Question

An electron traveling 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

Charged particle moves in a circular trajectory with constant speed $u$, since force always acts perpendicular to the motion. It is not able to change its speed. It just tends to come out after one circle is completed with speed $u$.