Question

Two long parallel wires are a distance 2a apart, as shown. Point P is in the plane of the wires and a distance a from wire X. When there is a current I in wire X and no current in wire Y, the magnitude of the magnetic field at P is B_o. When there are equal currents I in the same direction in both wires, the magnitude of the magnetic field at P is 

A) 2B_o/3 

B) B_o 

C) 10B_o/9 

D) 4B_o/3 

E) 2 B_o

Answer 1

The correct option is: (D) 4B_o/3 

Explanation:

The field from a single wire is given by μ_oI_a / 2πR. The additional field from wire Y would be based on this formula with R = 3R, so in comparison it has 1/3 the strength of wire X. So adding wire X’s field B_o + the relative field of wire Y’s of 1/3 B_o gives a total of 4/3 B_o.