# Homework Help: Direction of Magnetic Field

1. Jan 10, 2013

### e.pramudita

1. The problem statement, all variables and given/known data
https://dl.dropbox.com/u/63664351/Physics/ELEC1.PNG [Broken]

2. Relevant equations
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/wirfor.html
https://dl.dropbox.com/u/63664351/Physics/Magnetioc%20Force%20Between%20Wires.PNG [Broken]

3. The attempt at a solution
We want the tension to be zero.
But since the force of gravity going downward, then magnetic force in wire 2 must goes up.
But magnetic force in wire 2 goes up only if current going to the right and magnetic field goes entering the screen . My question. How do I know for sure that the magnetic field is entering the screen, not out of the screen?

Also from the relevant equation image there is something I don't understand in wire 1.
Isn't by right hand rule, if thumb is thrust and middle finger the current, then B must be out of the screen? but why in wire one the direction is looping?

Last edited by a moderator: May 6, 2017
2. Jan 10, 2013

### Staff: Mentor

You can determine the direction of magnetic field with the right-hand-rule (or just look at your illustration).
If it goes out of the screen, you need the opposite direction of current flow.

There is a shorter way to find the direction: Currents are attractive if they flow in the same direction (in parallel wires), which you can see in your image as well.