Two current carrying wires. Confusion

flyingpig

1. The problem statement, all variables and given/known data

Let there be two current carrying wires with current flowing in the same direction. According to my book, they attract by the right hand rule. Now here is my question, since the B-field is popping inwards for one of the wires and the other one is popping outwards in the middle between the two wires, wouldn't that mean the B-field is 0? If it is then how can the B-field come back outwards and inwards again?

cupid.callin

I cant understand what you are asking

flyingpig

Here is a picture



Uploaded with ImageShack.us

Now in the middle region is it 0?

cupid.callin

Yes it is

flyingpig

Then how can the Field come back in and out on the far right and left?

SammyS

Yes it's zero there, but what maters is what the B field is at the location of each wire.

cupid.callin

Why then can't? they are two independent fields ...

like for electric field ... if you place a conductor in its path ... it is zero inside it but again gains its strength outside it

flyingpig

But is the B-field only 0 at the mid point? Or is it everywhere between the two wires

SammyS

Think about this in three dimensions.
The ONLY place the B field is zero is right along the line that's midway between the wires.

cupid.callin

only at the mid point because only at that point their strengths become equal and opposite

Edit:

Mag field due to long wire,
[tex]\vec{B}= \frac{\mu_{o}}{2\pi}\frac{i}{d}[/tex]

flyingpig

Just one other question, I noticed the "finger" cross product rule is different from the wrap around right hand rule (not the curl one, like most calculus textbook teaches one) and this observation uses the wrap around the current rule

SammyS

Be more specific.

Are you referring to the rule which gives the direction of the B field around a current carrying wire, as "the wrap around right hand rule" ?

flyingpig

Yes that one.

Another question

If I want to find the B-field created by the wire on the left, why would I use the distance between the wires? Let d be the distance between the wires, so shouldn't I use d + x where x is a positive quantity.

SammyS

That's NOT a cross product at all. (Referring to the wrap around rule)

One question at a time, please! Your threads tend to get convoluted when you have multiple questions and/or topics in one post.

flyingpig

OKay I will make a new thread and ask it there

SammyS

No, it's OK now. Just didn't want to discuss two things in that one post -- plus it was a chance to give you a hard time. LOL

SammyS

Why would you want to use d+x?

You need the B field (produced by the left wire) at the location of the wire on the right. The distance from on to the other is d.

Let x = 0 if you like.