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In a solenoid, the field lines within are relatively constant and parallel, and so, the magnetic field is strongest as compared to the poles, at which the field lines diverge. In a permanent magnet, however, the magnetic field is strongest at the poles, despite being the point of divergence of the field lines. Why?

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2 Answers 2

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When we talk about a magnet's field being strongest at the poles, we're comparing the strengths of field at points outside the magnet. If we similarly restrict ourselves to points outside a solenoid, then the field is strongest at its ends (where the field lines have hardly started to diverge). We must compare like with like!

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  • $\begingroup$ Yes, I think that if you made a thin saw cut part way through the middle of a bar magnet and measured the field in the saw cut, the field would be stronger than at the ends. $\endgroup$ Dec 10, 2018 at 4:24
  • $\begingroup$ Yes indeed. But we don't normally consider this when talking about where a magnet's field is strongest, whereas we might well consider the field inside a solenoid because it is easily accessible. $\endgroup$ Dec 10, 2018 at 9:30
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The magnetic field around a solenoid depends on core, turns and current.

For a magnet, the flux lines repel each other so the field will be weaker at the sides. But they are concentrated at the poles, where they originate, so the field is stronger.

The field around a magnet:

Magnetic field

The field around an air wrapped solenoid:

Solenoid Magnetic field

Air is a poor conductor of flux. To get a similar magnetic field from the solenoid, we have to add a iron core.

The field around an iron core solenoid:

Iron core Solenoid Magnetic field

Iron is a conductor of flux, so we can use it to control the magnetic field

Horseshoe magnet

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