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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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27.8 The divergence theorem and Stokes’ theorem 897

Stokes’ theorem:

∮ ∫

v·dr= curlv·dS

C S

HeredrisanelementoflengthalongthecurveC.Recallthatdr = dxi +dyj +dzk.

WehaveusedthesymboldrratherthandsaswedidinSection27.2toavoidconfusion

with the element ofarea dS.

Example27.19 Acubeofside2unitsisconstructedwithfivesolidfacesandoneopenface.Itislocated

in the region defined by 0 x 2, 0 y 2, 0 z 2 and its open face is its top

face, bounded by the curveC, lying inthe planez = 2,asshown inFigure 27.26.

Throughout thisregion a vector field isgiven by

v=(x+y)i+(y+z)j+(x+z)k

(a) Evaluate ∮ C v·dr.

(b) Evaluate curlv.

(c) Evaluate ∫ curlvdS, and verifyStokes’ theorem.

S

Solution (a) The open face is highlighted in Figure 27.26. It is bounded by the curveC around

whichthelineintegral ∮ C v·drmustbeperformedinthesenseshown.Inthisplane

z = 2 and dz = 0,and hence

v·dr=(x+y)dx+(y+2)dy

We perform the lineintegral aroundC infour stages.

OnI,x = 0,dx = 0andhencev·dr = (y +2)dy.Notingthatyvariesfrom0to

2,the contribution tothe lineintegral is

∫ 2

[ ] y

2 2

y+2dy=

0 2 +2y = 6

0

z

E

z = 2 I bounding curve C

dS =

dxdzj

D

IV

III

II

B

dS= dxdzj

x

x = 2

A

F

y= 2

C

y

dS =

dxdyk

Figure27.26

Acubicalbox with open topbounded bycurveC.

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