A particle of charge 
and mass 
is travelling in a vacuum with speed 
. The particle enters, at a right angle, a uniform magnetic field of flux density 
, as shown in the figure below.
The particle leaves the field after following a semi-circular path of diameter 
.
State the direction of the magnetic field.
Explain why the speed of the particle is not affected by the magnetic field.
Show that the diameter of the semi-circular path is given by the expression
Use the expression in (c) to show that the time 
spent in the field by the particle is independent of its speed .
