Magnetic field at midpoint between opposite-current parallel wires

[SamTsui86]

Homework Statement

Two parallel conductors carry currents in opposite directions, as shown in Figure P19.56. One conductor carries a current of 10.0 A. Point A is the midpoint between the wires, and point C is 5.00 cm to the right of the 10.0 A current. I is adjusted so that the magnetic field at C is zero.

Find the value of the magnetic field at A.

Homework Equations

B = (uI)/(2pi(r))

The Attempt at a Solution

I actually got the solution. I know that I have to add the field produce by the two currents together. so (uI)/(2pi(.05)) + (uI1)/(2pi(.05)) = B
But can anyone explain why I have to add them together instead of subtracting them? I figured since that current are in opposite direction I would have to subtract them. Thank You

 

[MarkusNaslund19]

right hand rule.

 

[xander98989]

The magnetic fields are two parallel vectors. So combining two parallel vectors is addition. Subtraction would be necessary if the current in one wire was flowing in the opposite direction. This gave me pause too.