So I learned that the bar magnet makes a magnetic field and the field lines have a certain shape which kinda resembles 2 semi-ovals.
And if we pass current through a conductor then it makes a magnetic fields and the field lines are in the form of concentric circles around the conductor.
My question is- why? Why do magnetic field lines take the shape that they take? There must be a reason behind it. Why aren't the field lines of a normal bar magnet shaped like the field lines of a conductor carrying electricity? And vice-versa?
Please explain it to me..i know that it delves very deep into the fundamentals of the working of science but still…please tell me the reason if anyone knows it
Best Answer
Physics cannot answer why-questions ultimately. The natural sciences in general try to describe the world in terms of models, which consist of some fundamental objects and their relations. But why the relations are the way they are, cannot be answered by the scientific method. The only statement that physics does make about these fundamental relations is, that they are compatible with our observations and experiments.
So the question "Why do field lines take the shape they take?" cannot be answered finally. We could try answering it as follows: Because the (electro-)magnetic field obeys the Maxwell equations. But that is not a final answer, because we can regress and ask: "Why do the Maxwell equations hold for the electromagnetic field?", we now could answer that question in terms of quantum field theory and argue, that the electromagnetic field is a massless, spin-1 field coupling to a U(1) charge and therefore must obey the Maxwell equations in the appropriate classical limit, but again, this is not a fundamental answer to the why-question. In a way, this question reduces to the question "Why are the physical laws the way they are?" which is quite obviously outside the realm of physics.
As explained, in physics, why-questions can only be answered in reference to some set of fundamental assumptions (axioms). The second question "Why are the field lines of a current carrying wire and a bar magnet different?" is easy to interpret as a question which is to be answered with reference to our model of electromagnetism. Electromagnetism is governed by the Maxwell equations, which can be solved to give the electric and magnetic fields in terms of the current and charge distribution.
In this framework the answer is easy: Because a bar magnet and a current carrying wire have different current distributions. The bar magnet can effectively be modelled by a current perpendicular to the magnetization axis on the surface of the magnet$^1$, the current in the wire is obvious.
Given the Maxwell equations obtaining the shape of the field lines is an exercise in mathematics. One can also obtain them qualitatively by using simple rules, that can be derived from the Maxwell equations (namely, that the circulation of the magnetic field along some line is given by the current through a surface spanned by that line).
Note, that the "effective current" of the bar magnet is very similar to the current in a solenoid, so their magnetic fields will be similar (as noted in the comment by @mikuszefski).
${}^1$ by the way, a proper explanation for the "current distribution" in a permanent magnet is quite complex and requires quantum field theory.