Question

An electron travelling with a speed u along the positive x-axis enters into a region of magnetic field where $B=-B_0\hat{k}(x>0)$. It comes out of the region with speed v then

• A. $v=u$ at y > 0
• B. $v=u$ at y < 0
• C. v > u at y > 0
• D. v > u at y < 0

Answer 1

Answer: (B)

$v=u$ at y < 0

From Lorents equation: $F=-ev\hat{i}\times B_0(-\hat{k})=-eu{B}_0\hat{j}$
Hence, it will complete a semicircular arc and comes out of the region at a position y, such that y < 0.