Experts why we take the cos component in gauss law if according to this point field is always normal to the surface please explain:


2. At the surface of a charged conductor, electrostatic field must be normal to the surface at every point 

If  E were not normal to the surface, it would have some non-zero component along the surface. Free charges on the surface of the conductor would then experience force and move. In the static situation, therefore, E should have no tangential component. Thus electrostatic field at the surface of a charged conductor must be normal to the surface at every point. (For a conductor without any surface charge density, field is zero even at the surface.) See result 5.