The direction of magnet field

[wuwei]

for a infinite length length of current, only consider a part of it, is the direction of magnet field of a ring on the mid vertical plane the same magnitude and along the tangential direction? and for a closed surface , if the flux is 0?

 

[Dale]

only consider a part of it

You cannot do this. It is inconsistent with Maxwell’s equations. See https://en.m.wikipedia.org/wiki/Charge_conservation