I went through certain statements regarding magnetic fields and M.G lines which I am not able to understand.
First doubt: Why do magnetic field lines form closed curves?
I went through some answers over the internet and on Phys. SE, but I could not understand them as they were a little difficult for me. The reason I thought of when I tried to think about this question is that as you move the test North Pole away from the North Pole of the magnet, it gradually is attracted by the South Pole of the magnet, and therefore, the magnetic field line moves closer and closer towards the South Pole, so we can say that it forms a loop. However, I am not still able to satisfy myself. So is this reasoning correct? Or if not, can someone explain this to me in a more easier way?
Second doubt: Why do we say that the strength of the magnetic field is more where the lines are closer together?
Now when I try to think of this, I am just reminded that these lines are plotted using a test North Pole. So how can we say that where lines are closer, strength is more? Those lines just represent the direction, how is this justified? Isn't the strength of the field related to the closeness of the test North Pole to the magnet?
Third: Why do iron fillings acquire exactly the design of the magnetic field?
I am pretty sure that you all know which experiment I am talking about. Now, I just cannot understand the reason behind this. Shouldn't the fillings just stick to the cardboard in a random design, because they are just getting attracted, what else is happening?
Last doubt: The diagram of the magnetic field lines that we see (the 2D diagram with many curves), is that diagram 3D in reality?
I hope these are not stupid questions, I really can't understand these.