# Homework Help: One more question on Magnetic Fields

1. Feb 9, 2005

### andrew410

A metal rod having a mass per unit length p carries a current I. The rod hangs from two vertical wires in a uniform vertical magnetic field as shown in the figure below. The wires make an angle theta with the vertical when in equilibrium. Determine the magnitude of the magnetic field.

Figure: http://east.ilrn.com/books/sepsp06t/pse6e.29.64p.e.jpg

I'm not sure where I need to use the theta for in this problem. I know again that we need to apply the formula F=IL x B, but need help starting it.

Again any help is great! thx~!!! :)

2. Feb 9, 2005

### s_a

In this case there are two forces acting on the current carrying conductor, perpendicular to each other. Weight force (down), and magnetic force (left). The vector sum of these two forces points in the direction of the two supporting wires, diagonally down at an angle @ (theta) to the vertical axis. Using trigonometry (and perhaps a vector diagram to help you visualise):
tan@ = (magnetic force)/(weight)
= BIL/mg,

so you can find B. To determine the sign of B (positive or negative), you need to see if the diagram is consistent with the right hand rule.

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