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Magnetic force on a wire

by Sheneron
Tags: force, magnetic, wire
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Nov22-08, 03:12 PM
P: 360
1. The problem statement, all variables and given/known data
A packed bundle of 100 long, straight, insulated wires forms a cylinder of radius R = 0.500 cm.
(a) If each wire carries 2.00 A, what are the magnitude and direction of the magnetic force per unit length acting on a wire located 0.200 cm from the center of the bundle?

2. Relevant equations
[tex]F= ILB[/tex]

[tex]B = \frac{\mu_{0}I_{enc}}{2\pi r}[/tex]

3. The attempt at a solution

Here is what I did to get the solution:

I can find the number of wires in out to where the radius equals 0.002m.
[tex]\frac{\pi(0.002)^2}{\pi(0.005)^2}*100 = Number of Wires[/tex]

That gives me 16 wires and since each has 2 amps the current enclosed = 32 A.

Using ampere's law:

[tex]B = \frac{(4\pi * 10^{-7})(32)}{2\pi(0.002)}[/tex]

B = 0.0032

Knowning B I can find the force per unit length on one wire with 2 amps of current.

[tex]\frac{F}{L} = (2)(0.0032)[/tex]

That equals 0.0064 N/m

That answer is wrong and I can't figure out why. If anyone can help it would be appreciated. Thanks
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Nov22-08, 03:28 PM
P: 46
Do you mean .0064N/m
Nov22-08, 03:34 PM
P: 360
yes I did, sorry

Nov22-08, 08:09 PM
P: 360
Magnetic force on a wire

I can't figure out what could be wrong with this.
Nov22-08, 08:13 PM
P: 166
Looks okay to me too..
Nov24-08, 07:09 PM
P: 3
The instructions request the direction as well as the magnitude...
You have the magnitude: 6.34mN/m
I'd say you need to articulate the direction ("inward"), ,otherwise you are absolutely correct.
Nov24-08, 07:49 PM
P: 360
Well I got an answer of 6.4mN/m which is not the right answer. 3 options. Something is wrong in my logic. Something is wrong in my calculations. Or the books is wrong.

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