GR9277 #64



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Comments 
NoPhysicist3 20170323 12:31:21  The words \"certain region\" are VERY confusing. However, when choosing between B and E, one should keep in mind that it is unlikely for ETS to consider a correct answer containing ultimate statements. Therefore B is the only correct.   Naismith 20111010 04:30:08  What do they mean by "in a certain region" ? In my opinion, it is always possible to find a region small enough so that it doesn't contain any charges, therefore charge density. The charge then will be outside the region...
h.fei10 20121104 07:42:12 
That's not possible. The electric field pevades this certain region, so does the charge.

calcuttj 20140903 17:57:04 
Think about the field inside a cylinder of constant charge density.
The cylinder has radius R, constrain r < R such that
E*r^2d = r^3d
(d is the length of our Gaussian cylinder)
E = r
The region could be the z axis inside the cylinder
Not necessarily the only charge distribution to create E =kz, and this definitely doesn't prove there is ALWAYS a distribution to create a field like this, but it definitely disproves that the field is impossible
Now think about this. If there wasn't a charge density in the region. shouldn't the field be decreasing (i.e. E=k/z)?

calcuttj 20140910 16:36:07 
I made a mistake in my last comment, ignore it.

  r10101 20071027 16:32:37  Why does a small region of vacuum near the surface of an infinite charged plate (with constant = E normal to the surface) not satisfy this question, making answer (E) correct?
panos85 20071031 05:12:32 
It says , not . The electric field near the surface of a conductor is constant, while the field in this problem is not.

tonyhong 20081025 01:54:24 
this is a trap...

  sharpstones 20061201 10:25:11  how could you possibly construct a charge density that would make such an E field?
mhas035 20070404 23:58:56 
Remember that it says that the field is only in a certain region. We just need the region to be small with a relatively large charged plane of constant charge density.

evanb 20080624 11:56:52 
How about a uniformdensity infiniteplane slab. So, it would be thick and the region of interest would be from the middle of the slab to the edge of the slab.

 

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