Question

if we dont know the path traversed by electron then how can we say for sure 
that it is not circular

Answer 1

First of all, path of electron is not always parabolic in electric field or circular in magnetic field.

Lets talk about electric field first. Force on due to electric field is in a direction opposite to that of the field. Now if electric field is applied to an electron kept at rest in electric field then it will be accelerated on a straight line, directed opposite to the field. Like a rocket.


If electric field is applied to a moving electron along its line of motion then too it will keep moving on straight line. Its motion may be accelerated ir retarded depending on the direction of applied electric field.