Determining the north and south pole of a solenoid

[ellieee]

Homework Statement

(refer to pic) if I place a compass on top of a solenoid, and if the arrow on the compass deflects from pointing towards north pole to pointing towards the west pole, then does it mean that the magnetic field lines are moving in the anti clockwise direction and A is north pole while B is south pole?

Relevant Equations

nil

 

[Delta2]

Not sure what you mean, as there are no west poles in magnets, only north and south poles, but yes if the arrow of the compass is pointing towards A (or W as in your drawing) then the North pole of solenoid is at A.

 

[ellieee]

there are no west poles in magnets

right, I meant like the label "W" On the compass, what do u call that though?

 

[Delta2]

Not sure what type of compass you talk about, in the simple compass I know the arrow is always pointing towards North.

 

[Delta2]

I was wrong btw, if the compass arrow is pointing towards A then A is the south pole of the solenoid.

 

[DaveE]

Check out the drawing below. A magnet that is free to move (like a compass needle) will align itself with the field lines of the solenoid (which, let's say, can't move) so that the arrows in the center of the magnet point in the same direction as the field lines from the solenoid (i.e. south is attracted to north). However, since the field from the solenoid is curved, it doesn't always mean the the magnet points to the north pole of the solenoid, especially near the south pole. A compass needle has a north pole at the pointy end so it will be attracted to the magnetic south pole of the earth, which we all call the "north pole". No, I can't explain why.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html

 

[Delta2]

A compass needle has a south pole at the pointy end so it will be attracted to the north pole.

I thought that it had a north pole. BTW isn't the geographical north pole of earth, the south magnetic pole?

 

[DaveE]

I thought that it had a north pole. BTW isn't the geographical north pole of earth, the south magnetic pole?

Yes, you're right. I was just fixing that as you posted!

 

[Steve4Physics]

Homework Statement:: (refer to pic) if I place a compass on top of a solenoid, and if the arrow on the compass deflects from pointing towards north pole to pointing towards the west pole, then does it mean that the magnetic field lines are moving in the anti clockwise direction and A is north pole while B is south pole?
Relevant Equations:: nil

View attachment 283450

@ellieee, your diagram doesn't show the compass-needle, which is the most important part!

By ‘on top a solenoid' do you mean flat against one end of a solenoid - so the needle's axis of rotation lies on the cylinder's axis? If so, the needle will point in a random direction. That’s because the field is symmetric about the axis; there is no preferred direction for the needle to point in its plane of rotation.

Note that outside a solenoid, the magnetic field is the same as a simple bar magnet. The direction of the needle at different compass positions is shown here:
https://docbrown.info/ephysics/ephyspics/electromag01.gif

Or even better, here:
https://javalab.org/wp-content/uploads/magnetic_field_around_a_bar_magnet.png

The markings (N, S, E, W) on the compass-dial aren’t relevant (unless you also plan to use the compass for direction finding when you go walking!).

 

[rsk]

I thought that it had a north pole. BTW isn't the geographical north pole of earth, the south magnetic pole?

I hate this fact.
Teaching magnetism to high school students is fine: induction, Faraday, Lenz.
Teaching it in middle school and this thing always comes up and it scrambles my brain every time.
North (seeking) pole. South (seeking) pole.

 

[Delta2]

I hate this fact.
Teaching magnetism to high school students is fine: induction, Faraday, Lenz.
Teaching it in middle school and this thing always comes up and it scrambles my brain every time.
North (seeking) pole. South (seeking) pole.

I completely understand you, things like these can be the source of great confusion, especially to young students.

 

[kuruman]

I hate this fact.
Teaching magnetism to high school students is fine: induction, Faraday, Lenz.
Teaching it in middle school and this thing always comes up and it scrambles my brain every time.
North (seeking) pole. South (seeking) pole.

I found this "seeking" clarification misleading because it is not necessarily correct. It is more correct to say that the magnetic needle will always align so that its south-North direction (i.e. its magnetic moment) points in the same direction as the local magnetic field. I stress the word "local".

Study the diagram below. It's a schematic of the Earth showing its magnetic field lines as if they were generated by a bar magnet. The arrows in various places indicate the direction of the local magnetic field and the dashed circle represents the spherical Earth. Imagine walking on the dashed circle holding a compass needle. It will align itself with the field lines so that in the northern hemisphere the north pole of the needle will be "seeking" the south pole of the Earth, that's true enough. At the Earth's magnetic north, the needle's north will point straight down to the Earth's magnetic south. However, near the magnetic north of the Earth, which is near the geographic south, the needle's north will point away from the Earth's magnetic south so it's more correct to say that in the southern hemisphere the needle's south is Earth north "seeking".

The explanation above was deliberately constructed to show you how confusing this "seeking" business could be. The best way to understand it is to have a clear picture of (a) what the magnetic field looks like in 3d space and (b) where you are in that 3d space. Then apply the principle that the needle will line itself with the local field and consider the orientation of the needle at different locations, something like what is shown in the hyperphysics link, posting #6, but with the Earth instead of a solenoid. Generalizations trying to explain which way the magnetic needle will "always" point are fraught with pitfalls. This is one of the situations where a picture is worth a thousand words.

 

[rsk]

I found this "seeking" clarification misleading because it is not necessarily correct. It is more correct to say that the magnetic needle will always align so that its south-North direction (i.e. its magnetic moment) points in the same direction as the local magnetic field. I stress the word "local".

Study the diagram below. It's a schematic of the Earth showing its magnetic field lines as if they were generated by a bar magnet. The arrows in various places indicate the direction of the local magnetic field and the dashed circle represents the spherical Earth. Imagine walking on the dashed circle holding a compass needle. It will align itself with the field lines so that in the northern hemisphere the north pole of the needle will be "seeking" the south pole of the Earth, that's true enough. At the Earth's magnetic north, the needle's north will point straight down to the Earth's magnetic south. However, near the magnetic north of the Earth, which is near the geographic south, the needle's north will point away from the Earth's magnetic south so it's more correct to say that in the southern hemisphere the needle's south is Earth north "seeking".
View attachment 283567
The explanation above was deliberately constructed to show you how confusing this "seeking" business could be. The best way to understand it is to have a clear picture of (a) what the magnetic field looks like in 3d space and (b) where you are in that 3d space. Then apply the principle that the needle will line itself with the local field and consider the orientation of the needle at different locations, something like what is shown in the hyperphysics link, posting #6, but with the Earth instead of a solenoid. Generalizations trying to explain which way the magnetic needle will "always" point are fraught with pitfalls. This is one of the situations where a picture is worth a thousand words.

You may be surprised to hear that we do use a diagram like this - and that's exactly what the children find odd, the fact that the Earth's south magnetic pole is where its geographic north pole is and vice versa.

The "seeking" was in quotes because it's often the suggested explanation but not necessarily helpful.

FWIW we usually teach this topic to 11/12 year olds.

Thank you anyway.

 

[kuruman]

I do not find it surprising that you use a diagram like this, it is the correct way (in my opinion) to go about it. If the 11½-year olds find this odd, then they need to understand that it actually isn't and remove any preconceptions that might be forming in their minds. It's a fine opportunity to make them aware of the idea of a "field", something that has size and direction in 3d space.

Appeal to their common sense by using a field that they understand intuitively, namely gravity. Draw an Earth and put a dot above its circumference in the northern hemisphere. Ask them which way a rock would move if released from that point. I bet 100% will get it right. Now draw a diametrically opposed point and ask the same question. Draw a third and fourth point and ask again. Then draw (representative) gravitational field lines and deduce the statement that "Objects released from rest fall along the lines of gravity."

Now do the same with the dipolar-like magnetic field of the Earth. Draw an Earth and the magnetic field lines. Be sure to orient the Earth and the magnetic lines "sideways", i.e. with the north-south diameter along the chalkboard's horizontal. This is important for decoupling what they find "odd". Assert the principle that magnetic needles line up in the same direction as the magnetic field. Then pick points all over and ask which way would compasses point if placed there. Again, I am sure that most if not all of them will get it right because there is no reference yet to where the geographic North is. Anyone who asks "which way is North?" has not been paying attention.

Here comes the clincher. Ask, "if I were to draw continents on this Earth, roughly where do you think I should draw the US? What about Canada and why?"^* Let them grapple with it but guide their discussion. After they all get it right, say "now imagine being in the US holding a compass needle. Which way would it point? Tell me how to draw it" Finally ask, "so where do you think the geographic North is? Does the magnetic needle in the US point to Canada or in the opposite direction?" That might do it.
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^* this assumes that your 11½-year olds understand that the US is closer to the Equator than Canada.