# Find current given magnetic field

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1. Oct 29, 2016

### ooohffff

1. The problem statement, all variables and given/known data
Suppose a straight 1.90 mm diameter copper wire could just "float" horizontally in air because of the force due to the Earth's magnetic field [PLAIN]http://www.webassign.net/images/boldB.gif, [Broken] which is horizontal, perpendicular to the wire, and of magnitude 4 ✕ 10-5 T. What current would the wire carry?

2. Relevant equations
F = ILB

3. The attempt at a solution
It seems like a simple question but the phrasing is a bit confusing and I'm having trouble visualizing it. So the F vector is pointing up in the k-direction because it's pushing the wire up, right? The B vector is horizontal so let's say in the i-direction, but how is it perpendicular to the wire if the wire is also floating horizontally?

So far, all I could do is rearrange the force equation to
I=F/LB

Last edited by a moderator: May 8, 2017
2. Oct 29, 2016

### ooohffff

Also, I believe F should be equal to Fg since the wire is floating in the air.

Ah, also I think B should be going into the page?

Last edited: Oct 29, 2016
3. Oct 31, 2016

### ooohffff

So update:

I = 9.8N/ L*(4x10^-5 T)

How am I supposed to get L?

4. Nov 3, 2016

### rude man

Where did you get 9.8N from? You're implying (whether you know it or not) that your wire has mass of 1 kg. So what's the length of 1 kg of Cu wire with diameter 1.9mm?[/QUOTE]