Ampere's Law

1. The problem statement, all variables and given/known data
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

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

3. 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?

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

 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),

Ampere's Law

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?

 thank you very much enricfemi! :D