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4.Moving Charges and Magnetism
medium
An electron moving with a speed $u$ along the positive $x-$axis at $y = 0$ enters a region of uniform magnetic field $\overrightarrow B = - {B_0}\hat k$ which exists to the right of $y$-axis. The electron exits from the region after some time with the speed $v$ at co-ordinate $y$, then

A
$v > u, y < 0$
B
$v = u, y > 0$
C
$v > u, y > 0$
D
$v = u, y < 0$
(IIT-2004)
Solution
(d) The energy of a charged particle moving in magnetic field remains constant because the magnetic field does not do any work. Therefore kinetic energy is constant i.e. $u = v$.
The force on electron will act along negative $y$-axis initially. The electron will undergo circular motion in clockwise direction and emerge out the field. So $y < 0$.
Standard 12
Physics