The figure shows three situations when an electron moves with velocity `vec(v)` travels through a uniform magnetic field `vec(B)`. In each case, what is the direction of magnetic force on the electron

Options

A. `+ve` z-axis, `-ve` x-axis, `+ve` y-axis
B. `-ve` z-axis, `-ve` x-axis and zero
C. `+ve` z-axis, `+ve` x-axis and zero
D. `-ve` z-axis, `+ve` x-axis and zero

Answer

B

Explanation

1. `vec(F)_(m) = (-e)(-vhat(i) xx Bhat(j)) = -B e v hat(k)` i.e. `-z`- axis
2. `vec(F)_(m) = (-e)(-v hat(j) xx B hat(k)) = -B e v hat(i)` i.e. `-x`- axis
3. `vec(F)_(m) = (-e)(-v hat(j) xx B hat(k)) = 0`
Alternatively, use Fleming's left hand rule.