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A uniform magnetic field $B$ exists in the region between $x=0$ and $x=\frac{3 R}{2}$ (region $2$ in the figure) pointing normally into the plane of the paper. A particle with charge $+Q$ and momentum $p$ directed along $x$-axis enters region $2$ from region $1$ at point $P_1(y=-R)$. Which of the following option(s) is/are correct?
$[A$ For $B>\frac{2}{3} \frac{p}{QR}$, the particle will re-enter region $1$
$[B]$ For $B=\frac{8}{13} \frac{\mathrm{p}}{QR}$, the particle will enter region $3$ through the point $P_2$ on $\mathrm{x}$-axis
$[C]$ When the particle re-enters region 1 through the longest possible path in region $2$ , the magnitude of the change in its linear momentum between point $P_1$ and the farthest point from $y$-axis is $p / \sqrt{2}$
$[D]$ For a fixed $B$, particles of same charge $Q$ and same velocity $v$, the distance between the point $P_1$ and the point of re-entry into region $1$ is inversely proportional to the mass of the particle
$A,B$
$A,C$
$A,D$
$A,B,D$
Solution
$R^{\prime}=\frac{p}{q B}$
$\left(R^{\prime}-R\right)^2+R^2=R^{\prime}$
$\Rightarrow R^{\prime}=\frac{13}{8} R$
