What is a magnetic field line and how does a straight bopper wire produce magnetic field lines?
I think that at the level you are talking about, a magnetic field line can be understood in terms of an experiment in which a straight wire passes vertically upwards through a hole in a horizontal board. The wire should carry a current of a couple of amps or so. You place a sheet of paper on the board with the wire passing through the paper, then you place a tiny ‘plotting compass’ on the paper and mark the position of both ends of the needle on the paper. You then move the compass so that its N end is where its S end was previously, and mark the paper again. If you continue doing this long enough you find that you have drawn a circle with the wire as its centre. That circular line is a magnetic field line. It is a line which is everywhere tangential to the direction of the local magnetic field vector. If you have enough patience you can repeat the plotting experiment with the compass initially either further away from, or nearer the wire, and you will find that you draw another circle
