# Find current when linear mass density and current is given.

1. Jul 1, 2012

### arunashish

1. The problem statement, all variables and given/known data
A long straight wire is fixed horizontally and carries a current of 50 A. A second wire having linear mass density 1.0 * 10^{-4} is placed parallel to and directly above this wire at a separation of 5mm. What current should this second wire carry such that the magnetic repulsion can balance its weight?
Equations relevant
(B = μ0i)/2∏r
F = B * I * L * sina
My attempt
I have first got the magnetic field of Wire1 by the formula (B = μ0i)/2∏r.
Then i took out the Mg Field of Wire2 with the same formula taking current as I.Then i took the weight of Wire2 as Total Force and calculated---
Ftotal=B * I * L + B1 * I1 * L1 (taking L as 1m and where B1,L1,I1 as values of Wire2
But i dont get the right answer that is 0.49A.

Last edited: Jul 1, 2012
2. Jul 1, 2012

### Infinitum

Hi arunashish. Welcome to PF.

How have you attempted the problem? What do you think would be the equations relevant here?