(a) Upward (away from the plane)
Let's assume that the wires are placed horizontal on the plane. We can determine the direction of the magnetic field produced by each wire by using the right hand rule:
- Thumb: direction of the current in the wire
- Other fingers wrapped around the thumb: direction of the magnetic field around the wire
The two wires are carrying current in opposite directions: let's assume that the wire on the right is carrying current to the north, while the wire on the left is carrying current to the south.
For the wire on the right, we have:
- thumb (current): north
- Other fingers (magnetic field): at a point midway between the wires, the direction is upward (away from the plane)
For the wire on the left, we have:
- thumb (current): south
- Other fingers (magnetic field): at a point midway between the wires, the direction is upward (away from the plane) again
So, the direction of the combined magnetic field generated by the two currents is upward (away from the plane)
(b) They reinforce each other
As we described in part (a), at a point midway between the two wires the two magnetic fields are in the same direction: this means that they add together, so they reinforce each other.
(c) Cancel each other
Let's determine for example the direction of the two magnetic fields at a point on the right of the wire on the right. We have:
For the wire on the right, we have:
- thumb (current): north
- Other fingers (magnetic field): at a point on the right of the wire, the direction is downward (inside the plane)
For the wire on the left, we have:
- thumb (current): south
- Other fingers (magnetic field): at a point on the right of the wire, the direction is upward (away from the plane) again
So, we see that the two magnetic fields have opposite directions, so they tend to cancel each other.