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I have a question that, why are the magnetic field lines around a straight current carrying wire in the form of concentric circles?

If the wire is carrying current, it is acting like a magnet. So shouldn't the magnetic field resemble the magnetic field created by a bar magnet? Then why is it in the form of concentric circle and not the oval-shaped circles as seen in a bar magnet? Magnetic Field around a bar magnet1

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  • $\begingroup$ How long is the wire? $\endgroup$
    – Newbie
    Jan 15, 2022 at 6:36
  • $\begingroup$ @Newbie I'm curious — why would the length of the wire matter? $\endgroup$
    – Shane
    Jan 15, 2022 at 8:34
  • $\begingroup$ @Newbie length doesn't matter at all just it will contribute in increasing or decreasing the $STRENGTH$ of the magnetic field at some point. the magnetic field lines will still be in the circular shape as it was before. $\endgroup$
    – Tejas Dahake
    Jan 15, 2022 at 8:58
  • $\begingroup$ Try completing the field lines for the magnet with the parts inside the magnet. You will find they are loops. The permanent magnet acts as if it had a current flowing around it, not from N to S but in the direction around the body, on its surface. $\endgroup$
    – Andrew Steane
    Jan 15, 2022 at 9:50
  • $\begingroup$ Related : A generalization of the Biot-Savart law for a number n of wires with n≥3. $\endgroup$
    – Frobenius
    Jan 15, 2022 at 10:59

3 Answers 3

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They form circular in shape because it tells us that the magnetic field is constant in this circular path.

Since according to the Biot-Savart law the magnetic field strenght is inversely proportional to the square of distance and we have to define the same magnetic strength at a different point and that is only possible if the magnetic field lines are circular that means the magnetic field is constant but on the different point in the space. just in case if it was not constant then it will be formed in the oval shape or any other uneven shape as you said in the case of the bar magnet. otherwise magnetic field is every where in the space and the magnetic field lines only tells us in which direction the magnetic fields are going to be made at some point.

That's why they are circular in the shape.

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Let's assume that the wire here is very long and very straight. There is a nice argument based on symmetry which suggests that the magnetic field around the wire have to be concentric circles.

Let us suppose that the magnetic field were not concentric circles, but concentric squares or "off centre" circles. Now, we rotate the wire along its axis by a few degrees. Since the wire rotates, the orientation of the magnetic field in space should also change, right?

But, we can also argue that the magnetic field should not change, because the appearance of the wire has not changed. Two perfectly logical and valid lines of reasoning have produced contradictory results!

The only way to resolve this contradiction is to postulate that the magnetic field around the long, straight wire form concentric circles.

The bar magnet does not have the rotational symmetry of a long, straight wire, so the above reasoning will not apply to it.

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The field of the bar magnet is essentially the same of a current-carrying wire with a circle shape. A straight wire give a field of concentric circles. If you transform the straight line into a circle, a ring-wire, the concentric circles transform in the bar magnet pattern.

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