Integrate F = -(ysinz)i + (xsinz)j + (xycosz)k around thecircle cut from the sphere x2 + y 2 + z 2 = 5 and by the plane z = -1, clockwise asviewed from above
Question
Integrate
F = - (y sin z) i + (x sin z) j + (x y cos z) k
around the circle cut from the sphere
x² + y² + z² = 5
and by the plane z = -1,
clockwise as viewed from above.
Solution
To solve this problem, we will use the Stokes' Theorem which states that the line integral of a vector field around a simple closed curve C is equal to the surface integral of the curl of the vector field over the surface bounded by C.
First, we need to calculate the curl of F. The curl of a vector field F = P(x, y, z)i + Q(x, y, z)j + R(x, y, z)k is given by:
curl F = (∂R/∂y - ∂Q/∂z)i - (∂R/∂x - ∂P/∂z)j + (∂Q/∂x - ∂P/∂y)k
For F = -(ysinz)i + (xsinz)j + (xycosz)k, we have:
P = -ysinz, Q = xsinz, R = xycosz
So, curl F = (∂R/∂y - ∂Q/∂z)i - (∂R/∂x - ∂P/∂z)j + (∂Q/∂x - ∂P/∂y)k = (xcosz - cosz)i - (ycosz + sinz)j + (sinz + 0)k = (xcosz - cosz)i - (ycosz + sinz)j + sinzk
Next, we need to calculate the surface integral of curl F over the surface S bounded by the circle. The surface S is the part of the sphere x^2 + y^2 + z^2 = 5 above the plane z = -1. In spherical coordinates, this surface is described by ρ = sqrt(5), -π/2 ≤ θ ≤ π/2, 0 ≤ φ ≤ 2π.
The surface integral of a vector field F over a surface S is given by:
∫∫S F . dS
where dS is the vector normal to the surface S. For a sphere of radius ρ, dS = ρ^2 sinφ dφ dθ n, where n is the unit vector pointing outwards from the origin.
So, ∫∫S curl F . dS = ∫ (from 0 to 2π) ∫ (from -π/2 to π/2) ((xcosz - cosz)i - (ycosz + sinz)j + sinzk) . (sqrt(5))^2 sinφ dφ dθ n = ∫ (from 0 to 2π) ∫ (from -π/2 to π/2) ((xcosz - cosz)sinφ cosθ - (ycosz + sinz)sinφ sinθ + sinz sinφ) sqrt(5)^2 dφ dθ = ∫ (from 0 to 2π) ∫ (from -π/2 to π/2) ((ρsinφ cosθ cosz - cosz)sinφ cosθ - (ρsinφ sinθ cosz + sinz)sinφ sinθ + sinz sinφ) ρ^2 dφ dθ = ∫ (from 0 to 2π) ∫ (from -π/2 to π/2) (5sin^2φ cos^2θ cosz - 5sinφ cosθ cosz - 5sin^2φ sin^2θ cosz - 5sin^2φ sinθ sinz + 5sin^2φ sinz) dφ dθ = ∫ (from 0 to 2π) ∫ (from -π/2 to π/2) (5sin^2φ cosz - 5sinφ cosθ cosz - 5sin^2φ sinθ sinz + 5sin^2φ sinz) dφ dθ = ∫ (from 0 to 2π) ∫ (from -π/2 to π/2) (5sin^2φ (cosz - sinz)) dφ dθ = 5 ∫ (from 0 to 2π) ∫ (from -π/2 to π/2) sin^2φ (cosz - sinz) dφ dθ
This integral can be evaluated using standard techniques of integration.
Note: The above solution assumes that the z-component of the vector field is xycosz. If it is xycosz^2, the curl and the integral will be different.
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