- #1
CAF123
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Homework Statement
I am some trouble understanding the surfaces required to integrate over in the following questions. I can tackle them, I just don't understand some terminology.
Q1) A circle C is cut on the surface of the sphere ##x^2 +y^2 +z^2 = 25##
by the plane ##z=3##. The direction round C is anticlockwise, as perceived from (0,0,5). Evaluate ##\int_{C} z\,dx + x\,dy +x\,dz## via Stokes' Thm
Q2)Use Stokes' to evaluate $$\iint_{\sum} (∇ \times \vec{F}) \cdot \hat{n}\,dS$$ where ##\vec{F} ## is some vector field and ##\sum## is the surface defined by ##z=4-x^2-y^2, z≥0##.
Q3)Same as Q2) but ##\sum## is the curved surface of the cylinder ##x^2 +y^2 = 4## for ##z\,\in\,[0,3]##
The Attempt at a Solution
Q1) I can do it easily directly, but using Stokes' Thm, I said that the required surface is the portion of the sphere between z = 3 and z =5. I.e the part of the sphere bounded by C. I think this is correct, because it follows from the def of Stokes' Thm. However, in the solutions I have, they say the required surface is a disk of radius 4. Why?
Q2) So I said the required surface is the whole surface between z =4 and z=0. But in the answers they consider only the boundary of the paraboloid?
Q3) I said the surface was the curved part of the cylinder (ie the boundary of the circle ##x^2 +y^2 =4##, projected up to z =3) That is ##x=2\cos\theta## and ##y=2\sin\theta## and ##z=z## and the integral in the end is $$\int_{0}^{2\pi} \int_{0}^{3} ...\,dz\,d\theta.$$ But, and I have no idea why this is the case, the solutions say the required surface is the two surfaces ##x^2 +y^2 =4## at ##z=3## and ##z=0## (the two circles). This is not the curved part of the surface? What am i missing?