Consider a parallel-plate capacitor with circular plates distant d from each other. A hole is now drilled in the center of each circular plate, and then a wire carrying a steady current i is run through the holes. There is no connection between the wires and the plates, and the potential difference across the plates can be controlled externally. Derive an expression for the rate of change of this potential difference so that there is no magnetic field at a distance r from the wire. Calculate the value of this rate of change in the particular case where the distance r from the wire is 30mm, the current is 25mA, and the distance between the plates is 1mm.