If we're thinking about a case where bar magnet is placed on a flat surface, with its N-pole on the right and a compass is placed just above the middle of the magnet, in which direction will the compass needle point? Will it go in circles? Here's a diagram: My image

  • 3
    $\begingroup$ Ah yes, Maxwell's (comp)ass! $\endgroup$ Jun 16 at 13:47
  • $\begingroup$ Will it go round in circles? Will it fly high like a bird up in the sky? $\endgroup$
    – hobbs
    Jun 17 at 2:16
  • 2
    $\begingroup$ Don't think of the compass needle as "pointing" toward or away from any particular point. What it really does is, it aligns itself parallel to the ambient magnetic field lines, and magnetic field lines always are closed loops. Google for "bar magnet field lines" to see pictures of the field lines near a bar magnet. If you put a compass anywhere within the picture, the needle will try to line up parallel to those lines. $\endgroup$ Jun 17 at 12:22

Let's find out!

Here, I have a compass. The red end points to the Earth's North Pole, so that is the north side of the magnet by definition. Ignore the dial, since I've it rotated it to make the needle easier to see.

A magnetic compass. The red, north-pointing end currently points to teh left. The white, south-pointing end points to the right. The end of a magnet that seeks Earth's North Pole is defined as the north pole of the magnet.

Now, I don't have a bar magnet, but I do have a bunch of neodymium disc magnets that I can stack into a bar. I have written an "S" on one end of the magnet stack to indicate it is the south pole. We can tell this because the north pole of the compass is attracted towards it. (I've used tape to create a writing surface and to stop the magnets from rolling away. The magnets are very well stuck together.)

A stack of disc magnets (simulating a bar magnet) is placed near the compass. The visible end of the magnet stack is labeled "S" for south pole. The north pole of the compass points towards the magnets.

Just to confirm that we understand how both of these magnets work, if I flip the magnet stack, the compass needle flips.

A stack of disc magnets (simulating a bar magnet) is placed near the compass but rotated 180 degrees compared with the last picture. The visible end of the magnet stack is labeled "N" for north pole. The south pole of the compass points towards the magnets.

In preparation for placing the compass on top of the magnet, I'll show how the two will be arranged in the picture below. The north pole of the magnet stack will be pointing to the left. Notice which way the compass points.

The compass and magnet laying side by side--compass on the left, magnet stack on the right. The north end of the magnet stack points to the left. The south end of the compass points towards the stack (to the right).

Now, I'll pick up the compass and move it directly over the magnet.

The compass is held by hand directly above the magnet stack. The center of the compass needle is directly above the center of the magnet stack. The needle has reversed direction.

My fingers are visible holding the compass in place because the magnetic force between the two would cause one or the other to shift to the side. Notice that the compass needle has flipped to pointing the opposite direction from when it was to the side of the magnet.

In case you meant "above" in a different direction, here's a slightly different arrangement with the same result. The north pole of the magnet stack is still pointing to the left, which causes the north pole of the compass needle to point to the right. The picture is slightly tilted so the "N" on the magnet stack is visible.

The compass and magnet stack are both on the same surface. The compass needle is farther away from the camera than the magnet stack. The north end of the magnet stack points to the left. The north end of the compass needle points to the right.

The way magnetic field arrows are drawn is that they point away from north poles and towards south poles. With this convention, you can see in the previous pictures that the north end of the compass wants to align with the local magnetic field direction. What does the last picture tell you about how the magnetic field points near the middle of a bar magnet?

So, apparently, some commenters think I need to demonstrate that opposite poles of a magnet attract without assuming it. It's like people here don't trust me.

Since we're defining the north end of a magnet as one that seeks Earth's North Pole, we need to find where north is.

Compass aligned with North. Arrow is drawn to mark the northerly direction.

Now, in order to let the stack of magnets find its own north, I'll float it in a bowl of water (laboratory? Hah! I've got a kitchen!).

A bowl of water and a taped-up stack of magnets sitting on a plastic lid. All of this is on top of a paper with an arrow indicating north.

The magnets' boat is a plastic lid from a breadcrumb container.

Now, we float the magnetic boat in the bowl of water.

An animated gif showing the magnet rotating so that one end faces north. It oscillates a few times before coming to rest parallel with the drawn arrow pointing north.

We now label the ends of the magnet to follow the rule that the north end of a magnet points to the north.

The magnet stack floating in the water with the near side labeled with an "S" for south and the north pointing arrow pointing away from the camera.

Finally, we check that the new labels on the magnet match the labels used in the first part of this answer.

The labeled magnet stack is placed next to the compass showing that the north end of the magnet attracts the south end of the compass. The north-pointing arrow is visible to show that the magnet is pulling the magnet away from pointing to Earth's north pole.

As you can see, this arrangement is the same as the third picture in this answer, meaning the magnet was correctly labeled all this time. This also means that the Earth actually does have a magnetic south pole near the geographic North Pole.

  • 2
    $\begingroup$ I think the axis along which you place the compass in the final picture is not what the OP means. According to his question , in the final picture you must take the compass and put it flat against the paper, such that the bottom of the compass device i.e. the part reading ?type 7 nl" would be parallel to the axis of the magnets. Though i think the answer will still be same $\endgroup$ Jun 16 at 6:06
  • 1
    $\begingroup$ @silverrahul You might be right. I've added another picture. $\endgroup$
    – Mark H
    Jun 16 at 6:25
  • 11
    $\begingroup$ I love how this clearly illustrates that the Earth's north magnetic pole, is a "south" pole. $\endgroup$
    – PcMan
    Jun 16 at 12:05
  • $\begingroup$ Just to add more work: you should demonstrate that the side of the Nd magnet labeled "N" in fact points north by floating the magnet, and taking a picture of it's orientatation relative to the room. (and doing the same shot of the compass when the Nd magnet is far away) ;) $\endgroup$
    – Dave
    Jun 16 at 20:53
  • 1
    $\begingroup$ @Dave There! Happy? :) $\endgroup$
    – Mark H
    Jun 17 at 3:23

Mark H's answer is good, but I just want to add some clarity to what a compass does and what Earth's north and south mean, magnetically, and it's easier to do that with a picture, hence using an answer rather than a comment.

A compass in Earth's magnetic field points North, and a compass points in the direction of the magnetic field, which goes in arcs from the magnetic north pole to the magnetic south pole.

Presumably somewhere early in the history of physics before field lines were invented somebody got confused. Earth's magnetic field is such that the magnetic field lines, which by convention follow arcs from the north pole to the south pole, point north. That is to say, if Earth was a bar magnet, the south pole would be under the geogrpahic north pole, and the north pole would be under the geographic south pole.

enter image description here

  • 11
    $\begingroup$ You're right, but I wouldn't call the north-south quirk "confused." People were using magnets for a thousand years for navigation before Michael Faraday came up with the idea of "lines of force" (what we today call vector fields). For those thousand years, the rule was simply "the north end of the compass needle points north." I think continuity with the historical experience with magnets is worth the quirk of Earth's North Pole being a magnetic south pole. $\endgroup$
    – Mark H
    Jun 16 at 7:02
  • 1
    $\begingroup$ The North Pole having a south pole is an inevitable consequence of the north end of a compass magnet pointing North, unlike the negative charge on an electron which was arbitrary. $\endgroup$
    – Henry
    Jun 16 at 12:57

The compass will not spin in circles as it would violate conservation of energy. With no magnetic flux the compass might oscillate some at first, but would settle to point directly parallel with the bar magnet. When the compass is directly in the middle of the bar magnet, the North and South poles of the compass needle will be equally attracted to the South and North poles of the magnet. Earth's North pole is actually its magnetic south, so compasses are made to point to magnetic South. So the compass needle would be point towards the South pole of the bar magnet, not the North pole of the bar magnet as you might think.

  • $\begingroup$ Hi Adrian! Would you then consider Allure's answer to this question wrong? Would the needle point to the left? $\endgroup$
    – frankio10
    Jun 16 at 4:06
  • $\begingroup$ The compass would be equally attracted at each end so it would point parallel with the bar, not angled, unless it were off center. The compass arrow will point to the bar's South pole. See; en.wikipedia.org/wiki/North_magnetic_pole#Polarity $\endgroup$ Jun 16 at 4:06
  • $\begingroup$ @AdrianHoward you'd need an infinitesimally small compass for it to be parallel. For the size in the diagram I think the angle is appropriate. $\endgroup$
    – Allure
    Jun 16 at 4:10
  • $\begingroup$ @Allure I would not say you are wrong, parallel alignment would require exact centering of the magnetic fields which is an unstable condition in real situations. $\endgroup$ Jun 16 at 5:12
  • $\begingroup$ @AdrianHoward I convinced myself I was wrong, actually. The N point of the compass would be pulled one way, and the S point would be pulled the other way. If we assume the compass is exactly at the middle of the magnet, the two forces would be equally large, and the compass would still point parallel to the bar magnet. Hence I deleted my answer. $\endgroup$
    – Allure
    Jun 17 at 2:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.