Magnetic Field between two plates

[Jgamita10]

Homework Statement

Two very thick equally-magnetized square plates of
side 0.23m and of mass 20 g, are hung by threads
20.0 cm long from a common point. The plates repel and
deflect from each other through a very small angle.

a) Calculate the B field between the two plates, if the
distance between them is 0.123 cm?

b) Calculate the magnetic
field energy stored between the two plates.

Homework Equations

The Attempt at a Solution

I so far solved for the angle, and solved for the tension in the string using W=Mg=Tcosθ

 

[Simon Bridge]

That's great.
Please show your working, and you best attempt at the bit you got stuck on.