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Section 15.7 : Triple Integrals in Spherical Coordinates

1. Evaluate E10xz+3dV where E is the region portion of x2+y2+z2=16 with z0.

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Okay, let’s start off with a quick sketch of the region E so we can get a feel for what we’re dealing with.

We’ve given the sketches with a set of “traditional” axes as well as a set of “box” axes to help visualize the surface and region.

In this case we’re dealing with the upper half of a sphere of radius 4.

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Now, since we are integrating over a portion of a sphere it makes sense to use spherical coordinate for the integral and the limits are,

0φπ20θ2π0ρ4

Remember that φ is the angle from the positive z-axis that we rotate through as we cover the region and θ is the angle we rotate around the z-axis as we cover the region.

In this case we have the full upper half of the sphere and so θ will range from 0 to 2π while φ will range from 0 to π2.

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Plugging these limits into the integral and converting to spherical coordinates gives,

E10xz+3dV=π202π040[10(ρsinφcosθ)(ρcosφ)+3](ρ2sinφ)dρdθdφ=π202π04010ρ4sin2φcosφcosθ+3ρ2sinφdρdθdφ

Don’t forget to convert the x and z into spherical coordinates and also don’t forget that dV=ρ2sinφdρdθdφ and so we’ll pick up a couple of extra terms when converting the dV to spherical coordinates.

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Okay, now all we need to do is evaluate the integral. Here is the ρ integration.

E10xz+3dV=π202π0(2ρ5sin2φcosφcosθ+ρ3sinφ)|40dθdφ=π202π02048sin2φcosφcosθ+64sinφdθdφ Show Step 5

Next let’s do the θ integration.

E10xz+3dV=π20(2048sin2φcosφsinθ+64θsinφ)|2π0dφ=π20128πsinφdφ Show Step 6

Finally, we’ll do the φ integration.

E10xz+3dV=(128πcosφ)|π20=128π

Note that, in this case, because the limits of each of the integrals were all constants we could have done the integration in any order we wanted to. In this case, it might have been “simpler” to do the φ first or second as that would have greatly reduced the integrand for the remaining integral(s).