Page 1017 - College Physics For AP Courses

Chapter 22 | Magnetism 1005
Making Connections: Field Canceling
For two parallel wires, the fields will tend to cancel out in the area between the wires.
Figure 22.45 Two parallel wires have currents pointing in the same direction, out of the page. The direction of the magnetic fields induced by each wire is shown.
Note that the magnetic influence of the wire on the left-hand side extends beyond the wire on the right-hand side. To the right of both wires, the total magnetic field is directed toward the top of the page and is the result of the sum of the fields of both wires. Obviously, the closer wire has a greater effect on the overall magnetic field, but the more distant wire also contributes. One wire cannot block the magnetic field of another wire any more than a massive stone floor beneath you can block the gravitational field of the Earth.
Parallel wires with currents in the same direction attract, as you can see if we isolate the magnetic field lines of wire 2 influencing the current in wire 1. Right-hand rule 1 tells us the direction of the resulting magnetic force.
Figure 22.46 The same two wires are shown, but now only a part of the magnetic field due to wire 2 is shown in order to demonstrate how the magnetic force from wire 2 affects wire 1.
When the currents point in opposite directions as shown, the magnetic field in between the two wires is augmented. In the region outside of the two wires, along the horizontal line connecting the wires, the magnetic fields partially cancel.
Figure 22.47 Two wires with parallel currents pointing in opposite directions are shown. The direction of the magnetic field due to each wire is indicated.
Parallel wires with currents in opposite directions repel, as you can see if we isolate the magnetic field lines of wire 2 influencing the current in wire 1. Right-hand rule 1 tells us the direction of the resulting magnetic force.