A charged particle of mass m and charge q moves with a constant velocity  v = a i along the positive x-direction. It enters a region of magnetic field which is directed towards positive z-direction, from x = l, which is given by B = b k and moves in a circular path.

What is the initial acceleration of the particle?

-qab/m j

Velocity is along x-dir. Magnetic field is along z-dir. Direction of acceleration is the direction of the force. 

[v x B]direction = i x k = -j

Magnitude : qvB = ma ⇒ a = \(\frac{qvB}{m}\)