1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution I uploaded the problem which I want to solve. Getting a general expression for magnetic field created by plane is difficult, I solve it another method. X is still larger than W, we can think palne current as just a infinite wire. Furthermore, because of large X, magnetic field of z-direction around z=0 is considered as a constant. (magnetic field has x,y dependence and x is still larger than w. So, of course, magnetic field affected by x coordinate is dominant.) Consequently, z-dependence will be vanish and gradient potential has no z dependence. So, a force has a just x direction component. Is it right?