Chapter 9: Vector Calculus
Section 9.6: Surface Integrals
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Example 9.6.9
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Integrate the scalar on the surface defined over the first-quadrant part of the interior of the ellipse bounded by the lines and .
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Solution
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Mathematical Solution
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Figure 9.6.9(a) shows the portion of the ellipse over which integration is to take place.
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The region shaded in red lies under the line for . The region shaded in green lies under the ellipse where , for .
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The integrand of the surface integral is
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so that the surface integral itself is given from first principles in Cartesian coordinates by
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>
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use plots, plottools in
module()
local p1,p2,p3;
p1:=plot(sqrt(1-x^2)/2,x=1/sqrt(5)..1,color=black,filled=[color=green]):
p2:=plot(x,x=0..1/sqrt(5),color=black,filled=[color=red]);
p3:=display(p2,p1,scaling=constrained,labels=[x,y]);
print(p3);
end module:
end use:
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Figure 9.6.9(a) Sector of ellipse
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= ≐
It is also possible to make the change of variables , that is, to polar coordinates. To this end, write the surface as the position vector
=
so that
= =
and . Since the integrand of the surface integral is given by
the surface integral itself is given by
The upper limit on the inner integral is obtained by expressing the equation of the ellipse in polar coordinates. Thus, write
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Maple Solution - Interactive
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Table 9.6.9(a) provides a solution from first principles implemented in polar coordinates.
Initialize
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Tools≻Load Package: Student Multivariate Calculus
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Loading Student:-MultivariateCalculus
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Context Panel: Assign to a Name≻R
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Obtain
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Calculus palette: Partial-derivative operator
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Common Symbols palette: Cross-product operator
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Context Panel: Evaluate and Display Inline
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Context Panel: Simplify≻Assuming Positive
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Context Panel: Assign to a Name≻
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Obtain the integrand in polar coordinates
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Context Panel: Evaluate and Display Inline
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Context Panel: Assign to a Name≻
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Form and evaluate the surface integral
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Calculus palette: Iterated double-integral operator
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Context Panel: Evaluate and Display Inline
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Context Panel: Approximate≻10 (digits)
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=
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Table 9.6.9(a) Solution from first principles implemented in polar coordinates
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Maple Solution - Coded
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Table 9.6.9(b) provides a solution from first principles implemented in polar coordinates.
Initialize
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Install the Student MultivariateCalculus package.
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Loading Student:-MultivariateCalculus
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Define the surface as the position vector R.
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Use the CrossProduct, Norm, and simplify commands to obtain
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Form and evaluate the surface integral
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Except for the differentials, let G be the integrand in polar coordinates.
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Form the surface integral with the top-level Int command.
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Use the value command to evaluate the integral exactly.
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Use the evalf command to approximate the integral numerically.
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Table 9.6.9(b) Solution from first principles implemented in polar coordinates
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Table 9.6.9(c) provides a solution from first principles implemented in Cartesian coordinates.
Initialize
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Assign to the equation of the ellipse.
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Use the solve command to obtain the explicit Cartesian representation of the branches of the equation of the ellipse.
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Use the solve command to obtain the coordinates of the intersection of the ellipse with the line .
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Except for the differentials in , let G be the integrand in Cartesian coordinates.
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Form and evaluate the surface integral (See Figure 9.6.9(a))
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Form the surface integral with the top-level Int command.
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Use the value command to evaluate the integral exactly.
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Use the evalf command to approximate the integral numerically.
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Table 9.6.9(c) Solution from first principles implemented in Cartesian coordinates
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