I have understand that even inside of a bar magnet magnetic lines move from north to south from this que below. Why is the direction of magnetic field from South to North Pole inside a magnet?.

But my doubt is that , when a current is passed in upward ⬆️ direction through a mettalic conductor /wire , the magnetic lines of force are produced like concentric circles in anticlockwise direction. Then south pole is produced at front side of the wire while north pole is produced at back side of wire. We know that magnetic lines of force are directed from north to south pole , But in this scenario , though the lines of force are directed from north to south but at the same time it is also directed from south to north because it forms concentric circles . So pls explain me how can these lines are directed from south to north

If you have got my point , then pls answer me


2 Answers 2


Magnetic field lines are closed loops. A region in which the lines diverge from a region in which they are close together is called a magnetic North Pole; a region in which they converge, becoming closer together, is called a South Pole. These terms make sense when describing magnets and solenoids, as you should confirm for yourself by drawing or visualising the pattern of field lines. [Incidentally, inside the magnet or solenoid the lines run from South Pole to North Pole, emerging from the North Pole and returning to the South Pole. Note that there is no motion along the lines; I'm just describing their directions, that is the directions of the field at different points along them.]

A straight current-carrying wire has no magnetic poles, because there are no regions where the field lines diverge or converge. Even though the lines are closed loops, being concentric circles (as predicted by Ampère's law or the Biot-Savart rule) they are the same distance apart all round the wire.

  • $\begingroup$ If current carrying wire doesn't have any magnetic pole , then how there is magnetic lines of force like concentric circle $\endgroup$ Commented Dec 10, 2021 at 15:58
  • $\begingroup$ @Vaibhav Tiwari Magnetic field lines (which used to be called magnetic lines of force) are lines whose direction at each point along them is that of the magnetic field strength at that point. We account for the circular field lines around a long straight current-carrying wire by using Ampère's law or the Biot-Savart rule or simply the right hand screw rule; we do not call on the idea of magnetic poles – which is just as well because there aren't any in this case! As I've tried to explain, poles are features that arise with certain field patterns, but not all. Is this clearer now? $\endgroup$ Commented Dec 11, 2021 at 17:58
  • $\begingroup$ Can you make it more simple $\endgroup$ Commented Dec 12, 2021 at 7:49
  • $\begingroup$ @Vaibhav Tiwari Please say what in particular you don't understand. My basic point is that, for a long straight current-carrying wire, you have a magnetic field, and magnetic field lines, without having magnetic poles. $\endgroup$ Commented Dec 12, 2021 at 11:07
  • $\begingroup$ My doubt is , why lines of force are like concentric circles $\endgroup$ Commented Dec 13, 2021 at 19:20

We know that magnetic lines of force are directed from north to south pole

Yes. In the case of the Earth magnetic field, the magnetic South pole is close to the geographic North pole and vice versa. The North pole of a compass needle points to that direction, to the Earth magnetic South pole.

If a horizontal conductor has a conventional current flowing South to North, and a compass is placed over it, the needle deflects to East. That indicates that the $B$ field lines generated by the current points to East over the wire.

Here, thinking about South and North poles are confusing because the lines form circles around the wire, there is no poles. But anyway the convention that the lines are from North to South works. The vectorial sum of the Earth and the wire B-fields now points somewhere between North and East. And the needle aligns with that new direction.

If the compass is placed under the wire, the deflection is to the west, and the same reasoning applies.


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