1. Order of Kilopi
Join Date
Dec 2004
Posts
11,219

If a light wave is moving forward, and the positive direction of
the electric field is up, is the positive direction of the magnetic
field to the right or the left? How can that be determined?

-- Jeff, in Minneapolis
Last edited by Jeff Root; 2008-Jun-29 at 08:19 AM. Reason: clearer wording

2. More questions - a causal search has failed to find an answer:

1. Is it not true that a magnetic field is not "positive" or "negative" but north or south?

2. Is there a hard connection between positive electricity and a north or south magnetism?

3. Or they a function of whatever polarity the wave has at the time?

4. If (3) is true, in a coherent EM source (i.e. a laser, for example), do all of the waves have the same relationship between the + and - and N and S? How does that happen?
Last edited by Vanamonde; 2008-Jun-29 at 12:43 PM. Reason: to make sense

3. Order of Kilopi
Join Date
Sep 2004
Posts
5,448
right hand rule

If k is forward and E is up then B is to the right.

4. Order of Kilopi
Join Date
Sep 2004
Posts
5,448
Originally Posted by Vanamonde
More questions - a causal search has failed to find an answer:

1. Is it not true that a magnetic field is not "positive" or "negative" but north or south?

2. Is there a hard connection between positive electricity and a north or south magnetism?

3. Or they a function of whatever polarity the wave has at the time?

4. If (3) is true, in a coherent EM source (i.e. a laser, for example), do all of the waves have the same relationship between the + and - and N and S? How does that happen?
1. Positive B is a north pole.

2. In classical radiation there is. Direction of propagation, electric field and magnetic field are all perpendicular to one another.

3. technically says the same thing.

4 they have the same relationship because that is the definition of coherent. If they did not all have the same relationship, then the waves would tend to interfere with one another.

5. If you're standing on the equator of a planet with a magnetic field, facing the direction of rotation (east for us), does magnetic north always have to be to your left and magnetic south always have to be to your right? Or can it possibly be the other way around for other planets?

6. Originally Posted by Delvo
If you're standing on the equator of a planet with a magnetic field, facing the direction of rotation (east for us), does magnetic north always have to be to your left and magnetic south always have to be to your right? Or can it possibly be the other way around for other planets?
It doesn't and indeed can be the other way around. And it even has for us, with the magnetic pole reversals that happen every so often on geological time scales. We're due for another "soon" on those time scales. The internal dynamo mechanism is a complex one. It takes rotation to drive it but the dipole component does not have to align with the rotational axis. Magnetic north is not aligned exactly with true north and further does wander around now.

They have complex models of that dynamo mechanism which display these pole flip events. During the flip, the magnetic field doesn't go to zero everywhere, it's just the *dipole component* of the total field goes to zero for a bit before it changes sign. The quadrapole and higher moments are still there.

Really which we call "north" and "south" is just an accident of which way the earth's field happened to be during modern times when EM was discovered and the theory worked out.

If the earth's field was flipped, the minus sign in Maxwell would just be on the opposite curl equation. And that could be remedied by changing the direction of E, which would then flip positive and negative. And that's how it would've been, I think, if the earth's B field were backwards. We would have defined positive current to be in the opposite direction which would've resulted in the + and - sense of the charges being reversed.

-Richard

7. Originally Posted by publius
It doesn't and indeed can be the other way around. And it even has for us, with the magnetic pole reversals that happen every so often on geological time scales.
Oh gewd gawd, I've known that for years. I got too stuck in abstractness and forgot that I was going back to something monumentally basic there. It's like reading about the latest automotive technology and suddenly suggesting that maybe our pack sledges would be easier to move with heavier payloads if we mounted them up on some sort of spinning round things...

8. Order of Kilopi
Join Date
Jul 2005
Posts
9,761
Originally Posted by publius
Really which we call "north" and "south" is just an accident of which way the earth's field happened to be during modern times when EM was discovered and the theory worked out.
And, of course, the magnetic pole near the Earth's north rotation pole is a south magnetic pole. Otherwise the north pole of a compass needle wouldn't point towards it.

Grant Hutchison

9. Originally Posted by grant hutchison
And, of course, the magnetic pole near the Earth's north rotation pole is a south magnetic pole. Otherwise the north pole of a compass needle wouldn't point towards it.

Grant Hutchison
Yes, that's right and is something that is easy to get mixed up. The north pole of a magnet was defined as the one that pointed northward, thus making the pole in the north a south magnetic pole. The earth's B field lines (roughly) come out of the geographical south magnetic pole and go back down at the geographical north magnetic pole.

-Richard

10. Order of Kilopi
Join Date
Sep 2004
Posts
5,448
Originally Posted by grant hutchison
And, of course, the magnetic pole near the Earth's north rotation pole is a south magnetic pole. Otherwise the north pole of a compass needle wouldn't point towards it.

Grant Hutchison
I love that one. It tends to blow their minds until they really think about it

11. Originally Posted by grant hutchison
And, of course, the magnetic pole near the Earth's north rotation pole is a south magnetic pole. Otherwise the north pole of a compass needle wouldn't point towards it.

Grant Hutchison
I bet that the north pole of my compass needle is a magnetic south pole
And I also bet that the electron´s charge is positive, and it´s not 1 but 3, which makes quarks´ charge 1, instead of 1/3, hurray!

12. Yes I must confess that's the one thing the engineers got right and the physicists got wrong-- the electron charge should be positive.

13. Originally Posted by publius
It doesn't and indeed can be the other way around. And it even has for us, with the magnetic pole reversals that happen every so often on geological time scales. We're due for another "soon" on those time scales. The internal dynamo mechanism is a complex one. It takes rotation to drive it but the dipole component does not have to align with the rotational axis. Magnetic north is not aligned exactly with true north and further does wander around now.

They have complex models of that dynamo mechanism which display these pole flip events. During the flip, the magnetic field doesn't go to zero everywhere, it's just the *dipole component* of the total field goes to zero for a bit before it changes sign. The quadrapole and higher moments are still there.

Really which we call "north" and "south" is just an accident of which way the earth's field happened to be during modern times when EM was discovered and the theory worked out.

If the earth's field was flipped, the minus sign in Maxwell would just be on the opposite curl equation. And that could be remedied by changing the direction of E, which would then flip positive and negative. And that's how it would've been, I think, if the earth's B field were backwards. We would have defined positive current to be in the opposite direction which would've resulted in the + and - sense of the charges being reversed.

-Richard
Well just to put this on the record, something I leaned from electronics classes.

Electricity flows from the - pole to the + pole. - basically means you have more electrons there, then at the + side of a circuit, and dictates the flow of current.

Interestingly enough though, most circuits are designed as if the + was the source and the - was the ground. The same circuit will work either way if a PNP transistor is replaced by it's NPN transistor counterpart, and vis-versa.

14. Originally Posted by dgavin
Interestingly enough though, most circuits are designed as if the + was the source and the - was the ground. The same circuit will work either way if a PNP transistor is replaced by it's NPN transistor counterpart, and vis-versa.
Wouldn't you have to reverse the battery?

15. Yes, my bad, in that case the Ground symbol would be for the + pole and the Source the - pole.

16. Excellent discussion on remedial magnetism, guys, thanks.

And korjik, thank you for your answers to my questions. As far number 4, I have to let out a big one Homer "D'oh!". Of course, we cannot have self-interference in a coherent beam. I should have realized that.

17. Order of Kilopi
Join Date
Dec 2004
Posts
11,219
Regarding the original question:

If a light wave is moving forward, and the positive direction of
the electric field is up, is the positive direction of the magnetic
field to the right or the left? How can that be determined?

Can you provide a URL to a reliable web page which states the
right-hand rule which applies to electromagnetic radiation? I can
find the rule for currents in a conductor and the resulting force, but
the one page I found that has an illustration showing the directions
of the electric and magnetic fields in EM radiation appears to show
the positive direction of the magnetic field to the left, not the right.
Bottom of the page. I assume that the wave is travelling toward
positive X, away from the viewer.

http://www.ch.ic.ac.uk/local/physical/mi_5.html

I note that this diagram uses a color scheme which is exactly opposite
the one that I use, and the planes of the electric and magnetic fields
are exactly opposite the positions I conventionally place them in. *
If the blue were the E field and the red were the B field, it would fit
the rule korjik stated: If k is forward and E is up then B is to the right.

-- Jeff, in Minneapolis

* Edit to correct: I was wrong about the planes of the E & B fields
being in the opposite positions from those I conventionally use. Since
you've probably never seen any diagram I've drawn of this, it makes
no difference anyhow.
Last edited by Jeff Root; 2008-Jul-11 at 04:38 AM.

18. google "Poynting flux" or "Poynting vector".

19. Order of Kilopi
Join Date
Sep 2004
Posts
5,448
Originally Posted by Ken G
Yes I must confess that's the one thing the engineers got right and the physicists got wrong-- the electron charge should be positive.
Benjamin Franklin got it wrong. Dunno it if he was considered a physicist.

20. Originally Posted by korjik
Benjamin Franklin got it wrong. Dunno it if he was considered a physicist.
Maybe not a "physicist", in the narrowest sense, but definitely a scientist and philosopher who was much more recognized as such in Europe than he was in his own country!

21. Order of Kilopi
Join Date
Sep 2004
Posts
5,448
Originally Posted by Kaptain K
Maybe not a "physicist", in the narrowest sense, but definitely a scientist and philosopher who was much more recognized as such in Europe than he was in his own country!
It is not like he is unknown here. There is only so much a guy can be famous for

22. Originally Posted by korjik
It is not like he is unknown here. There is only so much a guy can be famous for
True, but what he is best known for here are, in probable order, publisher (Poor Richard's Almanac), statesman, and the guy with a kite.

23. Originally Posted by Jeff Root
Can you provide a URL to a reliable web page which states the right-hand rule which applies to electromagnetic radiation?
snip
http://www.ch.ic.ac.uk/local/physical/mi_5.html
Hate to point it out, but just below the illustration:
Originally Posted by your web page
One way to remember the relative direction of the force is by imagining the direction a right handed screw would move if it were turned through the angle θ from v to B
Emphasis mine. Fear not; I get this wrong about half the time myself.

24. Originally Posted by korjik
Benjamin Franklin got it wrong. Dunno it if he was considered a physicist.
He was quite straightforward about the fact that he was guessing. He had a 50/50 chance of being right. It's about the only thing he got wrong, really.

25. Order of Kilopi
Join Date
Nov 2003
Posts
6,197
Originally Posted by grant hutchison
And, of course, the magnetic pole near the Earth's north rotation pole is a south magnetic pole. Otherwise the north pole of a compass needle wouldn't point towards it.

Grant Hutchison
Wouldn’t it be the south pole of a compass needle that points toward the north pole of the earth? It’s not the “N” on the compass that points north, it’s the south pole of the needle that points north. The “N” represents the direction of the north pole of the earth.

26. Order of Kilopi
Join Date
Jul 2005
Posts
9,761
Originally Posted by Sam5
Wouldn’t it be the south pole of a compass needle that points toward the north pole of the earth? It’s not the “N” on the compass that points north, it’s the south pole of the needle that points north. The “N” represents the direction of the north pole of the earth.
The conventional names for magnet poles come from the geographical direction in which a magnetized needle orientates itself: "north" is actually short for "north-seeking" and "south" for "south-seeking". So the north end of an orientated compass needle is its north magnetic pole, which makes the north end of the Earth a south magnetic pole.

Grant Hutchison

27. Order of Kilopi
Join Date
Dec 2004
Posts
11,219
Originally Posted by grant hutchison
The conventional names for magnet poles come from the geographical
direction in which a magnetized needle orientates itself: "north" is actually
short for "north-seeking" and "south" for "south-seeking". So the north end
of an orientated compass needle is its north magnetic pole, which makes
the north end of the Earth a south magnetic pole.
If your first sentence is correct, then your second sentence is wrong.

If the north end of a compass needle is north-seeking, then it seeks
a north pole, and must itself be a south pole.

Are you sure it is Earth's north pole which is really a south pole, not
the north-seeking ends of compass needles which are really south poles?

-- Jeff, in Minneapolis

28. Order of Kilopi
Join Date
Jul 2005
Posts
9,761
Originally Posted by Jeff Root
If your first sentence is correct, then your second sentence is wrong.
Not so.

Originally Posted by Jeff Root
If the north end of a compass needle is north-seeking, then it seeks
a north pole, and must itself be a south pole.
The north end of the compass needle is called "north-seeking" (and therefore "north") because it seeks geographical north: it points towards the North Star, and lets you orientate your map with geographical north at the top.
So conventional magnetic polarity gets its terminology from the geographical direction in which a magnet aligns itself under the action of the Earth's magnetic field. For this to happen, the Earth's magnetic field must have its south pole in Earth's high northern latitudes.

Grant Hutchison

29. Order of Kilopi
Join Date
Apr 2004
Posts
7,307
Maybe that's why physicists named magnetic poles as they did. Otherwise, there would be a lot of confusion between the north geographical pole and the north magnetic pole. Better to put both in the same place!

30. Order of Kilopi
Join Date
Dec 2004
Posts
11,219
Originally Posted by Schneibster
Originally Posted by Jeff Root
Can you provide a URL to a reliable web page which states the
right-hand rule which applies to electromagnetic radiation?
snip
http://www.ch.ic.ac.uk/local/physical/mi_5.html
Hate to point it out, but just below the illustration:
Originally Posted by your web page
One way to remember the relative direction of the force is by imagining
the direction a right handed screw would move if it were turned through
the angle θ from v to B
Emphasis mine. Fear not; I get this wrong about half the time myself.
I read that quote when I found the page. I did not then and still do
not now understand what it is saying.

The quote is underneath the first illustration on the page, and refers
to "The Lorentz force on a positively charged ion". I am asking about
the positive direction of the magnetic field in electromagnetic radiation,
which is discussed at the bottom of the page, and diagrammed in the
last illustration on the page. That illustration appears to be in error,
showing a left-hand relationship of the magnetic field to the electric
field, rather than the right-hand relationship korjik asserted. Nobody
has either confirmed my interpretation that the illustration is wrong,
told me that my interpretation is wrong, or shown me a reliable correct
illustration or verbal description.

In searching for ""Poynting vector" as Ken suggested, I have found
lots of web pages which tell me clearly that the electric and magnetic
fields are at right angles to each other and to the direction of
propagation, but none that clearly state the positive direction of
the magnetic field relative to that of the electric field. I have no
doubt that it is unambiguously expressed in Maxwell's equations, but
I am as capable of interpreting those equations correctly as I am
capable of defusing a hydrogen bomb set to explode in 30 seconds.

So far the best I have found are the crude diagram on this page:
http://electron9.phys.utk.edu/optics...m1/emwaves.htm
which apparently confirms the right-hand rule and contradicts the
diagram at the bottom of the page I linked previously:
http://www.ch.ic.ac.uk/local/physical/mi_5.html
(note that the two diagrams put E and B on different axes),
and this statement:
From the viewpoint of an electromagnetic wave traveling forward,
the electric field might be oscillating up and down, while the
magnetic field oscillates right and left; but this picture can
be rotated with the electric field oscillating right and left
and the magnetic field oscillating down and up.
which can be read in such a way as to infer that the positive
directions of the E and B fields follow the right-hand rule. (From
the fact that it says "down and up" instead of "up and down".)

-- Jeff, in Minneapolis

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•