quadrupole term.
Transcribed Image Text:Problem 5. Electric quadrupole Four charged particles are located in the x-y plane each a distance d from the origin. The two charge particles on the x axis have charge q. The two on the y axis have charge -q. :Y b. a. Determine the electric field at (x,y,z) = (0,0,0). b. Determine the electric field at any point on the z axis, i.e. at (0,0, z). c. Find an exact expression for the electric field along the x axis, E(x,0,0), for x > d, -d < < d and x < -d. d. Find an approximate expression for your result in part c in the limit x >> d. You will need to evaluate all four terms to the same order in d/x. That means that for two of the terms, you will have to carry out the binomial approximation to second order, namely а(а — 1) (1+ 8)ª 2 1+ að + You will obtain a quadrupole field that falls off with distance as 1/distance“. Hopefully you are, by now, seeing pattern. In your next E&M class you will explore what is referred to as a "multipole" expansion of electric fields which involves a sum of terms that fall off with distance, r, as 1/r2+n and that involve multipole moments that reflect the geometry of the charge distribution. The monopole moment is the total charge. The dipole moment you explored in problem 2. The quadrupole moment and higher moments are more complicated. But observe that you can write the quadrupole field in terms of a product of q and an area. Also look back at the correction term in part d of problem 4. It is also a quadrupole term.
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