I don't understand why magnetic field gets reversed when the electric current is reversed. So I searched on the internet but couldn't find any answer. Could someone plz explain this phenomenon?(with reference to the the following article on quora if possible:Answer to Why don't stationary charges produce magnetic fields like moving charges? by Ashwin https://www.quora.com/Why-dont-stationary-charges-produce-magnetic-fields-like-moving-charges/answer/Ashwin-815?ch=15&oid=109456771&share=00a43b46&srid=ughGY2&target_type=answer)?
I don't know relativity,Lorentz invariance,etc, I have just started to learn electromagnetism. So please explain in as much layman's term as possible.
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$\begingroup$ Related : (1) Magnetic field due to a single moving charge. (2) A generalization of the Biot-Savart law for a number n of wires with n≥3. (3) Are magnetic fields just modified relativistic electric fields?. $\endgroup$– VoulkosCommented Dec 30, 2021 at 13:32
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$\begingroup$ (4) Charges and relative motion. $\endgroup$– VoulkosCommented Dec 30, 2021 at 13:32
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2 Answers
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Oerdsted (almost exactly 200 years ago) discovered that a magnetised needle (a compass needle) was systematically deflected by an electric current in a nearby wire. Perhaps you are happy to accept this as a starting point. We call the direction in which the North Pole of the compass points the direction of the magnetic field.
We find that for a current coming out of a piece of paper towards us, the field lines (lines whose direction at every point is the direction of the field at that point) are anticlockwise. Wouldn't it be odd if these lines were always anticlockwise, even if we reversed the direction of the current? It would suggest that anticlockwise has some special priority over clockwise in the world when we look down on our piece of paper, but it's the other way round if we look at the paper from underneath. Would this not be weird?
That the field direction reverses if we reverse the current (as Oersted found) doesn't have the same weirdness!
answered Dec 30, 2021 at 12:37
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I you have a vector $v$ then $-v$ is the vector in the opposite direction. If you look at Biot Savarts law: https://en.wikipedia.org/wiki/Biot%E2%80%93Savart_law and you reverse the current it means that you have to flip $l$ to $-l$ and that flips $B$ to $-B$.
answered Dec 30, 2021 at 12:34