2 wires running parallel end up attracting?

[johnwandering]

---

Members don't see this ad.

This is from #25 of BR chapter 8.

I do not understand this problem at all. They seem to just make up these magnetic field directions in the answer explanation without giving any explanation.

Why are there magnetic fields going in on one side and out on the other?
Why isn't it the other way around?


I feel like the BR Physics electrostatics is very poor. They leave out a whole Bunch of stuff. They never mention the V=Ed equation anywhere, and there's this F=i xl x B equation in the "equations to know" that is never mentioned in the chapter (still no idea what that is).

 

[SaintJude]

A couple of people aren't using TBR! (I'm using Kaplan & EK.)Can you post the questions?

 

[johnwandering]

Sure thing!

It's an independent question (no passage).

If 2 parallel wires carry currents in the same direction:
A.) the wires attract
B.) the wires Repel
C.) the wires rotate such that they become perpendicular
D.) the wires feel no net external force

 

[SaintJude]

It's A. It's a popular question often asked in intro physics classes b/c it requires you to use both hand rules. It's hard to explain w/o a visual so I scoured internet for this one below.

Focus on either current. Say we start with the left one--"lefty" If you use the RHR no.2 --the one that allows you determine direction of the magnetic field. You will see that lefty imposes a magnetic field on "righty" that pushes the magnetic field into the page.

Now switch RHR rules and focus on "righty". The external magnetic field (due to lefty) is into the page, so use your version of the RHR rule that includes force, current & magnetic field. The one I use is this: My fingers are pointing into the direction of the field and my thumb points into the direction of the current. Force points in direction of palm You'll see the force will be pointing towards the Lefty.

Repeat the process for lefty & see that both forces will be toward each other.

 

[johnwandering]

Ohhh wowwwww thanks

The BR doesn't mention the #2 right hand rule anywhere in the entire book.



Pretty hard to remember for a guy 7 years removed from a physics class.