3. According to Ampere Maxwell's law, if the electric flux density changes with time, a magnetic field will appear there. As shown in the figure, there is a parallel plate condenser consisting of circular pole plates of radius a connected to an infinitely long straight current. When a steady-state current I flows into it, find the magnitude B of the magnetic flux density at a position of radius r from the center of the capacitor. However, the following assumptions are made. (1) The electric field between the poles is uniform and orthogonal to the poles. (2) Due to the symmetry of the system, the magnetic field is circumferential and the magnitude of the field at r from the center is constant.

3. According to Ampere Maxwell's law, if the electric flux density changes with time, a magnetic field will appear there. As shown in the figure, there is a parallel plate condenser consisting of circular pole plates of radius a connected to an infinitely long straight current. When a steady-state current I flows into it, find the magnitude B of the magnetic flux density at a position of radius r from the center of the capacitor. However, the following assumptions are made. (1) The electric field between the poles is uniform and orthogonal to the poles. (2) Due to the symmetry of the system, the magnetic field is circumferential and the magnitude of the field at r from the center is constant.