So...

I'm having trouble with set 12 sample test questions. Currently I'm on 12.1

I'm thinking H = (I / 4pi(r^2))(dl X ur)

dl X ur is .707dzuy - .707dzux

I then try to solve for the R in the equation above using the given H value and the cross product to tell where the intersection is. No luck so far.

4/14/2007 11:26:47 PM

UPDATE: I got the mag (1.2 for r), but how do you know that its going in the -x, -y. Im guessing something with the cross product.

Right hand rule?

[Edited on April 15, 2007 at 12:17 AM. Reason : ...]

4/15/2007 12:11:09 AM

i'm impressed....12 o'clock on a saturday night and you're studying for a test

4/15/2007 1:08:00 AM

I always was one of the cool kids

EDIT: I got 12.1 and 12.2 except for the direction I talked about in the previous post.

[Edited on April 15, 2007 at 1:27 AM. Reason : more]

4/15/2007 1:24:22 AM

Has anyone gotten any of section 15?

4/15/2007 11:35:36 AM

I can do absolutely 0% of chapter 15, and i cant make much sense of his notes.

4/15/2007 3:36:44 PM

I got the last one in section 15, page 11 is almost exactly like it.

4/15/2007 4:24:58 PM

I'll be checking this thread hardcore all day tomorrow...

[Edited on April 15, 2007 at 4:35 PM. Reason : ]

4/15/2007 4:35:24 PM

In numbers 12-3, 5 I can get all of the values correct, but what I cannot find is why its in the Uz direction. How do I setup the curl calculation?

Also on 12-5, for inside outer conductor, I am getting

H = .002 (2) / 2*PI*.005 which is -25.4...off by alittle

[Edited on April 15, 2007 at 6:20 PM. Reason : ..]

4/15/2007 6:14:18 PM

UPDATE: I have all of 13 except 13-4b. Im gonna go get food then come back to tackle it. Leaving 14 - 15 for tomorrow

4/15/2007 7:54:17 PM

the direction has always seemed to match the direction of the current

4/15/2007 8:01:02 PM

I managed to figure out all of 15 except for #4 :/

4/15/2007 9:13:39 PM

^^ yes but these has to be a reason based on the cross product

4/15/2007 9:17:13 PM

for 15-4 recallculate the n. because u=uo and e=10eo. And you know H=E/n.


how do u 15.1 or 15.3?

[Edited on April 15, 2007 at 9:23 PM. Reason : ]

4/15/2007 9:20:38 PM

Hmm...seems like everyone has all this figured out.

I'm having trouble with 11-3. I get 2.5x^2 + .5x + .025

I set it up where d/dx (dV/dx) = -5

I'm also having trouble with 11-4.

[Edited on April 16, 2007 at 4:48 PM. Reason : b]

4/16/2007 4:41:56 PM

11-2

Isn't it almost the same as 11-1? I'm having trouble with it though. Isn't d^2 of the answer supposed to be -2? d^2 of 11-1 is 2, but 11-2 doesn't work out that way. Any help is appreciated.

4/16/2007 5:52:09 PM

look in the notes for info on cylendrical coordnates

4/16/2007 6:14:35 PM

^^^ On 11-3,

d/dx (dV/dx) = 0.5 and -0.5...that's why you are an order of mag off

[Edited on April 16, 2007 at 6:17 PM. Reason : more]

4/16/2007 6:17:21 PM

Would someone mind doing a step-by-step for 12-1 or 12-2?

4/16/2007 7:24:54 PM

^ Nevermind, saw the worked problem.

4/16/2007 7:27:18 PM

Im into 14 now, how do you get 14.1. I know the formula and all is in the turn ins, but I cannot seem to get the same answer. 0.45exp(-sigma/e)right?

4/16/2007 7:31:27 PM

On the worked problem 12-8, can someone explain y = 2+d? Thanks.

4/16/2007 7:34:27 PM

That is because you are looking at the distance between the 2 wires. If I start at the lower wire and move away from it...my distance increases. The distance that I am from the other (top) wire is thus 2 (max) - d (how far above wire 1 I am)

4/16/2007 7:37:09 PM

So, in 12-2...

Since x is between 3 and 1, x = 2+d as well, correct?
When I do this, I get x = 3.33 instead of 2.33.

4/16/2007 7:42:32 PM

its 2 - d...

What did you do for 14-2 and 3

[Edited on April 16, 2007 at 7:45 PM. Reason : .]

4/16/2007 7:45:10 PM

It's I / (2*pi*(2-d)), but x = 2+d...that's how it is worked in 12-8.

I haven't gotten to 14 yet.

[Edited on April 16, 2007 at 7:48 PM. Reason : more]

4/16/2007 7:47:21 PM

In the problem you are working its 1 + d since the first wire is at x = 1. The 2 - d comes from the wires being 2 apart.

4/16/2007 7:49:43 PM

I thought the lines were 2 apart since one is at x=1 and one is at x=3.

[Edited on April 16, 2007 at 7:50 PM. Reason : ]

4/16/2007 7:49:44 PM

Yes they are 2 apart, thats why you use 2 - d in the first part, but since x=1 in the second part you would use 1 + d. 1 + d = how far above x axis...

4/16/2007 7:52:03 PM

Thanks...I'm an idiot.

4/16/2007 7:52:23 PM

In 12-3,

inside H = I*r / (2*pi*a^2)
That doesn't work here. For some reason, it works if inside H = 2*I / (2*pi*r^2)
Can anyone explain why?

4/16/2007 8:04:26 PM

I need help with all of 15. I don't really understand any of it.

^Thats what I did and it worked...just gonna put on my notes sheet i guess.

[Edited on April 16, 2007 at 8:10 PM. Reason : g]

4/16/2007 8:10:25 PM

What you want to do for 12-3 is use equation 21 on page 12-14: (curl of H) = J

4/16/2007 8:18:16 PM

Did you ever figure out why you were off a little in 12-5 with 25.4? I am getting the same thing.

4/16/2007 8:18:59 PM

^ No I haven't

4/16/2007 8:19:43 PM

Inside the outside conductor,

(curl of H) = J

(curl of H) = I / A

(curl of H) = I / (pi * (r3^2 - r2^2))

(curl of H) = -26.525823848649 in the z-direction

4/16/2007 8:27:46 PM

Can you explain how you did 12-1 with a little more detail? Thanks a lot.

4/16/2007 8:28:23 PM

What I did in 12.1:

H = I / 2pir
-.418ux + .418uy = 1 / r
use ABS and find r = 1.69.

This tells you that the wire is 1.69 away from 6,6. By Pythagorean theorm that makes it 1.2 in both the X and the Y directions. Use dl X ur to get -x and -y and then end up with (4.8, 4.8). I am good up until this last little step.

Update: 15-2 Complete

Ones I don't know:

15-1, 15-3, 15-4, 15-5, 11-5, 14-2, 14-3

[Edited on April 16, 2007 at 8:47 PM. Reason : .]

4/16/2007 8:34:36 PM

Thanks a lot.

I have 11-5.

V(r) = K1*lnr + K2
Just solve for the two constants using the 2 boundary conditions.

It is the same as 11-4.

----------------------------------
Still working on:
13, 14, 15

[Edited on April 16, 2007 at 8:52 PM. Reason : more]

4/16/2007 8:51:52 PM

I'm sorry but how did you go from:

H = I / 2pir
to
-.418ux + .418uy = 1 / r

Where did the .418 come from?
.03 / (2*pi*.002) = 2.39

4/16/2007 9:02:13 PM

^ Find a value for I / 2pi...divide both sides by said value.

I'm gonna go watch an ep of south park...be back in 30

[Edited on April 16, 2007 at 9:05 PM. Reason : .]

4/16/2007 9:03:04 PM

In 13-1,

Since the normal vector is uz, you don't even consider the ux part of B(t), correct?
If so, my leading number is wrong. Instead of 2.51, I get 3.2. Why isn't is just (0.2*4*4)? The 4's being the diameter of the loop.

4/16/2007 9:36:50 PM

^ Its a loop...so the area is pir^2

15-1, 15-5 (I think i could get this one), 14-2, 14-3

[Edited on April 16, 2007 at 9:44 PM. Reason : f]

4/16/2007 9:41:49 PM

for 12.1:

how are u applying pythagorean theorm to get 1.2? I don't see the picture.

4/16/2007 9:49:57 PM

^ sqrt((6-x)^2 + (6-y)^2) = 1.69

x and y will both be 4.8

[Edited on April 16, 2007 at 9:52 PM. Reason : .]

4/16/2007 9:51:42 PM

oops...beat me to it

You know that the hyp is 1.7. So a^2 + b^2 = 1.7^2.

From H you know that the abs(a) = abs(b)

so 2a^2 = 1.7^2
solving for a = 1.2

What I can't tell you is why its -1.2, -1.2...Its something to do with a certain cross product

[Edited on April 16, 2007 at 9:53 PM. Reason : ..]

4/16/2007 9:52:55 PM

How is 13-2 diff. from 13-1?

[Edited on April 16, 2007 at 9:56 PM. Reason : ]

4/16/2007 9:53:19 PM

^ 13-2

you know that the area is 12.566, so that gives you 12.566uz DOT 0.2texp(-3t)uz

now take that and you can say that the FLUX = 2.51texp(-3t). The max is where the d/dt is equal to 0. So then you get (2.51 - 7.33t)exp(-3t)

Does that help any?

[Edited on April 16, 2007 at 9:57 PM. Reason : c]

4/16/2007 9:56:47 PM

Sorry...I was using the diameter instead of r...stupid mistake

4/16/2007 10:14:08 PM

I got 15-5:

it's like the example on pg 11. you know E and u know magH=mag E/n so magH=10/377

and so H=magH*cos(wt-By) Uy

the direction of H is Uy b/c E is in teh Ux and propigates in teh Uy so Ux (cross) Uy = Uz

and P = E (cross) H.

4/16/2007 10:22:18 PM