Electric Field #2?

Question

An electron is accelerated by a constant electric field of magnitude 280 N/C.
(a) Find the acceleration of the electron.
(b) Use the equations of motion with constant acceleration to find the electron's speed after 1.05 10-8 s, assuming it starts from rest.

Answer 1

The charge-to-mass ratio for an electron is 1.759*10^11C/kg.

a = (280 N/C)(1.759*10^11C/kg)
a = 4.9252*10^13 m/s^2
v = at = (4.9252*10^13 m/s^2)(1.05*10^-8 s)
v = 517,146 m/s
which exceeds the speed of light, so relativistic equations need to be used and acceleration can no longer be considered to be constant.