Problem 3.312

A thin wire ring of radius \(a\) and resistance \(r\) is located inside a long solenoid so that their axes coincide. The length of the solenoid is equal to \(l\), its cross-sectional radius, to b. At a certain moment the solenoid was connected to a source of a constant voltage \(V\). The total resistance of the circuit is equal to \(R .\) Assuming the inductance of the ring to be negligible, find the maximum value of the radial force acting per unit length of the ring.