25.08.2021 Views

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

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

27.5 Conservative fields and potential functions 875

Solutions

1 (a) (0,0, 0)

(b) (1,3, 2)

(c) 17

27.5 CONSERVATIVEFIELDSANDPOTENTIALFUNCTIONS

We have seen that, in general, the value of a line integral depends upon the particular

pathchosen.However,inspecialcases,theintegralofagivenvectorfieldFturnsoutto

be the same on any path with the same end points; that is, it is independent of the path

chosen.InthesecasesthevectorfieldFissaidtobeconservative.Thereisasimpletest

which tells us whether or notFis conservative:

Fisaconservative vector field if

curlF = 0

everywhere

Ifwearedealingwithatwo-dimensionalvectorfieldF(x,y) =P(x,y)i+Q(x,y)jthen

thistestbecomes:Fisconservative if ∂P

∂y = ∂Q . (SeeQuestion 7 inExercises 27.5.)

∂x

Example27.5 Show thatthe vector field

F =ze x sinyi +ze x cosyj +e x sinyk

isaconservative field.

Solution We find curlF:

i j k

∇×F=

∂ ∂ ∂

∂x ∂y ∂z

ze x sinyze x cosy e x siny

= (e x cosy −e x cosy)i − (e x siny −e x siny)j + (ze x cosy −ze x cosy)k

= 0

Wehaveshownthat ∇ ×F = 0andsothefieldisconservative.NotefromSection26.5

thatsuch a field isalsosaidtobe irrotational.

Therearetwoimportantpropertiesofconservativevectorfieldswhichwenowdiscuss.

27.5.1 Thepotentialfunction

Itcanbeshownthatanyconservativevectorfield,F,canbeexpressedasthegradientof

some scalar field φ. That is, we can find a scalar field φ such that F = ∇φ. The scalar

field φ issaidtobeapotentialfunctionandFissaidtobederivablefromapotential.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!