Question

Two long parallel wires lie in the xz plane. One wire passes through the point (−2 m, 0, 0) and the other through the point (+2 m, 0, 0). The wires carry equal currents in the positive z direction.

1. The magnetic field at (−3 m, 0, 0) is in the negative y direction.
2. The magnetic field at (−1 m, 0, 0) is in the positive y direction.
3. The magnetic field at (+1 m, 0, 0) is in the positive y direction.
4. The magnetic field at (+3 m, 0, 0) is in the negative y direction.
A. 1 and 2 are correct.
B. 1 and 4 are correct.
C. 2 and 3 are correct.
D. 3 and 4 are correct.
E. None of the above are correct.

Answer 1

D. 3 and 4 are correct

Step-by-step explanation:

P1(-2,0,0)

P2(2,0,0)

To know what is the direction of the magnetic field in the different points you take into account the right hand rule: the direction of B is perpendicular to the direction of the current, furthermore the direction of B is in the direction of your fingers when you wrapped the wire with your right hand:

1. In the point P(-3,0,0) the magnetic field is in the positive y direction. The magnetic fields generated by both wires in in the same direction.

2. For the point P(-1,0,0) the contribution of both wires to the magnetic field are opposite between them. However, the contribution of the P1 wire is greater than the contribution of the second one. Hence, B is in the negative y direction.

3. The same as before, but in this case the second wire has a greater contribution, hence, B is in the positive y direction.

4. The contribution of both wires to the magnetic field are in the same direction. In this case, in the negative y direction.

Hence:

D. 3 and 4 are correct