Magnetic Field / Force Problem

[bdh2991]

Homework Statement

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?

Homework Equations

LIB=mg

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?

 

[anathema]

I would like to clarify a few things before providing an answer. Firstly, the question does not specify the charge of the wire, so we cannot assume that it is negatively charged. Secondly, the direction of the magnetic field does not depend on the charge of the wire, but rather on the direction of the current.

In this case, the wire is carrying a current from east to west, so the direction of the magnetic field would need to be perpendicular to the current, i.e. from north to south. This is because according to the right-hand rule, when you point your thumb in the direction of the current, your fingers curl in the direction of the magnetic field.

To calculate the magnitude of the magnetic field, we can use the equation F = BIL, where F is the force required to counteract gravity, B is the magnetic field strength, I is the current, and L is the length of the wire. Rearranging this equation, we get B = F/(IL). Plugging in the values given in the question, we get B = (15g)(9.8m/s²)/(8A)(2m) = 9.19 mT.

Therefore, the magnitude of the magnetic field needed is 9.19 mT, and the direction should be from north to south. I hope this helps clarify any confusion and helps you in solving similar problems in the future.