# Magnetic field q

1. Feb 11, 2007

### debwaldy

1. The problem statement, all variables and given/known data
Two parallel wires 10 cm apart carry antiparallel currents of 1A and 3A respectively.
(i) what is the force per unit length(in magnitude and direction) between the wires
(ii) what is the magnetic field midway between the wires?
(iii) locate the point(s) where the magnetic fields cancel.

2. Relevant equations
F/L = 2*10^-7(I1)(I2)/d
H= I/2piR

3. The attempt at a solution

(i) i said F/L = 2*10^-7*(1)(3)/(0.1)= 6.0*10^-6 Nm^-1 and direction is repulsive.is this the correct method?

(ii)here i used the formula H=I/2piR
i used this on each wire and took the average value,although this doesn't seem correct:

H1= 1/2*pi*(0.05)= 3.183 A/m
H2 = 3/2*pi*(0.05) = 9.5492 A/m

and H= H1+H2/2 = 6.366 A/m

(iii) i wasn't too sure what to do here but i said that the magnetic fields would cancel wherever they were equal however this only gave me 0m as my answer....

any help would be great thanks