How is the EM-Force between current wires calculated in this problem?

[theperthian]

[SOLVED] EM-Force between current wires

Homework Statement

Problem 105 in chapter 27 Tipler and Mosca for those playing at home...

A long straight wire carries a current of 20 A, as shown in the figure. A rectangular coil
with 2 sides parallel to the straight wire has sides 5 cm and 10 cm with the near side at a
distance 2 cm from the wire. The coil carries a current of 5 A. (a) Find the force on each
segment of the rectangular coil due to the current in the long straight wire. (b) What is the
net force on the coil?

Homework Equations

good question
i used B= (mu)*I/(2*pi*r) [sorry, i can't use latex...]
and F=BIL

The Attempt at a Solution

The answers are F(2)= 100*10^-4N, F(4)= -0.286*10^-4N

For the two sides parallel (the 10cm ones),
I found B(a)=0.0002T
B(b)=0.0000571T
The other two wouldn't be effected cos they're perpendicular

F=BIL
F(a)=4*10^-4N
F(b)=1.14*10^-4N
...which are wrong

Is it something to do with the area and hence flux and shiznizzle?

 

[theperthian]

wait a minute team, you would use different current values in the two equations, because one is the field due to one wire, and the second is the force on the other wire right?? ooooooooh, of course...

bahaha, got to love how i only solved it AFTER i posted it.