Radial vs tangential electric field lines in a solenoid

[nosmas]

I was reading through an example question calculating the electric field in a solenoid and it made an assumption that the radial electric field must be equal to zero because there is no excess charge in the system. I don't understand when this is true or even why it is true, any suggestions?

Thanks

 

[Mindscrape]

This is the same kind of argument as current flow in a solid wire. In the solid wire, even though there are charges that will make some charge density, and probably to higher order there is some minimal radial field contribution (such as high voltage power lines), to first order the electric field flows with the current. That is to say that for a perfect conductor E=current flow/resistivity.