One conducting U tube can slide inside another as shown in figure, maintaining electrical contacts between…

One conducting U tube can slide inside another as shown in figure, maintaining electrical contacts between the tubes. The magnetic field $B$ 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 $B, \ell$ and $\mathrm{V}$ where $\ell$ is the width of each tube will be
  1. $\mathrm{B}\ell \mathrm{V}$
  2. $-\mathrm{B}\ell \mathrm{V}$
  3. zero
  4. $2 \mathrm{B}\ell \mathrm{V}$

Solution

$\left|\frac{d \phi}{d t}\right|=2 B \ell v$

Asked in: JEE Main 2005

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