GR9277 #9



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pam d 20110927 22:38:58  After having seen these practice tests a few times, I can tell anyone that reads this comment that coaxial cables are definitely something to know.   none 20081018 14:02:10  Or more theoretically, the system has axial symmetry, so on a circle centered around the cable, . . Enclosed is zero, so must be zero.   Poop Loops 20081005 13:07:57  If you don't remember what coaxial cables are for (shielding a current), then you can do it the following way:
The field has to go to Zero at r > inf (nothing else makes sense), which only leaves choices A (since it's always Zero), and C.
From here you have a good shot at guessing, but if you remember that C is the field for an unshielded wire (Ampere's Law) http://teacher.pas.rochester.edu/phy122/Lecture_Notes/Chapter31/chapter31.html
So A is the only choice left.
ramparts 20090809 16:44:15 
The other good way to eliminate C is that it's independent of a and b. The only reason to even suspect this answer is nonzero is because a and b are in different places (and presumably because there's more b surface area than a surface area), so it's ridiculous to have an answer that doesn't involve these two quantities.

  kevglynn 20061014 10:10:11  If you're like me, you've been through an entire undergraduate physics degree without ever hearing the Magnetic Field referred to as the "magnetic induction." I thought they were talking about electromagnetic induction or something else of which I hadn't heard.
Magnetic Induction is sometimes used for Magnetic field so as not to mix up H and B. I always learned that H was the Auxilliary Field, and B, the Magnetic Field, so be careful if you're like me, and don't slip because of mere semantics.
Furious 20071027 19:08:16 
Yeah man, I was in the same boat. I was sitting there going like, "Magnetic Induction, what does that have to do with this system." But in the end I remembered that the entire point of coax cables is to keep the E&M inside the wire, so I got this one right.

Jeremy 20071029 13:43:31 
I too was ignorant of the term, and yet, at some time, I must have read what Griffiths has on page 271: "Many authors call , not , the 'magnetic field.' Then they have to invent a new word for : the 'flux density,' or magnetic 'induction' (an absurd choice, since that term already has at least two other meanings in electrodynamics)." Oh well.

 

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