Two current carrying wires. Confusion

[flyingpig]

Homework Statement

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]

If it is then how can the B-field come back outwards and inwards again?

I can't understand what you are asking

 

[flyingpig]

Here is a picture

http://img269.imageshack.us/img269/7350/asasssc.th.png

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]

Here is a picture

http://img269.imageshack.us/img269/7350/asasssc.th.png

Uploaded with ImageShack.us

Now in the middle region is it 0?

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

 

[cupid.callin]

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

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]

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

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,
$$\vec{B}= \frac{\mu_{o}}{2\pi}\frac{i}{d}$$

 

[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]

Yes that one.

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.

Another question

[STRIKE]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.[/STRIKE]

 

[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]

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.

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.

 

[cupid.callin]

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

The thumb rule can be used to find the direction of cross product (if I'm getting correctly which rule is flyingpig talking about)

 

[SammyS]

The thumb rule can be used to find the direction of cross product (if I'm getting correctly which rule is flyingpig talking about)

Cupid.C,
Look at posts 12, 13, and 14.