# Homework Help: Magnetic Field / Force Problem

1. Apr 28, 2012

### bdh2991

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

A straight 15g wire 2m long carries a current of 8A. The wire is parallel to the ground and aligned so that the current goes from east to west. You want to support the wire against gravity by placing the wire in a uniform magnetic field. What magnitude and direction of field do you need?

2. Relevant equations

LIB=mg

3. The attempt at a solution

i understand that the force has to counteract the force of gravity (LIB=mg) and for my magnetic field, B, i got 9.19 mT. This may be a really dumb question but wouldn't the direction of the field depend on if it is positively or negatively charged??? and if it does depend on charge, how would you even know from the question?...i assumed it was a negative charge so in order to counteract gravity the magnetic field would have to be S to N. i ended up getting the question wrong but could someone explain this to me?