Question

Two long, parallel wires separated by 50 cm each carry currents of 3.0 A in a horizontal direction.
Find the magnetic field midway between the wires if the currents are in the same direction.
B= T
Find if they are in opposite directions.
B= T

Answer 1

Have you heard of Ampere's law? That is what you need to use.

http://en.wikipedia.org/wiki/Amp%C3%A8re%27s_circuital_law

The answer should be obvious for the same-direction case. For the opposite-direction case, double the field due to a single wire.