Solving Ampere's Law for Parallel Wires

[simplicity12]

Homework Statement

Calculate the magnitude of the magnetic field at a point midway between two long, parallel wires thar are 1.0 m apart and have currents of 10.0 A and 20.0 A, respectively, if the currents are:
a) In opposite directions and
b) in the same direction

Homework Equations

B=kI/(2 pi X r)
B is the magnetic field
k is the proportionality constant... 4pi X 10^-7
and r is the radius

The Attempt at a Solution

When it said that the currents move in opposite directions, i thought that the magnetic field at a point is B(net)= B(2) - B(1), and i got 4.0 X 10^-6 T. When it said that the currents move in the same direction, i did B(net)= B(2) + B(1), and i ended up getting 1.2 X 10^-5 T. But the answer at the back of the book is the opposite, the answer that i got for a turns out to be the answer for b and vice versa... are my equations wrong?

 

[enricfemi]

in fact , you are wrong in directions. try use you right hand to sure when to add or minus

 

[enricfemi]

But the answer at the back of the book is the opposite

so you calculation must be all right , conside more about it:

When it said that the currents move in opposite directions, i thought that the magnetic field at a point is B(net)= B(2) - B(1), and i got 4.0 X 10^-6 T. When it said that the currents move in the same direction, i did B(net)= B(2) + B(1),

 

[enricfemi]

you must distinguish between the diretions of currents and magnetic field .

remember the magnetic field is vector field,its directions has complicated relation with the currents.

does that help, my pretty baby?

 

[simplicity12]

thank you very much enricfemi! :D