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

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"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves

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?

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right hand rule

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

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.

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

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...

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

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

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!

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

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.

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Wouldn't you have to reverse the battery?

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

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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.

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I came for the astronomy but I do love the physics!

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.

__________________
http://www.FreeMars.org/jeff/

"The other planets?
Well, they just happen to be there, but the point of rockets is to explore them!"
-- Kai Yeves

google "Poynting flux" or "Poynting vector".

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!

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