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Old Mar 17th 2010, 06:19 PM   #1
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Uniform line charge of linear charge density + E field

A uniform line charge of linear charge density 9.0 nC/m extends from x=1m to x=5m. What is the total charge on the line?
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Old Mar 17th 2010, 06:30 PM   #2
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Originally Posted by chevy900ss View Post
A uniform line charge of linear charge density 9.0 nC/m extends from x=1m to x=5m. What is the total charge on the line?
Hey chevy900ss, welcome to PHF!
The answer is pretty straightforward. What's your attempt? A charge density of 9.0 nC/m means that in one meter you have 9.0 nC. How many meters do you have from x=1m to x=5m? Thus the result is...?
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Old Mar 17th 2010, 06:33 PM   #3
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5m-1m=4m
4x9= 36nC

Is this right?

Last edited by chevy900ss; Mar 17th 2010 at 06:36 PM.
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Old Mar 17th 2010, 06:38 PM   #4
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Originally Posted by chevy900ss View Post
5m-1m=4m
4x9= 36nC

Is this right?
Yes.
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Old Mar 17th 2010, 06:39 PM   #5
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Ok how do i figure out what the electric field is at x=10m?

Would i use the formula: E= (1/4 x pi x enot)(q/r^2)

Last edited by chevy900ss; Mar 17th 2010 at 06:49 PM.
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Old Mar 17th 2010, 07:03 PM   #6
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Originally Posted by chevy900ss View Post
Ok how do i figure out what the electric field is at x=10m?

Would i use the formula: E= (1/4 x pi x enot)(q/r^2)
In future please create a new thread for each new question.

You have to do an integration. What is the electric field due to a dx element situated at x meters from the point considered? Remember that the E field is a vector field and hence have a direction.
By the way, I don't understand your notation... what is "enot"?
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Old Mar 17th 2010, 07:10 PM   #7
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enot = 8.85x10^-12 C^2/Nm^2
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Old Mar 17th 2010, 07:17 PM   #8
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Im still confused on how to integrate.
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Old Mar 17th 2010, 07:18 PM   #9
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Originally Posted by chevy900ss View Post
enot = 8.85x10^-12 C^2/Nm^2
Ok, then no... the formula you wrote is not the good one.
What r would you choose?
Instead, dE=(kdQ)/r r where the bold letters represent vectors.
You have to integrate dE from r=5 to r=9. (why? Because r=9 is the distance between x=10m to x=1m and r=5 is the distance between x=5 to x=10.)
I'll help you a bit more... what is dQ, that is, the charge of a very small (differential in fact, of length dx) element of the charged line?
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Old Mar 17th 2010, 07:32 PM   #10
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i understand where r=5 and r=9 comes from, and know k= 9x10^9Nm^2/C^2.
For some reason though i cant seem to realize what value dQ is. Q is 9.0 nC/m right.
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