An electron is travelling at speed 
in a straight line in a vacuum. It enters a uniform magnetic field of flux density 
. Initially, the electron is travelling at right angles to the magnetic field, as illustrated in the figure below.
The path of the electron in the magnetic field is an arc of a circle of radius 
.
State and explain the direction of the magnetic field.
Show that the speed of the electron is 
.
A uniform electric field is now applied in the same region as the magnetic field. The electron passes undeviated through the region of the two fields, as illustrated in the figure below.
On the figure above, mark with an arrow the direction of the uniform electric field.
Use data from (a) to calculate the magnitude of the electric field strength.
.
