Expert: Daniel Mazur - 11/18/2007

Question
QUESTION: Hi:

Southern magnetic energy is emitted from the side of a magnet taking in the lines of force. Northern magnetic energy is emitted from the side of the magnet giving out the lines of force. This is analogous to the fact that negative electrodes give out electrons and positive electrodes take in electrons in a circuit. The current flows from negative to positive because electrons are negatively-charged. Similarly, the south side of a magnet is positive while the north side is negative.

Look at the visualizations in the following links [look at the arrows on the magnetic field lines] :

http://theory.uwinnipeg.ca/mod_tech/node92.html

http://www.tpub.com/neets/book1/chapter1/1i.htm

As you can see the magnetic lines depict the magnetic force being carried from north to south.

The Earth's "South" pole is really the North, while the "North" pole is actually the South because of this reason.

Quotes from http://en.wikipedia.org/wiki/Magnetic_South_Pole :

"The Earth's South Magnetic Pole is physically actually a magnetic north pole"

Quotes from http://en.wikipedia.org/wiki/Magnetic_North_Pole :

"Conversely, the Earth's South Magnetic Pole is physically a magnetic north pole"

Quotes from http://van.physics.uiuc.edu/qa/listing.php?id=404 :

"The magnetic field lines go out of the magnetic North pole and head towards the South pole"

See what I mean?

Electrons are negatively charged so they move away from negative electrodes to positive electrodes. Similarly, "magnetons" [the particles carrying magnetic energy] are "northly-charged", hence they move away from north-side of the magnet towards to south-side.

So,

Positive charge in electricity = Southern "charge" in magnetism

Negative charge in electricity = Northern "charge" in magnetism

It's an analogy.

From what you know, why are the magnetic lines always depicted with their arrows pointing to the south from the north? The arrows make it seem like the force starts at north and moves to the south, much like an electric current [made of electrons -- negatively charged] moves from a negative electrode to a positive electrode.


Thanks,

Green

ANSWER: Hello Mr.Green,

there are two parts to your question as I perceive it:
1) "why are the magnetic lines always depicted with their arrows pointing to the south from the north?"
Answer: No, they are not *always* depicted so. In the references you gave me, the field lines all are depicted with arrows from magnetic North to magnetic South. If you found a picture of Earth, where magnetic field lines are depicted with arrows going from *geographical North* to *geographical South*, then it is an error on the side of the Authors/Editors/Publishers. It is an easy error to make, so it may happen often, but it is still an error.

2) "Positive charge in electricity = Southern "charge" in magnetism
Negative charge in electricity = Northern "charge" in magnetism
It's an analogy."
Answer: I know you didn't ask this, but I feel obliged to tell you that your mention of analogy between electricity and magnetism is wrong. Very, very wrong, I am sorry to say. If you had teachers, who told you there's an analogy working the way you described it, they were very bad teachers. Please allow me to comment on the most strikingly erroneous notions you made, I am doing it in hope that it will help you in the long run.

* Electricity has monopoles, positive and negative charge, that can be easily separated. Magnetism does not have such monopoles, there are no "magnetons" you speak of – this is a scientific fact as the cumulative experimental evidence collected over centuries tells us so.  When you split a magnet down the middle, you don't end up with separate North and South poles, you will have created two new magnets, each with it's owns north and south poles. This is a simple observation, anyone can test it.

* There is not energy flowing just like that between an isolated positive charge and negative charge ("isolated" is the crucial word). Similarly, there is no energy being transferred between the poles of a magnet. Believe it or not, there isn't any energy flow, there is just some static *potential* energy associated with the existing, unchanging fields. If you build a conductive channel between electrically charged elements, then electrical current will flow. The potential energy will drop and the released of free energy can be used as work (mechanical or other). You can also release the field energy of a permanent magnet – if you break it into tiny little magnets and allow the latter to move, they will rearrange so that the “big” field will vanish and there will be only some tiny local variation of the total field of the small magnets. It is not a very useful process, however, so it is not being used to do work.

* The field lines of an electric field are in the direction of the force exerted on a test electric charge. The magnetic field lines are not! Magnetic field affects carriers of electrical (!) charge in motion and the FORCE exerted is always perpendicular to both the FIELD lines and the vector of the VELOCITY of the carrier (see http://en.wikipedia.org/wiki/Lorentz_force).

To sum up, magnetism is NOT an analogy of electricity. If you try to understand magnetism based on knowledge of electrostatics ASSUMING there is an analogy, it will only confuse you and you can never make it. Electromagnetism is one subject, which is only artificially being divided into "electrostatics" and "electrodynamics and magnetism". From your question I understand that you know the electrostatics part and you are trying to learn the rest by analogy. There is no analogy, the parts are *complementary*, they are "symmetrical" in some sense, but you need first to seriously study the laws governing each part to be able to see, which bits can be treated as alike, and which bits are different.

Electromagnetism is a time- and energy- consuming topic, albeit very well understood by physicists nowadays. I cannot substitute for a semester-long course here on these pages to explain properly. To do that, you must get a textbook or a personal tutor or both. I hope my answer gives you a good motivation to do that.

Good luck!
Daniel


---------- FOLLOW-UP ----------

QUESTION: Thanks for your response.

I was already aware that magnetic monopoles don't exist and that if you chop a big magnet into smaller magnet pieces, you will still get only dipoles -- no monopoles at all.

I was just wondering why magnets were depicted as with arrows pointing from the north to the south. To me it looked  similar to an electric path consisting of electrons flowing from the negative to positive.

I also already knew that the geographical "north" is really the "south" and that the geographical "south" is really the "north".


Thanks,

Green

Answer
O.K. I misunderstood you then, I apologize.
The arrows on the field lines are a matter of convention - once people decided to do it one way and that's it. Something similar did happen to electric current, which in metals is carried by negative carriers - electrons - and still we make the arrow in the direction of flow of some unreal positively charged particles. All these things are depending on convention. There may be stories behind them as to why the original scientist suggested things one way and not the other, but I do not know much about them.
Take care.
Daniel