Question

Two long straight horizontal parallel wires one above the other are separated by a distance 2a. If wires carry equal current in opposite direction what is the magnetic field in the plane of the wire at a point a) midway between them b) at a distance a above the upper wire .

Answer 1

Using the right hand rules for magnetic field, For a single wire 
If the distance is r , length is L.
(a)
The m,magnitude of the field ,
B = μ₀I/2πr 
Here we observe that the current is in the same direction for both the wires. Now for same current I & distance r = a , for both the wires.
∴ the net magnetic field = zero.

(b) Here we see that both wires creates magnetic field outside the area.

Now total field magnitude 

B = μ₀I/2πa + μ₀I/2π(3a)

B = 2μ₀I/2π(3a) .




Hope it Helps :-)