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

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878 Chapter 27 Line integrals and multiple integrals

B

y

1

B

C 2

C 1

C 2

C 1

A

Figure27.7 ∫

Theline integral

F·dscanbe

evaluated along different paths

between Aand B.

A

0 1

Figure27.8

Thepath ofintegration is the curveC 1

followed bythe lineC 2 .

x

torepresent the integral.So, foraconservative field wehave the result

F·ds=0

C

for any closed curveC.

Insummarywe have the following results:

Foraconservative field, F,all the following statements areequivalent:

∇×F=0

F·ds=0

∫ B

A

F·ds isindependentofthe pathbetween Aand B

Fisderivable from a scalar potential,thatisF = ∇φ

Example27.8 Evaluate

F·ds

C

whereFisthe vectorfieldy 2 i +2xyj and where:

(a) C =C 1

isthe curvey =x 2 goingfrom A(0,0)toB(1, 1).

(b) C =C 2

isthe straightlinegoingfromB(1, 1)toA(0,0).

(c) Deduce that ∮ F·ds = 0 where the closed line integral is taken around the pathC 1

and thenC 2

.

Solution Thesituation isdepictedinFigure 27.8.

(a) C 1

hasequationy =x 2 andsody = 2xdx.NotethatweareintegratingfromAtoB.

Therefore

F·ds=

C 1

(y 2 i +2xyj)·(dxi +dyj) =

C 1

y 2 dx +2xydy

C 1

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