The moving bar problem: an electromechanical damped oscillator

Abstract

The conducting bar sliding on rails through a uniform magnetic field is a standard textbook illustration of Faraday's law, almost always solved assuming the magnetic field produced by the induced current is negligible. We extend this classic problem by retaining the self-induced field: modelling the circuit as a rectangular loop of round wire of radius $d$, we compute in closed form its geometry-dependent self-inductance $L(x,l)$ and its gradient $dL/dx$ from the Biot--Savart law, including the flux inside the wire and at the corners. The bar then obeys coupled mechanical--electrical equations of motion containing, besides the familiar braking force $-B_0lI$, the inductance-gradient force $\tfrac{1}{2}I^2\,dL/dx$ familiar from electromagnetic launchers. In the absence of resistance the total energy $\tfrac12Mv^2+\tfrac12LI^2$ is exactly conserved; with resistance the system becomes an electromechanical damped oscillator that, in an appropriate regime, maps onto a series resistor--inductor--capacitor (RLC) circuit with equivalent capacitance $C_{eq}=M/(l^2B_0^2)$, the bar's momentum playing the role of the capacitor charge. Numerical integration of the full equations confirms these analytic approximations in their respective regimes and locates the crossover between over-damped and under-damped behaviour.