Can someone help me out? I'm not making any progress with any direction I go in. Thanks.

4/2/2007 9:16:26 PM

I'm in the same place as you. I wish I paid more attention in physics now!

I think for 12-5 you find the surface area of the circle ? does anyone know if that's right?

Did u get 13-4?

[Edited on April 2, 2007 at 9:28 PM. Reason : .]

4/2/2007 9:27:27 PM

I working on 13-4 now. Do you treat a loop (circle) the same as a rectangle? For example, if it is centered at the origin with r=0.5, would you integrate x from -0.5 to 0.5 and y from -0.5 to 0.5?

4/2/2007 9:39:39 PM

i would assume so.

4/2/2007 9:45:41 PM

ok i think i got 12-5a..not sure.


you just find the surface area and then solve for J.

4/2/2007 9:47:39 PM

Anyone else? jbl4me, daalians, neolithic, A Tanzarian, HUR?

4/2/2007 9:52:39 PM

I got 0.00001, 5e-6, and 0 for 13-4...no clue if these are correct.

4/2/2007 9:54:15 PM

When finding J or Ienc and setting up the integral...do you set up to integrate over the limits r: 0 to r and theta: 0 to 2*pi? This is not working for me. Thanks.

4/2/2007 10:40:47 PM

12-5a) Current density is just current per unit area. You know total current and you can find the cross-sectional area of the wire... Don't forget that the question asks for current density across all space.

12-5b) Essentially, you're going to use equation 17 on page 11 of note set 12. These two equations come from equation 16 on the previous page. It's probably worth your time to eventually figure out how to go from Eqn 16 to Eqn 17 in case Alexander asks a question that doesn't involve cylinders (IMHO).

12-7) A little more difficult to describe via internet...Each wire produces a magnetic field; you're trying to find the points where H1 + H2 = 0. Each wire has two equations for H--one for inside of the wire and one for outside of the wire. So, you're going to end up with four total equations which can be combined in 4 different ways.

^ If you're talking about the surface integral, then dS = r · dr · d-theta
And yes, integrate from 0 to r and 0 to 2·pi

[Edited on April 2, 2007 at 10:57 PM. Reason : ]

4/2/2007 10:43:35 PM

On 13-4, do you use cylindrical coordinates or rectangular coordinates?

4/2/2007 11:19:17 PM

^^ Could you explain 12-7 a little more? How do you go from 4 equations to finding points?

4/2/2007 11:36:51 PM

I imagine this is too late/early...

^ Each wire requires two equations to describe its magnetic field. One equation describes the field inside the wire and the second equation describes the field outside the wire. 2 equations x 2 wires means that you have 4 equations to work with. Don't forget that H is a vector. Use the right hand rule to determine in which direction the field points. Because of the orientation of the two wires, there is a pretty limited area where the total field from both wires can be zero. Use this fact (the limited area) to put your H field equations in terms of cartesian coordinates.

The problem asks you to find points where the total magnetic field equals 0, i.e. find where H1 + H2 = 0. There are 4 possible locations where these points can be:

- Outside wire 1 and outside wire 2
- Outside wire 1 and inside wire 2
- Inside wire 1 and outside wire 2
- Inside wire 1 and inside wire 2

You have equations to describe each of those situations...

^^ You can use either coordinate system, as long as you convert B and dS to the same system. Cylindrical coordinates are easier for this problem.

[Edited on April 3, 2007 at 8:16 AM. Reason : ]

4/3/2007 8:13:59 AM

shit i skipped last class and pretty fubar, i got 12-5 and part a of 13-4 but couldn't figure out 12-7

4/3/2007 9:55:19 AM

for 13-4 i just integrated the funtion over the cricle, so int(int(B,y,-sqrt(r^2-x^2),sqrt(r^2-x^2)),x,-r,r)

it doesn't work for th next hw problem, but its the same method as the example in the notes :/

4/3/2007 10:06:16 AM

How do you setup the distance variable for the equation in 12-7? I tried something obvious but I get a large radius for when they are both outside the cylinder (~7). If that is right, I'm not sure how to find where on that radius the point is.

4/3/2007 10:11:33 AM

I can get the four H equations...but not sure where to go from there.

4/3/2007 10:24:58 AM