Why do magnetic fields form closed loops?

[Padma]

I am sorry to be bothering you - I am a high school student and I need help with something I don't understand. It is understandable if we think of why magnetic fields form closed loops if we think of the North and the South Poles not being separate from each other. This has been given as a proof for the nonexsistence of monopoles but for things like the bar magnet where the 2 poles are separated by some distance why should they still form closed loops? I would be really gald if you could answer my question.

 

[topsquark]

I am sorry to be bothering you - I am a high school student and I need help with something I don't understand. It is understandable if we think of why magnetic fields form closed loops if we think of the North and the South Poles not being separate from each other. This has been given as a proof for the nonexsistence of monopoles but for things like the bar magnet where the 2 poles are separated by some distance why should they still form closed loops? I would be really gald if you could answer my question.

Field lines can be thought of as the path traced by a small "charge" placed in the field. So consider a bar magnet and a small (hypothetical) North pole monopole. Start this near the North pole of the bar magnet. What path is this little monopole going to trace? It will be repulsed by the North pole and attracted to the South pole of the bar magnet. So this little monopole will trace out some curve connecting N and S. And it will do so for (almost) any placement of our little monopole. This gives the magnetic field as curving from N to S.

There IS one exception: If we place our monopole directly above the N pole and directly in line with the S pole (ie. on the axis) the monopole will shoot away from the N pole and not curve around. There is a similar line near the S pole where the monopole goes straight into the S pole. These are the only field lines that don't curve.

-Dan

 

[Meir Achuz]

The lines of B are continuous loops. Inside a bar magnet B goes from the S end to the N end. It continues through the N end where you see it coming out.

 

[jmc8197]

I am sorry to be bothering you - I am a high school student and I need help with something I don't understand. It is understandable if we think of why magnetic fields form closed loops if we think of the North and the South Poles not being separate from each other. This has been given as a proof for the nonexsistence of monopoles but for things like the bar magnet where the 2 poles are separated by some distance why should they still form closed loops? I would be really gald if you could answer my question.

All magnetic fields are created by moving charges, whether they are moving in straight lines or orbits within an atom, as in your bar magnet. Secondly, a moving charge within a magnetic field experiences a magnetic force, such that moving charges will tend to attract one another if they are moving in the same direction; like currents attract, unlike repel. Each current loop will appear a North pole when viewed from one end, but a South when viewed from the other.

Think of your bar magnet as consisting of tiny current loops that orient themselves to line up with one another, so that you end up with a North at one end, a South at the other.

 

[Meir Achuz]

All magnetic fields are created by moving charges, whether they are moving in straight lines or orbits within an atom, as in your bar magnet. Secondly, a moving charge within a magnetic field experiences a magnetic force, such that moving charges will tend to attract one another if they are moving in the same direction; like currents attract, unlike repel. Each current loop will appear a North pole when viewed from one end, but a South when viewed from the other.

Think of your bar magnet as consisting of tiny current loops that orient themselves to line up with one another, so that you end up with a North at one end, a South at the other.

That is a good classical explanation, originally presented by Ampere,
and is useful for visualizing thje B field in and out of the magnet.
It is often used in elementary texts.
But, it is not what really happens in a bar magnet. The strong magnetization in a ferromagnet arises from the cooperative allignment of the intrinsic magnetic moments of the valence electrons in iron, and is unrelated to current loops.