9—Vector Calculus 1 2399.33 Theorem: Given that a vector field satisfies∇×~v= 0everywhere, then it follows that you can
write~vas the gradient of a scalar function,~v=−∇ψ. For each of the following vector fields find if
possible, and probably by trail and error if so, a functionψthat does this.Firstdetermine is the curl is
zero, because if it isn’t then your hunt for aψwill be futile. You should try however — the hunt will
be instructive.
(a)ˆxy^3 + 3yxyˆ^2 , (c)xyˆ cos(xy) +yxˆ cos(xy),
(b)ˆxx^2 y+yxyˆ^2 , (d)xyˆ^2 sinh(2xy^2 ) + 2ˆyxysinh(2xy^2 )
9.34 A hollow sphere has inner radiusa, outer radiusb, and massM, with uniform mass density in
this region.
(a) Find (and sketch) its gravitational fieldgr(r)everywhere.
(b) What happens in the limit thata→b? In this limiting case, graphgr. Usegr(r) =−dV/drand
compute and graph the potential functionV(r)for this limiting case. This violates Eq. (9.47). Why?
(c) Compute the area mass density,σ=dM/dA, in this limiting case and find the relationship between
the discontinuity indV/drand the value ofσ.
9.35 Evaluate
δijijk, mjknjk, ∂ixj, ∂ixi, ijkijk, δijvj
and show that ijkmnk=δimδjn−δinδjm
9.36 Verify the identities for arbitraryA~,
(~
A.∇
)
~r=A~ or Ai∂ixj=Aj
∇.∇×~v= 0 or ∂iijk∂jvk= 0
∇.
(
fA~
)
=
(
∇f
)
.A~+f
(
∇.A~
)
or ∂i(fAi) = (∂if)Ai+f∂iAi
You can try proving all these in the standard vector notation, but use the index notation instead. It’s
a lot easier.
9.37 Use index notation to prove∇×∇×~v=∇(∇.~v)−∇^2 ~v. First, what identity you have to prove
about’s.
9.38 Is∇×~vperpendicular to~v? Either prove it’s true or give an explicit example for which it is
false.
9.39 If for arbitraryAiand arbitraryBjit is known thataijAiBj= 0, prove then that all theaijare
zero.
9.40 Compute the divergences of
Axxˆ+By^2 yˆ+Czˆin rectangular coordinates.
Arrˆ+Bθ^2 θˆ+Cφˆin spherical coordinates.
How do the pictures of these vector fields correspond to the results of these calculations?
9.41 Compute the divergence and the curl of