Evaluate The Integral By Changing To Spherical Coordinates

The transition to spherical coordinates can significantly simplify the evaluation of integrals, particularly those over regions with spherical symmetry. This technique transforms a triple integral from Cartesian coordinates (x, y, z) to spherical coordinates (ρ, θ, φ), where ρ represents the radial distance, θ the azimuthal angle, and φ the polar angle. Understanding when and how to apply this transformation is crucial for efficient problem-solving in calculus and physics.

Why Spherical Coordinates?

Spherical coordinates excel when dealing with regions defined by spheres or cones, or when the integrand contains expressions like √(x² + y² + z²). The transformation simplifies the geometry and often the integrand itself, making the integral more manageable. However, it's essential to understand the transformation equations and the Jacobian determinant involved to avoid errors.

Spherical Coordinate System: A Refresher

Before diving into the evaluation process, let's briefly review the spherical coordinate system. A point P in 3D space can be represented by (ρ, θ, φ) where:

• ρ (rho): The distance from the origin to the point P (ρ ≥ 0).
• θ (theta): The same angle used in cylindrical coordinates; it's the angle in the xy-plane measured counterclockwise from the positive x-axis (0 ≤ θ ≤ 2π).
• φ (phi): The angle between the positive z-axis and the line segment connecting the origin to point P (0 ≤ φ ≤ π). Notice that φ ranges from 0 to π, not 2π, covering the entire 3D space.

Transformation Equations: Linking Cartesian and Spherical Coordinates

The bridge between Cartesian and spherical coordinates is established through the following equations:

• x = ρ sin φ cos θ
• y = ρ sin φ sin θ
• z = ρ cos φ

These equations allow us to convert points or equations from one coordinate system to the other. The reverse transformation is also useful:

• ρ = √(x² + y² + z²)
• θ = arctan(y/x) (Careful with the quadrant!)
• φ = arccos(z/ρ) = arccos(z/√(x² + y² + z²))

The Jacobian: Accounting for Volume Distortion

When changing coordinate systems in a multiple integral, it's crucial to account for the distortion of volume elements. This is where the Jacobian determinant comes into play. For the transformation from Cartesian to spherical coordinates, the Jacobian is:

J = ρ² sin φ

This means that the volume element dV = dx dy dz in Cartesian coordinates transforms to dV = ρ² sin φ dρ dθ dφ in spherical coordinates. Forgetting the Jacobian is a common mistake that leads to incorrect results.

Evaluating Integrals in Spherical Coordinates: A Step-by-Step Guide

Here's a detailed breakdown of how to evaluate a triple integral by changing to spherical coordinates:

Step 1: Visualize the Region of Integration

Understanding the region over which you're integrating is paramount. Sketch the region in 3D space, if possible. This helps in determining the limits of integration for ρ, θ, and φ. Look for symmetries, spheres, cones, or parts of spheres that might suggest spherical coordinates are a good choice.

Step 2: Determine the Limits of Integration

This is often the most challenging step. You need to express the boundaries of your region in terms of ρ, θ, and φ.

• ρ (Radial Distance): Determine the minimum and maximum values of ρ that span your region. This might involve solving equations of surfaces in terms of ρ. For example, if your region is bounded by a sphere of radius R, then 0 ≤ ρ ≤ R. If the region is outside a sphere of radius R, then ρ ≥ R.

• θ (Azimuthal Angle): Determine the range of θ values needed to cover your region in the xy-plane. This is similar to finding the limits for θ in cylindrical coordinates. If your region is a full circle, then 0 ≤ θ ≤ 2π. If it's a half-circle in the first and second quadrants, then 0 ≤ θ ≤ π.

• φ (Polar Angle): Determine the range of φ values needed to cover your region from the positive z-axis down. Remember that φ ranges from 0 to π. If your region is the entire space above the xy-plane, then 0 ≤ φ ≤ π/2. If it's the region inside a cone with its vertex at the origin and opening angle α with respect to the z-axis, then 0 ≤ φ ≤ α.

Step 3: Transform the Integrand

Replace x, y, and z in the integrand with their equivalent expressions in spherical coordinates:

• x = ρ sin φ cos θ
• y = ρ sin φ sin θ
• z = ρ cos φ

Simplify the resulting expression as much as possible. Sometimes, the integrand simplifies dramatically, making the integral much easier to evaluate. For example, if your integrand contains √(x² + y² + z²), it simplifies to ρ.

Step 4: Set Up the Integral in Spherical Coordinates

Replace dx dy dz with ρ² sin φ dρ dθ dφ (remember the Jacobian!) and write the triple integral with the limits of integration you determined in Step 2:

∫∫∫_V f(x, y, z) dx dy dz becomes ∫∫∫_V' f(ρ sin φ cos θ, ρ sin φ sin θ, ρ cos φ) ρ² sin φ dρ dθ dφ

Where V' represents the region V expressed in spherical coordinates. The order of integration (dρ dθ dφ) can sometimes be changed, but it's generally best to start with the innermost integral (usually ρ) and work your way outwards.

Step 5: Evaluate the Integral Iteratively

Evaluate the integral step-by-step, starting with the innermost integral. Treat θ and φ as constants when integrating with respect to ρ. Then, treat φ as a constant when integrating with respect to θ. Finally, integrate with respect to φ.

Example Problems: Putting the Theory into Practice

Let's illustrate the process with a few examples:

Example 1: Volume of a Sphere

Find the volume of a sphere with radius R using spherical coordinates.

• Region of Integration: The sphere is centered at the origin with radius R.

• Limits of Integration:

  • 0 ≤ ρ ≤ R
  • 0 ≤ θ ≤ 2π
  • 0 ≤ φ ≤ π

• Integrand: Since we're finding the volume, the integrand is simply 1.

• Integral Setup:

  ∫∫∫_V 1 dx dy dz = ∫₀^π ∫₀^(2π) ∫₀^R ρ² sin φ dρ dθ dφ

• Evaluation:

  1. ∫₀^R ρ² sin φ dρ = (ρ³/3) sin φ |₀^R = (R³/3) sin φ
  2. ∫₀^(2π) (R³/3) sin φ dθ = (R³/3) sin φ * θ |₀^(2π) = (2πR³/3) sin φ
  3. ∫₀^π (2πR³/3) sin φ dφ = (2πR³/3) * (-cos φ) |₀^π = (2πR³/3) * (-(-1) + 1) = (4πR³/3)

  Therefore, the volume of the sphere is (4πR³/3).

Example 2: Integrating over an Octant of a Sphere

Evaluate the integral ∫∫∫_E z dV, where E is the solid region bounded by the sphere x² + y² + z² = 9 in the first octant.

• Region of Integration: The region E is the portion of the sphere x² + y² + z² = 9 that lies in the first octant (x ≥ 0, y ≥ 0, z ≥ 0).

• Limits of Integration:

  • 0 ≤ ρ ≤ 3 (Since the sphere has radius 3)
  • 0 ≤ θ ≤ π/2 (First octant implies θ is between 0 and π/2)
  • 0 ≤ φ ≤ π/2 (First octant implies φ is between 0 and π/2)

• Integrand: z = ρ cos φ

• Integral Setup:

  ∫∫∫_E z dV = ∫₀^(π/2) ∫₀^(π/2) ∫₀^3 (ρ cos φ) ρ² sin φ dρ dθ dφ = ∫₀^(π/2) ∫₀^(π/2) ∫₀^3 ρ³ cos φ sin φ dρ dθ dφ

• Evaluation:

  1. ∫₀^3 ρ³ cos φ sin φ dρ = (ρ⁴/4) cos φ sin φ |₀^3 = (81/4) cos φ sin φ

  2. ∫₀^(π/2) (81/4) cos φ sin φ dθ = (81/4) cos φ sin φ * θ |₀^(π/2) = (81π/8) cos φ sin φ

  3. ∫₀^(π/2) (81π/8) cos φ sin φ dφ

     Let u = sin φ, then du = cos φ dφ. When φ = 0, u = 0. When φ = π/2, u = 1.

     ∫₀^(π/2) (81π/8) cos φ sin φ dφ = (81π/8) ∫₀^1 u du = (81π/8) * (u²/2) |₀^1 = (81π/16)

  Therefore, the value of the integral is (81π/16).

Example 3: Integrating over a Region Bounded by a Cone and a Sphere

Evaluate ∫∫∫_E (x² + y² + z²) dV, where E is the region bounded below by the cone z = √(x² + y²) and above by the sphere x² + y² + z² = 1.

• Region of Integration: The region E is bounded by a cone and a sphere.

• Finding the Intersection: Substitute z = √(x² + y²) into the sphere equation: x² + y² + (√(x² + y²))² = 1, which simplifies to 2(x² + y²) = 1, or x² + y² = 1/2. This means the projection of the intersection onto the xy-plane is a circle of radius 1/√2.

• Limits of Integration:

  • 0 ≤ ρ ≤ 1 (The sphere has radius 1)
  • 0 ≤ θ ≤ 2π (Full circle in the xy-plane)
  • The cone z = √(x² + y²) can be written in spherical coordinates as ρ cos φ = √(ρ² sin² φ cos² θ + ρ² sin² φ sin² θ) = ρ sin φ. This simplifies to cos φ = sin φ, or tan φ = 1. Therefore, φ = π/4. The region is bounded below by the cone, so π/4 ≤ φ ≤ π/2.

• Integrand: x² + y² + z² = ρ²

• Integral Setup:

  ∫∫∫_E (x² + y² + z²) dV = ∫₀^(2π) ∫(π/4)^(π/2) ∫₀^1 ρ² * ρ² sin φ dρ dθ dφ = ∫₀^(2π) ∫(π/4)^(π/2) ∫₀^1 ρ⁴ sin φ dρ dθ dφ

• Evaluation:

  1. ∫₀^1 ρ⁴ sin φ dρ = (ρ⁵/5) sin φ |₀^1 = (1/5) sin φ
  2. ∫(π/4)^(π/2) (1/5) sin φ dφ = (1/5) (-cos φ) |_(π/4)^(π/2) = (1/5) (-0 + cos(π/4)) = (1/5) * (√2/2) = √2/10
  3. ∫₀^(2π) (√2/10) dθ = (√2/10) * θ |₀^(2π) = (√2/10) * 2π = (π√2)/5

  Therefore, the value of the integral is (π√2)/5.

Common Mistakes to Avoid

• Forgetting the Jacobian: This is the most frequent error. Always remember to include ρ² sin φ in the volume element when converting to spherical coordinates.
• Incorrect Limits of Integration: Carefully visualize the region and determine the correct ranges for ρ, θ, and φ. A sketch is invaluable. Pay attention to which octant or part of space you're integrating over.
• Incorrect Transformation Equations: Double-check that you're using the correct equations to convert x, y, and z to ρ, θ, and φ, and vice versa.
• Sign Errors with Arctangent: When finding θ using arctan(y/x), be mindful of the quadrant in which the point (x, y) lies. You might need to add π or 2π to the result of the arctangent function.
• Not Simplifying the Integrand: Before setting up the integral, try to simplify the integrand as much as possible using the transformation equations. This can often make the integral much easier to evaluate.
• Assuming Symmetry Without Proof: If you're using symmetry to simplify the integral, make sure the integrand is also symmetric with respect to the same transformation.

When Not to Use Spherical Coordinates

While spherical coordinates can be powerful, they're not always the best choice. Consider the following situations:

• Regions with Cartesian Symmetry: If your region is a rectangular box, for example, Cartesian coordinates are usually simpler.
• Regions with Cylindrical Symmetry but not Spherical: If your region is a cylinder or involves expressions like √(x² + y²), cylindrical coordinates are often more appropriate.
• Complicated Boundaries: If the boundaries of your region are very complex and difficult to express in spherical coordinates, sticking with Cartesian coordinates might be easier, even if it means a more complicated integral.

Conclusion

Evaluating integrals by changing to spherical coordinates can significantly simplify problems involving spherical symmetry. By mastering the transformation equations, the Jacobian determinant, and the process of determining the limits of integration, you can effectively tackle a wider range of challenging integrals. Remember to visualize the region, double-check your work, and be mindful of common pitfalls. With practice, you'll develop the intuition to recognize when spherical coordinates are the optimal choice and how to apply them correctly. The ability to switch between coordinate systems is a valuable tool in calculus and beyond, empowering you to solve problems more efficiently and gain deeper insights into the geometry of functions and spaces.