Question -
Answer -
Energy of an electron beam, E =18 keV = 18 × 103 eV
Charge on an electron, e =1.6 × 10−19 C
E =18 × 103 × 1.6 × 10−19 J
Magnetic field, B =0.04 G
Mass of an electron, me =9.11 × 10−19 kg
Distance up to which the electronbeam travels, d = 30 cm = 0.3 m
We can write the kinetic energyof the electron beam as:
The electron beam deflects alonga circular path of radius, r.
The force due to the magnetic fieldbalances the centripetal force of the path.
Let the up and down deflection of the electron beambe 
Where,
θ =Angle of declination
Therefore, the up and downdeflection of the beam is 3.9 mm.