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BiGyElLoWhAt
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Homework Statement
A point charge +q is placed near a curved, charged, insulating rod as shown at the left below. (I'll just draw it, I don't have access to a camera at the moment) The charge is placed near the center of the curvature of the curved rod. For each of the five cases A-E, the charge density on the rod varies according to the graphs, but the total charge is the same.
Homework Equations
##F= K\frac{q_1q_2}{r^2}##
Not really sure, that's the only one I've used on this one.
The Attempt at a Solution
First off, I'm not really sure what's expected on this, we've only had one class, and this is due tomorrow (if I could get help in the next few hours I'd love you long time), maybe I'm going too far with this, I don't know.
It looks like i have an arc of a circle radius d, and the arc goes from -60deg to 60deg with a charge at the middle (see diagram)
Due to symmetry, all the y components cancel so that's nice.
It doesn't really say what the charge density function yields, so I assumed that the area under the curve from a to b was the total amount of charge from a to b measured in coulombs. Is that too big of a jump?
So given ## \rho (\theta)##,
We can say that (my assumption)
##\text{number}_{\text{charges from a to b}}= < \rho (\Delta \theta)>(\Delta \theta)
= \frac{\rho (a) +\rho(b)}{2}(b-a)##
and therefore my approximate force would be
##F = K\frac{q}{d^2}\Sigma \frac{\rho (a) +\rho(b)}{2} cos(\frac{a+b}{2})\Delta \theta##
1) Is my logic correct thus far? I mean, I think it makes pretty good sense, but... (continued after #2)
2) Can I simply jump from this riemann sum to an integral form of this equation? I think the best way I'd be able to "explain my reasoning" on this homework would be through the math.
...1 ctd.) ...[but]... I have my doubts about using the average charge density in my function... Can I do this as long as my function is continuous? I'm pretty sure I'm going to have to do some of these piece wise, but I don't know. I think I'm missing something here.
Let me provide an example:
Graph a is linear from -60deg to 0 and from 0 to 60. From -60 to 0 my density as a function of theta is ##\frac{1}{30}(\theta + 60)## and from 0 to 60 it just mirrors it back down to 60.
Don't ask me what the 2 at theta = 0 is, it's not labeled, it just goes up 2 ticks on the charge density (the graphs make me think this is potentially a qualitative analysis question, but I want to do it as quantitatively as possible.)
Using my logic and my function, the integral form of the force for this density would be:
##(2)(\frac{1}{30})K\frac{q}{d^2}\int_{-60}^0 \frac{\rho(0)+ \rho(-60)} {2}(\theta +60)cos(\theta)d\theta##
Here's my issue with this: I don't feel like my theta in the cosine is lining up with the other theta's, in my approximation, I was using the average charge density over an interval, and for the cosine theta I was using the angle smack dab in the middle of that interval, which is not exact, I know, but I'm not sure what to do about that.
Fact of the matter is I don't like my function, but I'm not really sure what to do about it. Any pointers? This is due tomorrow morning (it's 8:40pm, class is at 9am) $1 trillion to the first post haha =]
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