Question

why in phenomen of induction the flux of B trough a closed surface =0 while the flux of B trough an open surface is diffrent of 0 ?

Answer 1

I think, in case of a closed surface, the number of field lines entering the surface equals the number leaving it, therefore flux=B.dS-B.dS=0. but in case of an open surface the flux is simply B.dS. (there will be an integration of course).

Answer 2

around an open surface, you add up each little dl. that is the same as the circumfrence around the circle. Now when we are dealing with a closed surface, there is NO difference betweent he flux entering and the flux leaving. This no dE/dt. Does that make sense? Think of your bank account. If it is closed, nothing in or out and thus, no change.

Answer 3

I suggest that yo ustudy GIANCOLI'S Calculus based texbook. He does give some excellent examples of the problems you are having trouble with. One rap on Giancoli is that he is short on content but very heavy in problems. This maybe due to to the lack of patience of most students. They don't want to qualitatively understand the material. They want to get on with the problems.