One conducting $\mathit{\text{U}}$ tube can slide inside another as shown in the figure, maintaining electrical contacts between the tubes. The magnetic field $\mathit{\text{B}}$ is perpendicular to the plane of the figure. If each tube moves towards the other at a constant speed $v$, then the emf induced in the circuit in terms of $\mathit{\text{B}}$, $\mathit{\text{l}}$ and $v$ where $\mathit{\text{l}}$ is the width of each tube, will be