Who defined the positive direction of the magnetic field?

In summary: It's neither work nor formal study, so it isn't urgent. But I plan to write about science for lay people in the future, so it'd be nice to know.
  • #1
Jairo Amaral
8
1
This is about the history of electromagnetism. I already know about the convention that defined which electric charge is the "positive" one, which ended up making the electron a negative particle.

But what about the "positive" direction of the magnetic field? Does anybody here know
how it was arbitrated, and who did it? Specifically, did it come before the equation relating it to electric charges? Or was the direction defined by the equation itself?

Thank you in advance!
 
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  • #2
This doesn't answer your question, and I don't have a reference
for it, but it is an interesting part of the story: The "north" end of a
magnetic compass was defined as the end of the needle pointing
toward Earth's north pole. When the mechanism of magnetism
was puzzled out, it was realized that opposites attract, so the
end of a compass needle marked "north" is actually what is
called "south" on all other magnets.

-- Jeff, in Minneapolis
 
  • #3
Jeff Root said:
... the end of a compass needle marked "north" is actually what is called "south" on all other magnets.
No. The compass needle is marked just like other magnets. The Earth's magnetic poles are marked opposite to the usual convention.
https://en.wikipedia.org/wiki/North_Magnetic_Pole#Polarity
 
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  • #4
A.T. said:
The compass needle is marked just like other magnets.
This can be tested by hanging a bar magnet on a string, balancing it at its center of mass so it rotates freely in a horizontal plane. It should end up with the "N" end pointing north.
 
  • #5
Jairo Amaral said:
But what about the "positive" direction of the magnetic field?
Say you know mass m and charge of the particle q and throw it into magnetic field with velocity ##\vec{v}##. Motion of the particle will be decided by force [tex]\vec{F}=q\vec{v}\times\vec{B}[/tex]. Observing the motion, you can decide direction and magnitude of B. This is one of ways to let you know the conventional direction of magnetic field.
 
  • #6
sweet springs said:
Say you know mass m and charge of the particle q and throw it into magnetic field with velocity ##\vec{v}##. Motion of the particle will be decided by force [tex]\vec{F}=q\vec{v}\times\vec{B}[/tex]. Observing the motion, you can decide direction and magnitude of B. This is one of ways to let you know the conventional direction of magnetic field.
Hi! This is the convention. I'm asking how it appeared.

Was it like "I arbitrate the positive direction is that way, and now will derive an equation to agree with it", or was it like "I arbitrate the equation is thus, and now we'll use it to find the directions"?
 
  • #7
Jairo Amaral said:
Was it like "I arbitrate the positive direction is that way, and now will derive an equation to agree with it", or was it like "I arbitrate the equation is thus, and now we'll use it to find the directions"?
The equation gives us convention. You may use F= - qvXB if it is shared with all the other people. Why plus not minus sign is a matter of science history that I am not good at.
 
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  • #8
Jairo Amaral said:
This is about the history of electromagnetism. I already know about the convention that defined which electric charge is the "positive" one, which ended up making the electron a negative particle.
Please relate who you think defined the electrical polarity convention. In USA we learn it was Benjamin Franklin but I have read that other countries make different historical (and ideological) claims.
Jairo Amaral said:
But what about the "positive" direction of the magnetic field? Does anybody here know
how it was arbitrated, and who did it? Specifically, did it come before the equation relating it to electric charges? Or was the direction defined by the equation itself?...
My understanding, probably from reading Isaac Asimov and other science historians, is that J. F. C. Gauss and W. Weber developed, or at least confirmed, polarity conventions while inventing electrodynamics.

Franklin and Gauss were both such prolific inventors in so many areas, historians may give them precedence from familiarity. I can look for definitive sources specifically for magnetic polarity, if this is important.
 
  • #9
Klystron said:
Please relate who you think defined the electrical polarity convention. In USA we learn it was Benjamin Franklin but I have read that other countries make different historical (and ideological) claims.

I read it was both Benjamin Franklin and William Watson. I didn't know there was more than one version.

Klystron said:
Franklin and Gauss were both such prolific inventors in so many areas, historians may give them precedence from familiarity. I can look for definitive sources specifically for magnetic polarity, if this is important.

It's neither work nor formal study, so it isn't urgent. But I plan to write about science for lay people in the future, so it'd be nice to know. It it's too much trouble for you to get the exact answer, could you indicate me books that might have this? I'd love to read them anyway.
 
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  • #10
sweet springs said:
The equation gives us convention. You may use F= - qvXB if it is shared with all the other people. Why plus not minus sign is a matter of science history that I am not good at.

So we'll all discover that, and we'll give it enough publicity so it isn't forgotten again.
 
  • #11
Jairo Amaral said:
Hi! This is the convention. I'm asking how it appeared.

Was it like "I arbitrate the positive direction is that way, and now will derive an equation to agree with it", or was it like "I arbitrate the equation is thus, and now we'll use it to find the directions"?

There is no positive direction involved here. The direction a compass needle points, by definition, is called north.

Who came up with that naming convention? My guess would be Sir William Gilbert. You should be able to verify that with a google search.
 
  • #12
Mister T said:
There is no positive direction involved here. The direction a compass needle points, by definition, is called north.

Even though it started with just north and south, at some point a positive direction was established, since we're using it right now. And the positive direction of the magnetic field points to the south pole of the magnet. I'm trying to find out how it all started.
 
  • #13
Frequent geomagnetic reversal is observed in geological history. Some thousand or millions year past or future our current convention N would point out the south and S would point out the north. Further confusion is that compass painted N is polar S, painted S is polar N :wink:

Electric dipole qd is usually modeled as plus charge at z=+d/2 and minus charge at z=-d/2
Similarly magnetic dipole ##q_m d## is composed of plus magnetic charge at z=+d/2 and minus magnetic charge z=-d/2. The magnetic dipole corresponds to the Earth. z=d/2 corresponds North pole and z=-d/2 corresponds South pole.
 
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  • #14
sweet springs said:
... compass painted N is polar S...
No, it's not. See post #3.
 
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  • #15
Jairo Amaral said:
Even though it started with just north and south, at some point a positive direction was established, since we're using it right now.

What do you mean by your claim that you're using it right now? Can you give us some context?
 
  • #16
jtbell said:
This can be tested by hanging a bar magnet on a string, balancing it at its center of mass so it rotates freely in a horizontal plane. It should end up with the "N" end pointing north.

Likewise, one can place a bar magnet on top of a slab of styrofoam or the like, and float it on the surface of a tub of water. The end labelled "N" will be attracted to the ends of other bar magnets labelled "S". There is a south magnetic pole near Earth's geographic north pole.
 
  • #17
Mister T said:
What do you mean by your claim that you're using it right now? Can you give us some context?

In the equations for electromagnetism, the magnetic fields have positive and negative directions, not north and south directions.
 
  • #18
Once you've chosen a convention for which is the positive charge and which is the negative, it's almost reflexive to choose the convention for the magnetic field direction such that the force is ##q\vec{v}\times\vec{B}## instead of ##-q\vec{v}\times\vec{B}## - why introduce a gratuitous minus sign?

But note that there's yet another arbitrary convention here - the right-hand rule for the cross-product.
 
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  • #19
sweet springs said:
The equation gives us convention. You may use F= - qvXB if it is shared with all the other people. Why plus not minus sign is a matter of science history that I am not good at.
It is not plus versus minus. That would be too easy. It is ##qv \times B## versus ##B \times qv##. The physics does not dictate the order in which we place terms in a formula. [Which is to restate @Nugatory's point in different words]
 
  • #20
That's what I'm talking about. If the direction of the magnetic field was defined first, then it's just a matter of placing the cross product in the convenient order.
 
  • #21
A.T. said:
No, it's not. See post #3.
Oh! Thanks. I will restate that
North pole has minus magnetic charge and South pole has plus magnetic charge.
N pole:-, S pole:+.
Needle N has +magnetic charge and needle S has - magnetic charge.
N needle:+, S needle:-
N,S sayings of magnetic pole and print on compass needle is reverse in magnetic charge they have.

N pole:-, S pole:+. I feel here again discrepancy between the the first definition and later discovery as well as direction of electric current and later found inverse movement of minus charged electrons in wire.
 
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  • #22
Jairo Amaral said:
In the equations for electromagnetism, the magnetic fields have positive and negative directions, not north and south directions.

Are you sure of the equivalence? For example, north is defined as the direction in which the north end of a compass points. Is there an equivalent definition of positive? That is, is positive defined as the direction in which the north end of a compass points?
 
  • #23
sweet springs said:
I will restate that
North pole has minus magnetic charge and South pole has plus magnetic charge.
N pole:-, S pole:+.
Needle N has +magnetic charge and needle S has - magnetic charge.
N needle:+, S needle:-
N,S sayings of magnetic pole and print on compass needle is reverse in magnetic charge they have.
All you have to remember is that the compass needle is marked just like any other magnet.
 
  • #24
A.T. said:
All you have to remember is that the compass needle is marked just like any other magnet.
May I add that except Earth magnet or geomagnet ? North pole has polarity of compass needle S.
 
  • #25
sweet springs said:
May I add that except Earth magnet or geomagnet ? North pole has polarity of compass needle S.
Yeah, or you remember it as:

"The compass needle is marked just like other man-made magnets are usually marked."

The polarity of the Earth follows from that.
 
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  • #26
Thanks. So I summarize how to decide the direction of magnetic field as follows.
The compass needle end directing North has positive magnetic charge.
The compass needle end directing South has same amount but negative magnetic charge.
Exactly same as in electric dipole, these positive and negative magnetic charge generate magnetic dipole field starting from plus charge to minus charge outside the needle. Direction of magnetic field is thus related with geomagnetism. By this convention Earth magnetic field emerge from South pole , travel through outside the solid Earth and enter into North pole.

As for science history question "when, where, who made this convention?", I am not good at all. I would feel happy if some historian teach us.
 
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  • #27
sweet springs said:
Thanks. So I summarize how to decide the direction of magnetic field as follows.
The compass needle end directing North has positive magnetic charge.
The compass needle end directing South has same amount but negative magnetic charge.
Exactly same as in electric dipole, these positive and negative magnetic charge generate magnetic dipole field starting from plus charge to minus charge outside the needle. Direction of magnetic field is thus related with geomagnetism. By this convention Earth magnetic field emerge from South pole , travel through outside the solid Earth and enter into North pole.

As for science history question "when, where, who made this convention?", I am not good at all. I would feel happy if some historian teach us.
Except for the niggling problem that there is no such thing as a magnetic charge.
 
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  • #28
Yeah. I admit pair of magnetic charge that could generate magnetic field as well as electric charge generate electric field, is just a conventional thing to describe magnetic field outside the magnet.
 
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  • #29
@Jairo Amaral I (finally) remembered the early natural philosopher associated with magnetism:

"research of magnetic fields began in 1269 when French scholar Petrus Peregrinus de Maricourt mapped out the magnetic field on the surface of a spherical magnet using iron needles." excerpt from this History.

Peregrinus's improvements of the 13th C. astrolabe can be compared to 19th C. analogue computers. The biographical article contains many differences from what I learned at my first college; perhaps from new primary sources or translations. Hope this helps.
 
  • #30
Jeff Root said:
This doesn't answer your question, and I don't have a reference
for it, but it is an interesting part of the story: The "north" end of a
magnetic compass was defined as the end of the needle pointing
toward Earth's north pole. When the mechanism of magnetism
was puzzled out, it was realized that opposites attract, so the
end of a compass needle marked "north" is actually what is
called "south" on all other magnets.

-- Jeff, in Minneapolis
Wrong, I'm afraid! 'Magnetic North' is used to distinguish a location from the location of 'True North'. The term has NOTHING to do with its polarity. Since the north(seeking) pole of the compass is attracted towards that location, its polarity is south. To summarise, the POLARITY of the PLACE called Magnetic North is south.
 
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  • #31
This is irrelevant to your question but I can't help it. When geometric algebra is used for magnetic fields there are no arbitrary conventions. One gets a different sign in Maxwell's equations. I thought but never proved that a left hand rule would be the natural convention.
 
  • #32
Faraday was the first to study electromagnetism so the answer may lie there. He was however not a mathematical guy. I seem to recall that the most popular theories were by Hertz and maybe Gauss. At the time, quaternions were in vogue. Maxwell used quaternions and was found later to be correct. So maybe the convention came from Hamilton. Later Heaviside's cross product took over. My guess is that by then the convention was already established.

Can quaternions be defined with a different sign convention? Beats me.
 
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  • #33
Jeff Root said:
the
end of a compass needle marked "north" is actually what is
called "south" on all other magnets.
Take a bar magnet and place it on a wooden platform so you can float it in a tub of water. You will find that when free to rotate the north end of the bar magnet points in the same direction as the north end of a compass needle. A compass needle is just a bar magnet mounted in a such a way that it's free to rotate.
 
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  • #34
The magnetic field of the Earth can be modeled as a magnetized needle with its north magnetic pole (red) near the geographic South pole (see figure). Compass needles line up with the local field lines to minimize the potential energy. Thus, the north magnetic poles of the compass needles on or near the Earth's surface will point in the general direction of the Earth's geographic North.

Earth's Field.png
 
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  • #35
It's drawn correctly but, from a physics point of view, it's incorrectly named in the above diagram: the pole in the north of the Earth is the magnetic south pole and vice versa. It's of course the usual terminology to call the south pole of the Earth (seen as a magnet) the "northern magnetic pole". That always leads to the confusion and the discussion we have right here.

The following picture from Wikipedia is even more explicit:

https://upload.wikimedia.org/wikipedia/commons/2/2b/Geomagnetisme.svg

It's much clearer, not to mention the Earth and compass needles first but just use magneto statics a la Ampere and define everything in terms of vectors (magnetic field, ##\vec{B}##, and, if you want to deal with permanent magnets, magnetization).
 
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<h2>1. What is the positive direction of the magnetic field?</h2><p>The positive direction of the magnetic field is the direction in which a north magnetic pole would move when placed in the field. It is conventionally represented by an arrow pointing towards the north pole of a magnet.</p><h2>2. Who defined the positive direction of the magnetic field?</h2><p>The positive direction of the magnetic field was first defined by French physicist and mathematician André-Marie Ampère in the early 19th century. He established the convention of using the right-hand rule to determine the direction of the magnetic field around a current-carrying wire.</p><h2>3. Why is the positive direction of the magnetic field important?</h2><p>The positive direction of the magnetic field is important because it allows us to establish a consistent and universal convention for describing the direction of magnetic forces and fields. This convention is used in various fields of science and engineering, such as electromagnetism and magnetohydrodynamics.</p><h2>4. How is the positive direction of the magnetic field related to the Earth's magnetic field?</h2><p>The Earth's magnetic field is aligned with the positive direction of the magnetic field, with the north magnetic pole located near the geographic South Pole and the south magnetic pole near the geographic North Pole. This means that the Earth's magnetic field is directed from south to north, which is opposite to the direction of the magnetic field of a bar magnet.</p><h2>5. Can the positive direction of the magnetic field be reversed?</h2><p>Yes, the positive direction of the magnetic field can be reversed by reversing the direction of the current flow or by flipping the orientation of a magnet. This is known as magnetic reversal and has been observed in the Earth's history, where the north and south magnetic poles have switched positions multiple times.</p>

1. What is the positive direction of the magnetic field?

The positive direction of the magnetic field is the direction in which a north magnetic pole would move when placed in the field. It is conventionally represented by an arrow pointing towards the north pole of a magnet.

2. Who defined the positive direction of the magnetic field?

The positive direction of the magnetic field was first defined by French physicist and mathematician André-Marie Ampère in the early 19th century. He established the convention of using the right-hand rule to determine the direction of the magnetic field around a current-carrying wire.

3. Why is the positive direction of the magnetic field important?

The positive direction of the magnetic field is important because it allows us to establish a consistent and universal convention for describing the direction of magnetic forces and fields. This convention is used in various fields of science and engineering, such as electromagnetism and magnetohydrodynamics.

4. How is the positive direction of the magnetic field related to the Earth's magnetic field?

The Earth's magnetic field is aligned with the positive direction of the magnetic field, with the north magnetic pole located near the geographic South Pole and the south magnetic pole near the geographic North Pole. This means that the Earth's magnetic field is directed from south to north, which is opposite to the direction of the magnetic field of a bar magnet.

5. Can the positive direction of the magnetic field be reversed?

Yes, the positive direction of the magnetic field can be reversed by reversing the direction of the current flow or by flipping the orientation of a magnet. This is known as magnetic reversal and has been observed in the Earth's history, where the north and south magnetic poles have switched positions multiple times.

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