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"The Exploration of the Earth's Magnetosphere"

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19. "Why does this happen?" (electromagnetic induction)

I'm 17 years old and studying physics and recently learned about electromagnetic induction. It's pretty basic and I understand that when a magnetic field or flux cuts a wire at 90 degrees it generates a current and the same when a wire cuts the field lines. My question is why does this happen? could you please explain or just point me in the right direction of some good web sites.
thanks Ziad

["Electromagnetic Induction" means the creation of an electric current in a conductor moving across a magnetic field, a phenomenon discovered and studied by Faraday--see http://www.phy6.org/earthmag/dynamos.htm.]

Reply

Dear Ziad

The question "why" is usually the hardest and the last to be answered. People observe phenomena, use them to deduce laws, and only afterwards (maybe) see the underlying pattern and how it all fits together. I will not fully answer your question here--it's too long since I studied these things, and I mainly remember that the explanation is complicated. I will only try to point you in the right direction.

First: it makes no difference if the wire moves and the source of the magnetic field is fixed, or vice versa. Only relative motion matters. That is one of the things relativity established, and I discussed it briefly at

http://www.phy6.org/stargaze/Srelativ.htm

Second: the electric field E and the magnetic field B (or "magnetic induction"; I won't quibble here about preferring B to the related quantity H) are two tightly coupled quantities. These letters are printed in bold face, to show they are vectors, with direction in space as well as intensity. Maxwell's equations of electromagnetism

(see http://www.phy6.org/stargaze/Sun5wave.htm )

and Einstein's relativity involve both of them jointly. Here B at some point is the force on a magnetic pole of unit strength, IF it were placed at the point, and E is the force on an electric charge of unit intensity, at the same point. Faraday speculated--and Maxwell showed this made mathematical sense--that in both cases this was best viewed not as a "force-at-a-distance," exerted by distant magnetic sources or electric charges, but rather as a modification of space, and that the pole or charge was reacting to the modification at that point. Such modified space was gradually named "field."

In this formulation, what a changing magnetic field or flux creates is not a current but an electric field. If an electric conductor occupies the location, the electric field causes a current to flow, because an electric field in a conductor drives a current. But the basic relation is not between magnetic fields and electric currents, it is between magnetic and electric FIELDS.

Still with me? It turns out that the components of magnetic and electric fields belong to a single symmetric pattern, and Faraday's law of induction is necessary to maintain that pattern. In relativity, they express different components of the same 4-dimensional tensor (that's one level up from a vector), and transforming fields from one moving frame to another, magnetic fields in one frame contribute part of the electric field in the other, and vice versa.

Why that pattern and not another one? I don't know, maybe because it has a pretty symmetry. Ask the guy upstairs.

20. What would a Compass on the Moon point to?

Dear Dr. Stern,

I am student teaching in an 8th grade science class. We have been studying the earth's magnetic force, and the question was asked by one of the students, "What would a compass on the moon point to?" I know that the moon has no internal magnetic force, but does have some areas on the surface that are magnetized. Would a compass point to these? Thank you for your help with this question.

Reply

Your information about the moon is correct, but your students should bear in mind two points. One, we are used to regarding the compass needle as a sensitive instrument. It may be so, compared to other everyday indicators of magnetic forces, but space-borne electronic magnetometers, such as have mapped magnetism in space, are many thousands of times more sensitive. I would suspect that the weak magnetization of some areas on the moon is not nearly strong enough to move the needle of an ordinary compass, even in the weaker gravity.

You can find more about such magnetometers in http://www.phy6.org/earthmag/magmeter.htm

Even if the technical description of such instruments is beyond your students, they may be interested in the way they were used by Dr. David Cohen at MIT.

And secondly, the magnetic force, on Earth or on the moon, is usually not horizontal. That is discussed very briefly in
http://www.phy6.org/earthmag/upto1600.htm
and in more detail at the end of
http://www.phy6.org/earthmag/NSTA1A.htm

An ordinary compass only responds to the horizontal part of the force, and in a complicated magnetic field, this can be highly variable--towards magnetized patches, away from them, or neither. I do not have a magnetic map of the moon, but your students may look at one of Mars, which is also magnetized in patches, and appreciably more strongly than the Moon (though it lacks a core field like the Earth's). See map at the very end of
http://www.phy6.org/earthmag/planetmg.htm
and the link given there (since then that map has been made more accurate).

Your idea is interesting but FAR off the mark:

The electric charge and magnetic field needed to keep the Earth orbiting the Sun are very different from what is observed and MUCH stronger. The electric charge needed to generate a force comparable to gravity would be ENORMOUS and would immediately attract ions or electrons from space, which would cancel it out (and bombard us with penetrating radiation). Also, the observed magnetic field is deformed by the solar wind to where it could not give forces for orbiting the Sun in a plane. Furthermore, magnetic forces act very differently from gravity. And what about the motion of the Moon?

It just won't work.

23. All magnetism due to different arrangements of magnetic poles?

Dear Dr. Stern Instead of describing magnetism as the force between two currents, it might be easier to understand if it was described as the force between two charge pairs.

When I say charge pair, I'm referring a positive and negative charge of equal magnitude separated by a distance. (+)...(-)

When you are far away from a charge pair, they look like they are together to any other charges out there. So there is little force if any. When far away from a charge pair, the static electric force is 0.

When another charge get closer to a charge pair, one side is closer to it than the other. This slight difference in static electric force from the difference in distance is called the magnetic force.

If these two pairs spin together into the page with their poles like this, they attract.

(+)...(-) pair one

(-)....(+) pair two

Like this they repel. One will try to flip around to line up with the other.

(-)...(+) pair one

(-)...(+) pair two I think like this there is a slight attractive force if they are spinning into the page. It's not stable though.

If the charges are yo-yoing through each other in the pair, then there is a strong attraction and it's stable.

(-)...(+) (-)...(+)

When you pass a current through a wire, the electrons move away from the protons to jump from atom to atom. This gives the separation.

A good example of magnets in action are water molecules in snowflakes. Water molecules have a plus side, and a negative side, separated by a slight distance. Science calls them polar bonds, but in reality they are just magnetic.

Reply

Dear Jim

What you are describing seems very close to the theory of magnetism which existed up to 1820, though you might be confusing electric charge and magnetic charge. If you have (+) and (-) electric charges and connect them with a wire, an electric current briefly flows. If you have (N) and (S) magnetic poles (north- and south-seeking, on a compass needle) and connect them with a conducting wire, nothing happens. Charles Augustin Coulomb proved by a clever experiment, in the years after 1777, that two electric charges attract or repel with a force proportional to 1/r^2, and so do two magnetic charges. See the end of http://www.phy6.org/earthmag/to1820.htm

For gravity the "inverse square" law is always one of attraction, as Newton showed from the motion of the Moon and planets, and Henry Cavendish proved in the lab in 1796, using a very sensitive version of Coulomb's instrument. Thus scientists around 1800 believed that nature had 3 basic forces, very similar to each other, originating in "charges" and decreasing with distance r as 1/r^2.

The way you superpose magnetic charges is also very similar to spherical harmonic analysis of the magnetic field of Earth, as described briefly in http://www.phy6.org/earthmag/gauss.htm

Two electric charges close to each other (as you describe) are an electric dipole, and do exert forces on an isolated electric charge very much like the forces a magnetic "dipole" would exert force on an isolated magnetic pole. However, that force is electric, not magnetic.

In any case, the theory of magnetic poles completely failed to explain Oersted's experiment http://www.phy6.org/earthmag/oersted.htm

and that was what brought on the interpretation of magnetism as a force exerted by electric currents. That view still holds, and nicely fits the fact that isolated magnetic poles (unlike isolated electric poles) can never be produced, also with the theory of electromagnetic waves http://www.phy6.org/stargaze/Sun5wave.htm

and with relativity. The use of magnetic poles and spherical harmonic analysis (strictly as mathematical devices) is still OK in regions where no electric currents flow, e.g. for the external magnetic field of Earth.

24. Magnetism to replace gravity in a space station?

Do you have any idea if the magnetic lose its magnetic property in the space. I am developing virtual space station and I would like to so calculations of how can I have some furniture fixed in their position such as chairs, tables in this station and if there is any technique that can be applied to gives the spaceman the ability to walk in the space station instead of flying.

I would really be very happy by getting any information.

Reply

Magnets operate in space just as well as on Earth. However using magnets to keep people in position is questionable, because they attract all sorts of iron objects, can confuse instruments and an astronaut with magnetic shoes encounters resistance at each step. For many uses, weightless "flying" is preferred. And you need no chairs! On the space station "Skylab (a unit of which can be visited in the Smithsonian Museum in Washington--see picture in

http://www.phy6.org/stargaze/Sskylab.htm

http://www.phy6.org/stargaze/Srotfram.htm

25. Magnetic reversal due soon? And are volcanoes a factor?

Question 6-B (above) on reversals of the magnetosphere interest me. Well, I don't really worry about any effects of a reversal, but I think it's quite interesting what this Ricky wrote: "It has been brought to our attention that the magnetic poles of the earth reverse on an average of about every 500,000 years. The last change was about 700,000 years ago, so it would appear that we are long overdue."

Mmh, other scientists have studied those large volcanos like that Yellowstone national park-giant. They say, that its caldera was formed ca. 630.000 years ago and before this ca. 1,2 million years ago. They also say that its eruption was overdue.

Well, there could possibly be a relation between those eruptions of a super volcano and those reversals (if Ricky and those volcano-scientists are right). I mean what are 630,000 or 500,000 years? I don't think, anyone can define the time so precisely... Maybe a large volcano can cause this effect?

Give it a try ... maybe you can find out more of this if you brainstorm with a volcano specialist. Me, I'm too busy (unfortunately) as I am not a scientist - only a technician.

But reading your work is quite interesting, though.

Reply

Your message reminds me of what a friend once told me about Las Vegas (a place I have yet to visit). In a casino he saw a lit tabulation of how many times each roulette wheel had come up (say, in the past week) with each of its numbers. The interesting thing he noted was that, if, say, the frequency of the number "15" was unusually low, people did NOT avoid betting on 15, having concluded some uneven feature of the wheel made the ball less likely to stop there. No, MORE money was bet on 15, because players argued "it was overdue to be hit."

I do not think volcanism is a factor. It is a relatively shallow process, while the Earth's magnetic field originates deep down.

Gary Glatzmeier and his colleagues, in California, have used a computer to model variations of the magnetic field (Glatzmaier, Gary A., Robert S. Coe, Lionel Hongre and Paul H. Roberts, The role of the Earth's mantle controlling the frequency of geomagnetic reversals, Nature, 401, 885-90, October 28, 1999). They believe (like most geophysicists) that the magnetic field in the core is produced by flows in the liquid iron there, and those flows are driven (as are volcanoes, true) by heat released in the Earth's interior, from radioactivity and from other processes. One big question is how heat flows out to the surface--does more come out near the poles or more near the equator, etc. By changing these and other conditions, they sometimes obtained frequent reversals, sometimes infrequent ones, sometimes none at all. Furthermore, they also obtained "excursions" which almost ended in reversals, before the magnetic polarity decided to stay in its original direction. Paleomagnetic data have found evidence for similar events.

So... it seems complicated. Paleomagnetic data even found "superchrons" in the distant past when NO reversals occured for tens of millions of years. Go figure.

26. Can magnetic reversals affect the human mind?

I have a master of arts in philosophy and am interested in the Earth's magnetic field for a romantic science-fiction novella with a surrealistic angle, which I am thinking I may write. I have heard from someone the following theory, whose origin unfortunately I could not track. It claims:

The earth has a magnetic field and every human has a magnetic field likewise.
Since the earth magnetic field is declining, the rational capacities of humans with a "weak personality" are diminished.
A general effect of the diminishing earth magnetic field is that the population is turning more and more psychological unstable, slightly turning bit by bit more and more crazy.

Do these claims have any validity?

Reply

You may have an idea for an interesting story, but magnetism would not explain it. As far as I know, magnetic fields do not affect either mind or body. If they did, we would have a hard time finding the reason, since the body has neither appreciable electric currents nor magnetic materials.

Also, it is not true that humans generate a magnetic field. I remember an experiment (never mind how it came about, it's a long story) when the psychic Uri Geller and a friend visited my NASA center to give a lecture. They were invited to a facility used for testing satellites in very weak magnetic fields, like those in space, to detect magnetic fields produced by currents or magnetic materials on the spacecraft, which could later distort magnetic observations in space. In that facility (it still exists) large current-carrying coils reduced the magnetic field in the center to 1/10000 its usual value, as indicated by a magnetometer. Geller and his friends shook hands at it and tried their best to change the magnetic field registered by it, but nothing happened, while starting a car out on the parking lot did create a clear signal.

I am afraid that if you want to have in your story a "population turning more and more psychological unstable, slightly turning bit by bit more and more crazy," magnetism is not a believable cause. A US researcher named Calhoun once ran an interesting experiment, by growing laboratory mice in a large cage where they had everything--food, clean water, clean padded places to live etc.--everything but unlimited space. At first the population grew at a steep rate, but as it filled the cage and became crowded, the mice indeed turned "more and more psychological unstable, slightly turning bit by bit more and more crazy." Not having any quiet place to retreat, living in constant encounters with other mice, some stopped breeding, some attacked each other...and in the end the population "crashed." I am afraid, however, that this is too realistic and scary for a work of fiction.

27. When and where can I see "Northern Lights"?

When and where am I most likely to see Northern Lights?

Reply

What matters most is "where," or as they say in the real estate business--location, location, location. Your best bet is Fairbanks, Alaska; you may also see northern lights in Winnipeg, Canada, or even International Falls, Minnesota, but they could only be near the northern horizon.

Scientists call the phenomenon the "polar aurora." Earlier it was named "aurora borealis" meaning "northern dawn" in Latin, since in Europe it was mostly seen as a glow near the northern horizon. However, it also occurs near the south pole, so "polar aurora" is now preferred. Aurora is caused by electrons energized in the Earth's magnetic environment, the magnetosphere, and guided earthward along magnetic field lines (lines of force) along which they slide, a bit like beads sliding along a wire. They move at about 1/5 the speed of light and in many ways they resemble electrons beamed at the screen inside the picture tube of a TV receiver. Where picture-tube electrons produce light when they hit the screen, auroral electrons do so when they hit the fringes of the atmosphere, about 100 kilometers (60 miles) up.

The field lines on which this happens are the ones coming down along a circle centered near the magnetic pole ("auroral circle" or "auroral oval"), and that is where the accelerated electrons end up. The circle can expand and contract, but it usually passes near Fairbanks.

When? Don't expect to see aurora in the Alaskan tourist season, during the summer, it just does not get dark enough. Just as near-polar winter nights are long and dark, summer days are long and bright, and even after the Sun sets, twilight persists. September has some darkness, October much more--March and April are also OK, and so is winter, if you do not mind the cold. When? The brightest auroras come from the night side of the magnetosphere, so you should see them around midnight, or later because of Alaska's time zone. If you stay in a hotel, ask the desk clerk to wake you when a good display occurs.

What about the 11-year sunspot cycle? Occasionally, but especially in years of peak sunspot activity, the Sun sends out blobs of hot gas, which hit the magnetosphere and agitate it, causing "magnetic storms." At such times the "auroral circle" expands and reaches the lower 48 states of the US and mid-latitude Europe. The agitation also produces fine auroras, as happened on 5 November 2001, when the US was favorably located. However, since Fairbanks is located near the normal auroral circle, auroras are observed there throughout the 11-year cycle. The main factor is the slanting of interplanetary magnetic field lines (which come from the Sun)--southward slant, auroras likely, northward slant, not so much. That slant varies randomly, though satellites monitor it, and you can read their latest reports on the web.

You find much more in the various chapters and links of "The Exploration of the Earth's Magnetosphere", http://www.phy6.org/Education/Intro.html. That collection includes sections on the aurora, magnetic field lines, the event of 5 November 2001 and one about "space weather," with links telling where to find predictions of the likelihood of aurora.

28. Magnetic reversals due to comet impact?

I just recently studied geology in my science class. What caught my attention was why the Earth's magnetic polarity has changed so much and no one knows why. My teacher told us when you hit a magnet with a hammer it changes polarity.

I believe it possible that, like the impact of a hammer to a magnet, a comet hits the Earth and changes the Earth's polarity, by either dispersing the liquid iron core of the Earth enough to change the polarity, or by hitting so hard that the poles just change.

(from Australia)

Reply

I do not know in which of my files you found my e-mail address, but I would encourage you to go to "The Great Magnet, the Earth," home page

http://www.phy6.org/earthmag/demagint.htm

By the way, suggest to your teacher to read it too, because I do not think hitting a magnet with a hammer reverses its polarity. It may destroy some of it, by disorienting magnetic domains, but that's all. Your teacher may like the suggested lessons on the Earth's magnetism, starting in http://www.phy6.org/earthmag/NSTA1A.htm

About a comet hitting... a comet or meteorite may have been involved in creating the magnetism of Mars, which is very irregular and whose sources do not seem to be far below the surface. See end of the last section in "The Great Magnet."

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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: earthmag("at" symbol)phy6.org .
Last updated 6 June 2002
Re-formatted 9-27-2004