Homework Help: Question about magnetic fields

1. Feb 9, 2005

andrew410

A conductor suspended by two flexible wires as in the figure below has a mass per unit length of 0.0800 kg/m.

What current must exist in the conductor in order for the tension in the supporting wires to be zero when the magnetic field is 2.60 T into the page?

Figure: http://east.ilrn.com/books/sepsp06t/pse6e.29.14p.e.jpg

I know you have to use the formula F=IL x B to get the current but I don't know how to put it all together...
I think the force is the tension so you can make that zero. I don't know how to get the length because it only gives you the mass per unit length.
Maybe I'm doing this wrong...I don't know...

2. Feb 9, 2005

s_a

The magnetic force (F) per unit length (L) is:
F/L = BI

It's weight (W) per unit length is:
W/L = mg/L
= (m/L)*g
= 0.08*9.8 N/m
= 0.784 N/m

To have zero tension in the supporting wires, the conductor's magnetic force must cancel out it's weight (i.e. W=F)

BI = 0.784 N
-> I = 0.784/2.6 A
= 302 mA (current must go from left to right, using the right-hand rule)

3. Feb 9, 2005

andrew410

thx for the help and the quick response! :)