“B.dl = μ0 i (t)

“B.dl = μ0 i (t) ( . ) to find magnetic field at a point outside the capacitor. Figure . (a) shows a parallel plate capacitor C which is a part of circuit through which a time-dependent current i (t) flows . Let us find the magnetic field at a point such as P, in a region outside the parallel plate capacitor. For this, we consider a plane circular loop of radius r whose plane is perpendicular to the direction of the current-carrying wire, and which is centred symmetrically with respect to the wire [Fig. . (a)]. From symmetry, the magnetic field is directed along the circumference of the circular loop and is the same in magnitude at all points on the loop so that if B is the magnitude of the field, the left side of Eq. ( . ) is B (2π r). So we have B (2πr) = μ0i (t) ( . )