# A positive charge ' q ' of mass ' m ' is moving along the $\;+ x\;$ axis .We wish to apply a uniform magnetic field B for time $\;\Delta t\;$ so that the charge reverses its direction crossing the y- axis at a distance d . Then :

$(a)\;\normalsize B=\large\frac{mv}{qd}\;and\;\normalsize \Delta t=\large\frac{\pi d}{v} \qquad(b)\;\normalsize B=\large\frac{mv}{2qd}\;and\; \normalsize \Delta t=\large\frac{\pi d}{2v}\qquad(c)\;\normalsize B=\large\frac{2mv}{qd}\;and\;\normalsize \Delta t=\large\frac{\pi d}{2v}\qquad(d)\;\normalsize B=\large\frac{2mv}{qd}\;and\;\normalsize \Delta t=\large\frac{\pi d}{v}$