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Calculate the curl of electric field E if the corresponding magnetic field is B ( t ) = t x , t y , −2 t z , 0 t < .

curl E = x , y , −2 z

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Notice that the curl of the electric field does not change over time, although the magnetic field does change over time.

Key concepts

  • Stokes’ theorem relates a flux integral over a surface to a line integral around the boundary of the surface. Stokes’ theorem is a higher dimensional version of Green’s theorem, and therefore is another version of the Fundamental Theorem of Calculus in higher dimensions.
  • Stokes’ theorem can be used to transform a difficult surface integral into an easier line integral, or a difficult line integral into an easier surface integral.
  • Through Stokes’ theorem, line integrals can be evaluated using the simplest surface with boundary C .
  • Faraday’s law relates the curl of an electric field to the rate of change of the corresponding magnetic field. Stokes’ theorem can be used to derive Faraday’s law.

Key equations

  • Stokes’ theorem
    C F · d r = S curl F · d S

For the following exercises, without using Stokes’ theorem, calculate directly both the flux of curl F · N over the given surface and the circulation integral around its boundary, assuming all are oriented clockwise.

F ( x , y , z ) = y 2 i + z 2 j + x 2 k ; S is the first-octant portion of plane x + y + z = 1 .

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F ( x , y , z ) = z i + x j + y k ; S is hemisphere z = ( a 2 x 2 y 2 ) 1 / 2 .

S ( curl F · N ) d S = π a 2

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F ( x , y , z ) = y 2 i + 2 x j + 5 k ; S is hemisphere z = ( 4 x 2 y 2 ) 1 / 2 .

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F ( x , y , z ) = z i + 2 x j + 3 y k ; S is upper hemisphere z = 9 x 2 y 2 .

S ( curl F · N ) d S = 18 π

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F ( x , y , z ) = ( x + 2 z ) i + ( y x ) j + ( z y ) k ; S is a triangular region with vertices (3, 0, 0), (0, 3/2, 0), and (0, 0, 3).

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F ( x , y , z ) = 2 y i 6 z j + 3 x k ; S is a portion of paraboloid z = 4 x 2 y 2 and is above the xy -plane.

S ( curl F · N ) d S = −8 π

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For the following exercises, use Stokes’ theorem to evaluate S ( curl F · N ) d S for the vector fields and surface.

F ( x , y , z ) = x y i z j and S is the surface of the cube 0 x 1 , 0 y 1 , 0 z 1 , except for the face where z = 0 , and using the outward unit normal vector.

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F ( x , y , z ) = x y i + x 2 j + z 2 k ; and C is the intersection of paraboloid z = x 2 + y 2 and plane z = y , and using the outward normal vector.

S ( curl F · N ) d S = 0

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F ( x , y , z ) = 4 y i + z j + 2 y k and C is the intersection of sphere x 2 + y 2 + z 2 = 4 with plane z = 0 , and using the outward normal vector

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Use Stokes’ theorem to evaluate C [ 2 x y 2 z d x + 2 x 2 y z d y + ( x 2 y 2 2 z ) d z ] , where C is the curve given by x = cos t , y = sin t , z = sin t , 0 t 2 π , traversed in the direction of increasing t .

A vector field in three dimensional space. The arrows are larger the further they are from the x, y plane. The arrows curve up from below the x, y plane and slightly above it. The rest tend to curve down and horizontally. An oval-shaped curve is drawn through the middle of the space.

C F · d S = 0

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[T] Use a computer algebraic system (CAS) and Stokes’ theorem to approximate line integral C ( y d x + z d y + x d z ) , where C is the intersection of plane x + y = 2 and surface x 2 + y 2 + z 2 = 2 ( x + y ) , traversed counterclockwise viewed from the origin.

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[T] Use a CAS and Stokes’ theorem to approximate line integral C ( 3 y d x + 2 z d y 5 x d z ) , where C is the intersection of the xy -plane and hemisphere z = 1 x 2 y 2 , traversed counterclockwise viewed from the top—that is, from the positive z -axis toward the xy -plane.

C F · d S = −9.4248

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[T] Use a CAS and Stokes’ theorem to approximate line integral C [ ( 1 + y ) z d x + ( 1 + z ) x d y + ( 1 + x ) y d z ] , where C is a triangle with vertices ( 1 , 0 , 0 ) , ( 0 , 1 , 0 ) , and ( 0 , 0 , 1 ) oriented counterclockwise.

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Use Stokes’ theorem to evaluate S curl F · d S , where F ( x , y , z ) = e x y cos z i + x 2 z j + x y k , and S is half of sphere x = 1 y 2 z 2 , oriented out toward the positive x -axis.

S curl F · d S = 0

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Practice Key Terms 2

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Source:  OpenStax, Calculus volume 3. OpenStax CNX. Feb 05, 2016 Download for free at http://legacy.cnx.org/content/col11966/1.2
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