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Electromagnetism}Gauss Law

Recall Gauss Law \vec{E} \cdot d\vec{A} = q/\epsilon_0. Thus, E(4\pi r^2)=\int_0^{R/2}A r^2 4\pi r^2 dr = 4\pi A (R/2)^5/5. Solving for E, one has E=A/5(R/2)^3, as in choice (B).

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Comments
tsharky87
2009-11-06 08:46:03
I was confused about the fact that I seemed to be getting a 1/160, so here's an explanation for those of you who were as confused as azot and I:

I was getting my r's confused in Yosun's solution, I just took r=R and canceled away. The reason this is wrong is that the radius that you are taking the area with when multiplying by \vec{E} is actually R/2. So if you set r=R/2 you should arrive at the solution.
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jw111
2008-11-05 15:43:14
in addition, if you set A=1, R=1, epsilon=1

it will be easier

E*4pi*\frac{1}{2^2}=\int r^2*4pi*r^2 dr

then
E \frac{1}{2^2} = \frac{1}{5} \frac{1}{2^5}

E= 1/40
NEC
gliese876d
2008-10-11 20:56:52
hmm. I'm just wondering... I had the following formula memorized for similar situations:

E=\frac{Qr}{4piepsilonR^3}

Where little r is the radius of the point from the center and R is the total radius of the sphere
Now using this formula I figured Q would equal \frac{4piR^3rho}{3}

so I used this in the numerator for Q and also plugged in r= R/2 and you get

\frac{AR^3}{6epsilon}

which is *close* to the right answer, but not quite... where does this standard formula fall apart in reasoning for this particular situation?
NEC
hisperati
2007-10-31 20:29:18
To find the charge we need to integrate a charge density, that is per unit volume. The answers correspond to multipliying by a surface area ( I am not talking about the left side of Guass's Law, where it is obvious surface area is correctly used). Why isn't the charge density multiplied by \frac{4}{3}  pi  R^3  ?
blah22
2008-03-14 18:01:46
Yea I'm confused too
blah22
2008-03-14 18:27:59
Ah I see, for anyone else confused by this in the future, it's just a method of getting the volume of a sphere through integration. Divide the sphere into lots of shells of thickness dr, each with surface area 4 \Pi r^2.

Thanks so much Yosun!


NEC
azot
2007-10-23 06:04:01
the answer to this problem is wrong .
E shoud be equal to A*R^3/32*5
azot
2007-10-23 06:05:00
sorry I was wrong :) Everything is ok

kevinjay15
2007-10-23 18:24:01
ETS is never wrong.
NEC

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