Physics Help Forum Uniform line charge of linear charge density + E field

 Electricity and Magnetism Electricity and Magnetism Physics Help Forum

 Mar 17th 2010, 05:19 PM #1 Junior Member   Join Date: Mar 2010 Posts: 11 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?
Mar 17th 2010, 05:30 PM   #2
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 Originally Posted by chevy900ss 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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 Mar 17th 2010, 05:33 PM #3 Junior Member   Join Date: Mar 2010 Posts: 11 5m-1m=4m 4x9= 36nC Is this right? Last edited by chevy900ss; Mar 17th 2010 at 05:36 PM.
Mar 17th 2010, 05:38 PM   #4
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 Originally Posted by chevy900ss 5m-1m=4m 4x9= 36nC Is this right?
Yes.
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Isaac
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 Mar 17th 2010, 05:39 PM #5 Junior Member   Join Date: Mar 2010 Posts: 11 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 05:49 PM.
Mar 17th 2010, 06:03 PM   #6
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 Originally Posted by chevy900ss 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)

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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 Mar 17th 2010, 06:10 PM #7 Junior Member   Join Date: Mar 2010 Posts: 11 enot = 8.85x10^-12 C^2/Nm^2
 Mar 17th 2010, 06:17 PM #8 Junior Member   Join Date: Mar 2010 Posts: 11 Im still confused on how to integrate.
Mar 17th 2010, 06:18 PM   #9
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 Originally Posted by chevy900ss 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)/r³ 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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 Mar 17th 2010, 06:32 PM #10 Junior Member   Join Date: Mar 2010 Posts: 11 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.

 Tags charge, density, field, line, linear, uniform

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