physics
posted by Tom .
An electron enters a region where there is a magnetic field with a strength of 0.5 T which points out of the page (electron has an initial speed of 150 m/sec and is moving to the left). Describe motion of charge in an magnetic field (In particular, what is the shape of the path, what is its size, and what direction is it in?)

m•a = F
a = v^2/R is the normal (centripetal) acceleration,
F= q•v•B•sinα is Lorentz force , here, sinα = 1,
F =e•v•B,
e =1.6•10^19 C is the charge of electrom,
m =9.1•10^31 kg is the mass of electron.
m• v^2/R = e•v•B,
m• v/R = e•B,
R = m•v/e•B= =9.1•10^31•150/1.6•10^19•0.5= =1.71•10^9 m.
The çath is the circle of ràdius
R = 1.71•10^9 m.
If the electron moves to the left, and magnetic field points out of the page, the Lorentz force is directed downwards, therefore, the electron is rotating clockwise about the magnetic field lines. 
Thanks Elena