In Figure 19-14b the distance \(r\) is one-half of the length \(L\) of the near or far side of the loop, and \(\phi\) is the angle between the direction of the near or far side of the current loop and the force exerted on either the top or bottom of the loop. This angle phi is also equal to the angle between the direction of the magnetic field and an imaginary line that’s perpendicular to the plane of the current loop. We call this imaginary line the normal to the plane of the loop.