An electron is accelerated from rest through a voltage of 440 V and then enters

Question

An electron is accelerated from rest through a voltage of 440 V and then enters a region of constant magnetic field with the magnetic field perpendicular to the velocity of the electron.
(a) What is the speed of the electron when it just enters the region of magnetic field?
? m/s
(b) In the region with the magnetic field, the electron follows a circular path in this region. What is radius of the circular path if the magnitude of the magnetic field is 0.39 mT?
?cm

I don't know where to start with this one.
F=ma
F=qvBsin =90 degrees when B is perpendicular to v which it is in part a

a=v2/r

r=mv/Bq

Explanation / Answer

Voltage V = 440 V Mass of the electron m = 9.11 x 10 -31 kg Charge of electron q = 1.6 x 10 -19 C Speed of the electron v = [ 2Vq / m]                                     = 12.432 x 10 6 m / s (b). Magnetic field B = 0.39 mT                                  = 0.39 x 10 -3 T In magnetic field for circular motion , Bvq = mv 2 / r From this radius of the path r = mv / Bq                                              = 0.1814 m

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