Site Map Q&A (subj) Glossary Timeline For Teachers Review Article Central link

Listed below are questions submitted by e-mail to the author of "The Great Magnet, the Earth." Some of them (marked ***) came in response to an earlier site "The Exploration of the Earth's Magnetosphere" and are also found there in the question-and-answer section. Only some of the questions that arrive are listed, either because they keep coming up again and again--on the reversal of the Earth's magnetic field, for instance--or because the answers add extra details, which might interest other users.

Items covered:

If you have a relevant question of your own, you can send it to
earthmag("at" symbol)phy6.org

84.     Why no inverse-square law for magnetism?

A student's question: if magnetism and electricity are unified in theory, then why does the electrical force between two charged particles follow the inverse square law but magnetic attraction or repulsion does not. Any reasonable explanations suitable for high school general physics will be welcome.

Good question, indeed! And it would not have been asked, say, around the year 1800, because at that time, a perfect symmetry DID seem to exist. The experiments of Charles Augustine Coulomb around 1777--see section 5 in

http://www.phy6.org/earthmag/mill_4.htm
--seemed to indicate that not only Newton's gravity, but also the force between electric charges and magnetic poles decreased like the inverse square of the distance. Then in 1796 Henry Cavendish, using a similar apparatus (but with much, much greater sensitivity) demonstrated that in the laboratory, too, gravity decreased as the inverse square of distance.

But this nice symmetry did not last. Every Eden has its serpent.

In 1820 Oersted and Ampere showed that an electric current ALSO created a magnetic force, except that here its direction followed closed circles or ovals around the flow of current. See
http://www.phy6.org/oersted.htm
True, as one correspondent pointed out, the force exerted by one current element on another still decreased like the inverse square of distance, but its direction was complicated. The truth is, here was a completely different kind of force.

The distributions of magnetic forces in space--"magnetic fields"--can be modeled by flows of water, say in the middle of an ocean, far from any surface or shore. Two kinds of sources may exist. Imagine a pipe stuck into the ocean, spewing water in all directions: the flow-vectors of water away from it are distributed like the force-vectors of gravity from a compact mass, or the magnetic forces from a single magnetic pole. Another pipe sucking in water creates a mirror image of that pattern, with all the flows converging inwards, instead of outwards. Electric forces (from + and - charges) or magnetic ones (from N and S poles) create this kind of pattern.

But a second mode of flow also exists: the flow could be a circulating swirl. That is how magnetic forces around an electric current are structured.

And a theorem, due to the German physicist Von Helmholtz, states that these two classes of motion are (by and large) distinct. Either you have inflow and outflow, or you have swirl. Magnetic forces belong to the swirl family, electric forces (of static charges, no time variation in any of this) are inflow-outflow.

There is more, and lots of fine print, and relativity has its say, too. But basically, this is the level students may safely understand.

85.     Sources of magnetic fields in space near Earth

I've got a question about earth's magnetic field. Diagrams show earth's magnetosphere stretching out opposite the sun from the incoming solar wind. I think the Nova episode "magnetic storm" gives me the impression that the Earth's magnetic field lines move like a flag flapping in the wind (which doesn't mesh with my impression of what field lines are.)

Is the deformation a result of the vector sum of the magnetic field from the solar wind itself and earth's magnetic field? Otherwise I can't get my head around how the magnetic field is deformed?

Perhaps my understanding of fields is limited, but I think of the magnetic field around a magnet is a map of the force the magnet applies. I can understand how a secondary field might add to this to make a new field (vector sum) but it seems to be portrayed as if the earth's field is changed--which I can't understand without changes in the currents which give rise to the field in the first place.

(A chemistry/physics teacher with more background in chemistry.)

 If two or more ions start out located on the same field line, they will always share the same field line.

Solar wind ions move from the Sun, attached to solar field lines. Before leaving the Sun, they had nothing to do with terrestrial field lines, so one expects that they won't do so in the future, either.

For those ions to share a field line with ions of the Earth's magnetosphere, would require some splicing of solar and terrestrial field lines: such splicing ("magnetic reconnection") apparently does exist and is important in creating storms and substorms in the magnetosphere, driven by energy of the solar wind. But it is a phenomenon limited to regions where the magnetic field is very weak, and most of the solar wind just flows around the magnetosphere, leaving a well defined boundary ("magnetopause") between it and the solar wind.

That separation can be viewed as a mechanical process--the solar wind hits and obstacle and flows around it. In magnetic terms, however, it is achieved by electric currents on the magnetopause, due to the encounter between the magnetic fields of the Sun and the Earth.

Those currents create an extra magnetic field: inside the magnetosphere, it cancels ("shields out") most of the effects of the solar wind. Outside the magnetosphere it "shields out" the Earth's field, excluding it from interplanetary space.

The solar wind also exerts a pressure (like ordinary wind!), and this helps shape the boundary. It is closest to us at noon, while on the night side, where the compressed plasma encounters no opposing force, it stretches into a long tail. And because magnetic field lines stay attached to plasma ions which form that tail, they too get deformed.

That tail creates currents of its own, where two bundles of field lines lie next to each other--a bundle coming out of the south polar region of Earth and an opposites bundle flowing into the north polar region. Even though both come from Earth, they must be kept separate, and an electric current flowing between them (from dawn to dusk, in the "plasma sheet") maintains that separation. And yes, sometimes the bundles flap a bit, though less so than most flags.

So what we observe in space is the sum of several current systems in space, plus the Earth's field. I have not yet mentioned here the currents associated with the polar aurora ("Birkeland currents"). It's all fairly complex, fairly variable, and we are still trying to understand many of the features.

Thanks for asking! You will find much more in "Exploration of the Earth's Magnetosphere,"
http://www.phy6.org/Education/Intro.html.
And I'm sorry if the answer is more complicated than what you expected!

86.     Force and Energy

Why is it that a magnet can lift and hold an object with greater mass suspended for years and not lose any of it's energy?

Because Force and energy are different things! Force only expends energy if it overcomes resistance over a distance, performing work. See

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

Force which does not move the object on which it acts can do so without needing energy. A concrete floor can hold the weight of a brick placed on it for many years without expending energy. However, if the brick were to be lifted, moving against the force of its weight, energy would be needed.

87.     Technical questions on magnetic energy and heating rate

(1)     Do your notes anywhere describe the total energy in the earth's B field. That is, integration somewhere of the B2/2μ0 over the dipole field?

(2) Related question. Where can I find info. on the rate of heat generation by (a) radioactive decay in the earth, (b) tidal interaction with moon?

Ignoring solar heating of the surface, where can I find discussion of thermal balance of the earth: radioactive decay & tidal heating vs. conduction and radiation of that heat. All I've found was in Hartmann's book of a 1.00005 inbalance and a cryptic note of a factor of 20,000 difference in insolation and heat loss.

(1) My web pages do not discuss the total energy of the Earth's field, or of any magnetic field. Their exposition on the Earth's magnetic field is completely non-mathematical, and even "From Stargazers to Starships" is meant for the level of high school, and therefore deliberately avoids any calculus. The formula you mention goes far beyond that, being a consequence of Poynting's theorem.

Two additional comments. (a) The energy of the magnetic field should be integrated not just over the dipole field, but also over higher harmonics, which increase in importance with depth. (Also, of course, integration over all space includes the Earth's core, where currents flow and where the field distribution is not known). (b) The energy integral outside the Earth is involved in the so-called Dessler-Parker-Sckopke formula, relating the energy of a magnetic storm to the disturbance field at the Earth. If this interests you, I can send more.

(2) I do not know enough about heating--you have to search. Never heard of Hartmann's book.

Radioactive heating mostly comes from rocks in the crust--I think that if it were distributed evenly through the volume, the temperature inside the Earth would rise much more towards the center, creating vaporisation and instability (see
here for a dissenting opinion). The heat flux through the surface of the Earth has been measured, even in regions distant from volcanism.

I do not know about tidal heating, which is significant for the inner moons of Jupiter, as was predicted (I think) by Peale, S., Cassen, P. & Reynolds, R. Science 203, 892 (1979), a short time before Io's volcanoes were discovered. See "How tidal heating in Io drives the galilean orbital resonance locks" by Charles Yoder,
http://www.nature.com/nature/journal/v279/n5716/abs/279767a0.html

You will have to search yourself for estimates of the importance of tidal effects in heating the Earth.

Happy Holidays, on this (possibly) longest night of 2006!

88.   Complex (non-dipole) parts of the Earth's Field

When you talked about the reversal of the earth's magnetic field, you stated that the "two-pole (dipole) component of the field (which now dominates it) may go through zero, but the complex parts of the field will be relatively high, and because of them, while the overall field will be weaker, it won't vanish."

My question is...:   What are these complex parts? What are they made of? Where are they located? Thank you for your time and happy new year...

The magnetic field observed at the surface is the distant signature of electric currents flowing in the Earth's core. The pattern of those currents is far from symmetric, and is in fact constantly changing, on the scale of decades.

The fact that the field we observe at the surface is mostly a north-south "dipole" pattern comes in part from the fact that the field of more complicated patterns decreases faster with distance, and in part because actually the dipole pattern is dominant, even in the core (though not by as much as surface observations may lead one to believe). See:

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

As evidence, find a magnetic map and trace there the magnetic equator: it bends around, not at all a circle halfway between the magnetic poles.

About observations, see section #14 in this collection, also section 5 (on Coulomb) in
http://www.phy6.org/earthmag/mill_3.htm

89.     What causes sunspots?

I am taking a class called Science Research and Technology (SRT) in which I am doing a project on the cause of sunspots. Scientists say that sunspots are caused by disturbances in the sun's magnetic field, but what disturbance and the exact cause isn't clear. Could you please tell me what you think on this topic

Before telling you anything about sunspots, I should say that scientists do not understand them well enough. How deep do they go? Once they were thought to be associated with magnetic fields near the surface, now astronomers believe they go quite deep. We cannot study very well what goes on inside the Sun.

Anyway, I once wrote a fairly thorough review of sunspots at

http://www.phy6.org/earthmag/mill_5.htm

What is stated there is still believed: sunspots are caused by the uneven rotation of the Sun (see image there), the equator rotating faster than the polar regions. That stretches out magnetic field lines, crowding them together and making their magnetic field stronger. Strong magnetic field (under the surface) pushes away the solar gas, which therefore gets less dense, so that regions of strong field tend to float up to the top, the way oil floats to the surface of water. Where it breaks the surface, sunspots occur.

But we still do not understand a lot--why exactly the Sun rotates unevenly, why the north-south magnetic polarity reverses every 11-year cycle, how sunspots slow down the flow of solar heat (which makes them dark).

To learn more, you will have to read more of my web pages, and look at other resources.

90.     Magnetic shielding

What is the easiest way to make an active magnetic shield e.g. to cancel magnetic field on area of some 20x20x20 cm that is accessible. The magnetic field in that volume should be as much as possible close to zero.

Your question does not give enough details, such as

(1)     At a typical Earth location, the field intensity is 50,000 nT (nanotesla). How low do you want the shielded field be?

(2)     How much effort do you want to invest? Soft iron can shield out magnetic fields, but to shield a relatively large volume (as you want to do) you probably need to use an electric current. For instance, a large pair of coils ("Helmholtz coils", look it up) can cancel much of the Earth field. You will have to wind them around large plywood frames and probably feed them current from an automobile-type battery.

Actually, you may need not one pair of coils but two or three. The magnetic field of the Earth is not horizontal but is inclined downwards (by a "dip angle"), so unless you incline the axis of your coils in exactly the same direction, you will need two or three pairs of coils, with axes perpendicular to each other. You can calculate for each pair the current you need (and hence the number of windings, depending on the wire of course), but in addition you may have to fine-tune the current by using a variable resistor.

(3)     Most important; how will you measure the field and verify that it is indeed as low as you wish?

Creating moderate shielding is probably not too hard (you still need a meter). Creating thorough shielding is a big project.

91.     Can a lightning surge clean-wipe your hard disk?

Hi! Not sure if you can advise or not.

To record a threatened species of bats in an old conveyor belt tunnel. I used a video recording unit recording onto a Hard Drive, placed on top of an iron ore stock pile, containing 60% of crushed iron ore, sand size particles. The computer was inside a water-proof box. To keep the temperature of the equipment down I surrounded the sides of the box, up the sides 2/3 of its height, with the iron ore.

The location is just south of Darwin, Australia, an area with lots of lightning and thunder storms. On return from the field the unit had had no data on the hard drive, in fact the hard drive had no formatting, and I could not recover any data. When I left it, it had about 20 hours of data that I had downloaded as I left the site. All the electronics still worked.We tried to recover data for 2 days and the recovery programs found nothing.

Is it possible that a magnetic field from the lightning wiped the drive? Could the iron ore assist in this?

If so, I guess it is just luck that the drive was wiped and the electronics are OK?

Yes, Darwin is famous for its lightning storms. When my daughter studied atmospheric science (about 20 years ago) her summer job sent her down there to help a study of thunderstorms, because that was where some of the biggest ones occurred. She sent back a photo album with some spectacular pictures.

I am afraid lightning can create a brief but very strong magnetic signature, and yes, it can wipe a magnetic memory clean, and even ruin a computer. See the story at the end of

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

Iron ore will not help (and by the way--after your incident, did you check if it became magnetized?). What you need is completely enclose your recording unit inside an earthed conducting enclosure--aluminum is best (airplanes do get hit by lightning and their circuits continue functioning!). This keeps electric currents and voltages on the outside. You are lucky the computer electronics still worked!

The trouble is with that word "completely". How are you going to get any signal into a completely enclosed box? It will depend on what you want to observe. Perhaps you need a window through which your camera observes the bats, with a big aluminum flap covering it, and a microphone triggering a small motor--when thunder is heard, it automatically closes the window and reopens it only after half an hour of no thunder. You lose some data, of course.

Even so, if lightning strikes within 10 meters or so, you will get a big magnetic pulse from its electric current, which can affect your instrument. Maybe an array of grounded lightning rods can keep it away. It is not simple. You may enjoy reading about magnetic shielding in

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

especially the 2nd half. You might also ask local experts. Good luck.

92.     A billion-Tesla field on Earth?

Is there a possibility that billion Tesla magnetic field be generated on Earth"

93.     Measuring the Earth's Magnetic Field

I'm a university student in physics. As a project I would like to measure the declination, the inclination and the magnitude of both the horizontal and vertical component of the earth magnetic field. On your site I found some information on measuring these variables, but only the way how it could be measured was mentioned, and not the physics behind these methods. I would be very grateful if you could provide me with some clues to accomplish this!

Moreover I would like to measure the variations in the magnetic field due to the solar wind and other effects. Do you know an easy way to measure these variations?

94.   Orientation of ancient magnetized rocks

I have a question for you . . .If I were to find rocks from around the world that were formed around the same time and all aligned with the magnetic pole at the time, of which was then plotted on a map and each indicated that there were several magnetic poles at that time, what is the explanation for this? Why would these minerals be pointing in so many different directions.

I wanted to thank-you. You had answered my question perfectly. I know that my question was not the most reader friendly, which I wrote pretty late in the evening, feeling quite delirious after attempting to do endless Internet research on this question. Ultimately, I ended up relying on the magnetic reversal and plate tectonic theories for further explanation.

95.   Why is southern end of compass needle heavier?

I am doing an assignment and one question asks me to find out why one end of a surveyor's compass is heavier than the other.

I'm having real trouble finding anything at all. Do you have any basic information or some sites that might be of use to me?

96.    Dynamo theory

I am a year 12 student and Physics is one of my subjects. For a research investigation I am focusing on the Earth's magnetic field and how it is created. Looking at various sites, it appears that the magnetic field is created by an electrical current, created in a way similar to the one by which currents are induced in an electric generator. Is this the correct assumption? Does the relative motion between the solid inner core and liquid outer core induce a current, which creates a magnetic field? For this to occur wouldn't there have to be a magnetic field of some degree already existing (is that what the solid iron core has created--does it act as a magnet?). What is the origin of heat in the inner core?

I have read the articles on the dynamo effect on your website and found them very helpful and interesting. If you have any more information about the fundamental principle of the generation of the magnetic field please let me know.

As you have seen, fluid dynamos are complicated. The magnetic properties of iron play no role--the temperature is too high (above iron's "Curie point") for iron to be magnetic. What matters is that molten iron conducts electricity, and that something--some heat source, probably--sets it motion, to help heat travel to the Earth's surface (see here for a similar process in the Earth's atmosphere). In the presence of high electrical conductivity, magnetic field lines are deformed by the flow, as if they were embedded in the flowing fluid.

The idea of a dynamo process caused by internal flows in an electrically conducting fluid first arose in trying to explain the magnetism of sunspots. See
http://www.phy6.org/geomag/mill_5.htm
and in particular the deformation of solar field lines, schematically shown in the illustration there. The full flow pattern is not known--we know not what happens below the surface--but it now seems the Sun's magnetism is not confined to the upper layers, as was once believed. Also, if you wish, look up
http://www.phy6.org/stargaze/Simfproj.htm
where another result of this embedding is discussed.

Sorry, but this is a big subject! The relative rotation of the core (it apparently exists, but is very slow) plays a minor role. More important is the way inward and outward flows are deflected by the Coriolis effect, due to the Earth rotation
http://www.phy6.org/stargaze/Srotfram.htm
(discussed there halfway down). The result is mathematically very complex, though computer simulation has been tried on some simplified systems, and it does show magnetic pole reversal and other features (in some cases--not in others).

97.     How can an intensely hot Sun be magnetic?

I am, a student of STD 9 from India. I was going through your website and was highly benfitted by it. But I got a query .. How can the Sun be magnetic when its surface temperature itself is about 6000 degree Celsius ? And what is the proof that this magnetism is present on earth ? If it is present, how come it does not induce magnetism in all ferromagnetic substances ?

From your e-mail I can see that you have reached "The Great Magnet, the Earth" but have not gone very far in it. Please do so now!

You will find that magnetism in nature is primarily produced by electrical currents, not by ferromagnetic substances like magnetized steel. There is no limit to the temperatures at which electrical currents can flow--on the contrary, at high temperatures electrons are torn off atoms, and the resulting plasma (see
http://www.phy6.org/ Education/wplasma.html) can conduct electrical currents with very little resistance.

The questions you have asked were already raised by others. Interestingly, dynamo theory started in 1919 by an article by Sir Joseph Larmor titled "How could a rotating body such as the sun become a magnet?" The answer is complicated and is still a frontier of research.

The Sun's field is very weak when it reaches Earth, and is shielded out by currents in the Earth's magnetosphere. Its effects can be measured, but are masked (in casual observations) by the much stronger magnetism of Earth itself.

98.     Building one's own hybrid car

I am wondering how to make an electromagnet create D.C. current. I need enough to move 2 motors, the size is undetermined because I am not sure how big I need yet. Big enough to move a car( if I can ever find any) thank you.

What I have is a Volkswagen kit car that I'm going to try to make all electric while recharging itself in motion.

99.     Is volcanism related to magnetic changes?

Is there any study that correlates the change in movement of the earth's magnetic field with the quantity, size etc. of volcanoes, or even change in land elevations? It appears that if the solid core 'wobbles' and causes eddies and change in flow in the liquid core, that there should be some consequence on the surface of the earth.

100.     Nuclear reactor at the Earth's Center?

[This follows a phone call from Marvin Herndon on 25 July 2007. He is the author of a theory, arguing for a fission reactor near the center of the Earth. He later sent a link to his new paper "Nuclear Georeactor Generation of Earth's Geomagnetic Field," submitted to Current Science and posted in PDF format at
http://www.arxiv.org/ftp/arxiv/papers/0707/0707.2850.pdf

The fission regon which he proposes is very compact, with radius around 7 kilometers. He sidestepped the question how it is that most evidence suggests that almost all the heating of the Earth's interior
originates in the Earth's crust.]

(1) Loadstone is not ordinary magnetite. See
http://www.phy6.org/earthmag/lodeston.htm

(2) Gauss did not prove that the Earth's magnetic field originated at or very near the center. All he showed is that it predominantly originated inside the Earth, not outside it

In fact, the Gauss analysis suggests it would be very difficult for the field to originate within 10 km of the center, as you seem to imply. The reason is that the dipole field goes like 1/r3, so extrapolating a surface field of 0.5 gauss to a distance of 10 km gives you something like 100,000 gauss. No permanent magnet can produce such a field--besides, the center is too hot for permanent magnetism. The source therefore must involve electric currents, and the force on that current would be enormously large.

(3) I am not familiar with geochemistry, but understand you seem to say that the core of the Earth is not mostly iron, but maybe silicate. Most geophysicists I know assume otherwise, arguing that dense iron sinks to the bottom, as in a blast furnace, and that iron is a very common element (based on observations--e.g. red Mars--and also on iron having the most stable nucleus).

But there is more. We know the moment of inertia of Earth (0.33 m RE 2, against 0.4 m RE2 for a sphere with uniform distribution) suggesting that it is considerably more dense near the middle. See under "moment of inertia of Earth" in Google. The value observed fits with a core consisting mostly of iron.

101.     Protecting Magnetically encoded Tickets

My name is Danielle and I live in Adelaide, Australia. I am wondering how to protect my bus tickets, the effectiveness of which are destroyed by magnetism. I have done some research on the net only to find that gold foil of a nanometre thick is effective but was wondering if there was a method more in the range of an everyday person?

This is not my field, really, but I think erasure is not likely with tickets used just once. With many uses--possible, depending probably on what the reader machine does to the ticket..

Magnetic materials have a property called "coercivity"--a magnetic field intensity, below which no magnetism is induced. If it is exceeded, the magnetization rises (very quickly or gradually, depends on the material) to a saturation level. Look up in the (excellent!) "Hypephysics" web collection at

http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magperm.html

Note where the rising curve crosses the "H" axis. That is the intensity where magnetization begins, then rises to saturation. To erase it you need a reverse field at least as intense as the value where the descending curve crosses the same axis.

I asked Google on "accidental erasing of magnetic strips" and found for instance

http://www.magneticsmagazine.com/e-prints/qualtec.pdf

which suggests cards are not likely to degrade with use.

To be safe, don't let the magnetic strips of cards in your wallet rub against each other. As for gold foil--never heard of it. Gold is a good conductor of electricity and therefore is likely to shield out alternating magnetic fields, by creating eddy currents which produce an opposing magnetic fields. However, wrapping your card in aluminum foil should be just as effective. See also

http://www.phy6.org/earthmag/magnQ&A6.htm#q91

102.     Location of the Magnetic Pole

Who decides where the south and north magnetic pole are placed? How big is their area ? And is it located by using a compass or by other equipment? Where does the compass point to when it's inside the pole area? And does the compass point differently in different parts of the pole area?

Several different definitions exist for the magnetic pole. The one preferred in space research defines the magnetic axis by the dipole component of the Earth's magnetic field, based (these days) on analysis of satellite data. One could also use the "best" dipole axis, allowing the center of the field to be off-set from the Earth's center, but I know of no one using this definition.

In earlier days when all magnetic measurements are done on the surface, many people preferred the "dip pole" where the magnetic force is exactly vertical (it becomes more and more inclined as one gets close to that place). That was the definition when the north magnetic pole was located in 1831--see section 9 in

http://www.phy6.org/earthmag/mill_4.htm

The latter definition also depends on complex added parts of the field. Since 1831 the magnetic pole (by that definition, and probably by others too) has migrated into the arctic ocean and is now getting quite close to the geographic pole.

103.     Currents that Generate the Earth's Magnetism

About "the currents responsible for the Earth's magnetic field lose energy to electric resistance", did you mean electric current? And how does the electric current being generated in the core?

Yes, I meant electric currents. As to how they are generated--it's a long story. This file is part of a collection of web pages "The Great Magnet, the Earth" giving a historical review of the Earth's magnetism, Its home page is

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

and the "dynamo mechanism" generating such currents is discussed in section 12 and beyond, and also in the technical review "A Millennium of Geomagnetism" included in the collection. Please, find your answer there!

More questions:

I read the review you showed me and now understand about the generation of electricity. How strong should the energy source be? You just wrote that the energy loss to electrical resistance is a very slow process.

And what do you mean by "any self-sustaining dynamo in the Earth's core cannot have an axis of symmetry"?

And lastly, why did you say that the energy source of Earth's core must also be able to drive motion?

I do not know how strong--experts in that area may be working on that, and I am not sure how well even they can specify it. People have used computers to simulate a convecting liquid core (with and without inner core, but with parameters different from those believed to exist--those may have to wait for better computers). Depending on the distribution of heat conductivity outside the core, they usually got "dynamo magnetic fields", sometimes with polarity reversals now and then, sometimes without.

The fact that a dynamo cannot be completely symmetrical was a mathematical property discovered by Thomas Cowling, and it complicated the mathematical handling of the dynamo problem. Cowling's theorem was published in the 1930s, and solutions for flows through magnetic configurations, which also generated the currents required for maintaining those same magnetic configurations--those were only derived by Stanislaw Braginsky around 1960 (and they turned out to be possible even with only slight lack of symmetry).

As for fluid motion, it is an essential component of the dynamo process--on the sun too, for instance. No believable mechanism for generating currents has been proposed in which there was no circulating flow

Hay un mito urbano en Mexico, mi país: Hay una zona en Durango y Chihuahua del que no salen ni entran comunicaciones. Existe esto aquí o en otros lados del mundo? Gracias.

105.     Deriving Dynamo Models from Equations?

I came across your website on the earth's magnetism when researching around the topic, and your website has helped me with many queries. I am currently in sixth form college and doing a coursework report on geomagnetism. Having written up my report, I found that it lacked physics depth, especially in the form of equations. I wanted to know are there any equations to show simply how fast a dynamo would have to go to create a magnetic flux of the Earth's? I have come across some detailed equations, involving Lorentz equations, which i found were touching degree level - are there any simplfications?

106.    Taking Hard Disks across the Magnetic Equator

I would appreciate if you could answer my question or direct me to where I may find some.

It concerns effect of crossing equator (in my case Europe - Australia) with a hard disk containing data. I seem to remember reading that it modifies data on magnetic media.

My personal experience seems to confirm it. When leaving Australia in 1995, in cabin luggage flying to Europe where I now stay, I carried a 40Mb HDD. It worked in Australia, but in Europe it refused to boot and I had to reformat it and reinstall software.

This year I am moving back to Australia with 800Gb of data which I am reluctant to lose. Searching the internet produced no result, though maybe different phrasing of the search can help.

Having studied TV repair, I am aware that some TVs (earliest ones?) had to be degaussed and realigned (earliest ones?) because of similar effect. For example, TV's produced in Japan and sold in Australia.

On other hand I am aware that people travel with portables, so it may not be that data on HDD is running any risk. So I don't know: can travel between Northern an southern Hemispheres cause damage? The casings of drives made of aluminium alloys probably do not shield magnetic recording on HDDs from the magnetic field of Earth,

107.     Human effects on Earth Magnetism

My son is a physics major at Adelphi University in New York. Although I am not a college graduate, I do take an interest in his studies and we often discuss them. After watching a television program on earth's changing magnetic field I googled it and found your question and answer site.

My question is: Can changes in the conductivity of the earth's biosphere have an effect on its magnetic field? Changes in ocean salinity due to melting ice caps, or increase in particulate and heavy metal pollutants must have an effect on its conductivity. Mining metals from the ground and removing oil and water must also have some effect. I know electrical current will take the path of least resistance. Are magnetic fields also effected by resistance and conductivity? Can human activity be affecting earths magnetic field?

108.     Harry Paul Sprain's machine

Thank you for returning my phone call....and for our conversation this afternoon, about a device using magnets, claimed to rotate indefinitely.

As mentioned, there are links to two issued patents (give a search engine the name "Harry Paul Sprain").

An early prototype can be viewed on YouTube

Do you think this device works?

109.    Reversal of Magnetic Poles

I was reading all the questions and answers and I saw that you didn't answer one woman's question since she asked it in the wrong way. She asked "what would happen if the magnetic poles were reversed?" and you answered that they would get weaker and probably get more complex.

I think what she was really asking, and it's what I want to know, is "what happens when they do reverse?

1.Will the earth turn or spin to adjust itself to the new magnetic shift
2. Will this adjustment happen in seconds or years?
3. Is the earth too massive to do that?
4. Will we feel it?
5. Will it cause other geophysical things to happen on earth?

I know you can't predict the future and as a scientist you can say an absolute yes to anything. Please give me your honest and best answer!

110.     Magnetometers and MRI

I am just working on a presentation about geoarcheology and found your site very useful to understand the fluxgate magnetometer. However, there is one issue where I've trouble to follow, you write:

" Materials exist--certain ferrites--where saturation occurs abruptly and completely, at a stably defined level. If a large enough alternating current is driven through a coil wrapped around a core of such material, the core's magnetic polarity flip-flops back and forth, and saturation occurs in each half of the cycle, in symmetric fashion."

I've a materials science background but never had much to do with magnetic materials, but if I think of the magnetic hysteresis loop if I come close to the saturation magnetisation the variation in applied field causes only a small variation in flux density, instead of an abrupt change. Thus this seems to be every hard to measure accurately. I could imagine that soft magnetic materials have a steeper slope than hard magnetic materials and they are thus more suitable, however, its not really abrupt. Can you help me here, are these special materials (what kind of?) or did I understand something fundamentally wrong?

PS: Do you know of a good source of information about proton magnetometers?

Magnetic materials are not my field, but my site mentions a few resources at the end of

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

To the best of my recollection, fluxgate magnetometers in general have a nearly rectangular hysteresis curve, and drive the material deep into saturation. The signal is not the onset of saturation or the saturation field, but the difference between saturations when magnetized in opposite directions. Indeed, they often use a ring-shaped core, whose opposite sides carry equal windings in opposite directions. Thus when the same current passes both coils, the two saturation fluxes tend to cancel, and reversing the current mainly leads to saturation occurring sooner in one or the other branch.

The above references also tell about proton magnetometers--though please remember, my sites are all at the high school level! Those instruments are mentioned briefly in discussing Nuclear Magnetic Resonance (or "MRI, Magnetic Resonance Imaging") used in medicine. They involve precession of protons (a process somewhat similar to precession of the equinoxes) and are described near the end of

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

Please also realize that proton precession magnetometers have been largely replaced by magnetometers based on the Overhauser effect--somewhat similar but much more sensitive, thanks to chemical tricks. About 7 years ago, such a magnetometer was used (very successfully) to map the Earth's magnetic field from the Danish "Oersted" satellite.

111.  Earth--conductor or insulator?

SIR, I want to ask you 1 question. Whys is it that Earth some times behaves like a conductor and some times behaves like a dielectric. Please answer this question.
I'm a student of telecommunication, from Pakistan.

112.     Effects of Earth's magnetic field on electronic gadgets

What if any are the effects of earth's magnetic field on electronic gadgets? Generators have powerful manmade magnets in them. Transistors and nanotechnology all have electromagnetic technology in them. Are these dependent on the magnetic field of the earth to work or are they otherwise effected so they stop working?

What about the electrical field of the earth? Will it switch from east to west too? How, if at all will man-made radio, micro waves be effected?

113a.    Rotation of Magnetic Field Lines (1)

Hi
I've read much from your site on how the earth's magnetic field lines co-rotate with it: I did understand that one of the reasons the field lines also rotate is because the magnetic and geographic meridians are not exactly aligned. The other reason is that the earth and its atmosphere (at least the ionosphere) are conducting.

I want to dwell on this second reason; can you please succinctly explain to me how the conducting nature of this earth and plasma make it possible for the field lines to co-rotate with the Earth?

(The farthest I can understand is that as the earth's plasma moves with it, magnetic fields are induced as a result of the associated currents). Thanks.

Let us concentrate here on a perfectly symmetric situation--a spherical Earth with its conducting ionosphere and a dipole field, rotating around its dipole axis.

The non-dipole parts of the field create a rotating non-symmetric pattern which is best handled separately.

In this symmetric situation, if the surrounding space is empty of matter, the magnetic field lines (strictly speaking) does do NOT rotate, any more than geographic lines of longitude rotate with Earth.

HOWEVER...if the rotating object from which the magnetic field B originates conducts electricity (i.e. the Earth is covered by a conducting ionosphere), and IF the space around it is also filled with conducting matter--say, with plasma--there will be an ELECTRIC FIELD set up, caused by the relative motion of two conductors. This is discussed for instance in

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

The relative motion creates an electric current J whose flow through B produces a force F (bold face letters denote vectors). The force F  opposes the relative motion of the two conductors (an example of "Le Chatelier's principle," a general rule that "side effects oppose their cause") and tends to make the surrounding matter share the rotation. F=0 only if both Earth and the surrounding space ROTATE TOGETHER. In space near Earth, where currents can flow without resistivity, this is more or less what happens.

(Far away from Earth other sources of electric field overcome the one due to rotation)

This "co-rotation" (first proposed by Vincent Ferraro in the early 1900s) is what we mean by "rotation of magnetic field lines." The matter threaded by magnetic field lines moves with the rotation, and in general, if two particles (ions or electrons) share the same line at one time, they continue sharing it throughout the motion, even if the line is deformed (by secondary magnetic fields due to J). This "sharing" is the main reason for introducing the concept of "moving magnetic field lines." Without it we would have to bring the electric field E into consideration, because electric forces (in the rotating frame of reference) are the real cause of co-rotation. Moving magnetic field lines and field line sharing allows one to calculate the deformation of the field WITHOUT having to introduce E.

For more, go over the entire section

http://www.phy6.org/Education/wimfproj.html

Now see if you can explain this to your teachers!

113b.    Rotation of Magnetic Field Lines (2)

Hi, thanks a million for that great answer: I understood it perfectly well, and one more thing I would like to know is if this co-rotation of the earth's field with it have consequences or implications? What are they? Thanks.

Hi, Daniel
The Earth's atmosphere which you and I breathe is held close to Earth by gravity, by the weight of its air. Above 100 kilometers or so, however, many atoms and molecules are ionized--they become electrically charged by sunlight ripping of an electron (or sometimes, by loosely capturing such a free electron). This creates the ionosphere, which conducts electricity and reflects radio waves. Short-wave communication with distant ships and countries was first made possible by such reflection.

The reflection depends on electron density, which peaks around a million per cc (cubic centimeter) or less, at about 220 kilometers. Many of these electrons and ions (and certainly those of the radiation belt) are too fast to be held by gravity, but magnetic forces keep them tied to Earth and with the electric field due to the rotation of the Earth, the whole top of the ionosphere rotates with you and me.

This extended ionosphere is known as the "plasmasphere" of the Earth, and its density gradually decreases with height, until at 4-5 Earth radii it is 10-50 electrons per cc. The outer edge ("plasmapause") is irregular and variable. Various factors determine it--one may be the electric field of the solar wind, which stretches out the long magnetic tail on the night side. Another may be local electric fields ("interchange instabilities") which help ions and electrons escape, overcoming gravity through their velocity.

The upper ionosphere is important in certain modes of radio propagation, but for most people, its rotation is just interesting. But you might note, the same forces also cause co-rotation in the envelope of a pulsar.

114. Magnetism of the human body

You have clained that the magnetic field of a human being is minimal; however, some effective acupuncture protocols are said to create electrical current to effect the body's magnetic field or to set up an additional magnetic field, which influences the body's magnetic field, in order to produce healing effects.

Is "the body has a very weak magnetic field" a contradiction to the explanation for the action or process of an efficacious acupuncture protocol? Or as an alternative, could you please explain what is happening. Is the acupuncture protocol setting up an additional magnetic field or is it producing an electrical current that influences the magnetic field? Or is there some other explanation for the efficacy? (In this example we are just using a long needle, not a machine with electrical current.)

115.    Rapidly reversing magnet

I have an invention I'm working on that requires a electro magnet to switch on and of about 6000 times a minute. Could you suggest a core that will work in this and not get magnetized?

I have been fascinated by magnets since I was 9 years old. I am now 23 and still playing with them. I also help with science experiments concerning electro magnetism and electricity.

116.   Earth's core of frozen magnetic oxygen?

Although it is very difficult to know, high temperature and high pressure of the interior of the Earth are two paradigms that scientist have to shift in order to go farther.

Have the scientists ever considered the possibility that Earth's inner core could be the coldest zone of the planet?

A possible theory about the interior of the Earth suggests that the Inner Core is a huge bubble of solid oxygen mass (magnetic-paramagnetic) floating in the center of Outer Core ( liquid oxygen). The inner core is the coldest zone of the planet. The innermost core is nothing else than an empty space of 1326 kilometers of diameter in the center of Inner Core.

It is a clever theory, but it fails in several fundamental ways:

(1) Heat always flows from higher temperature to lower temperature--that is established in many experiments, and in many kitchens. If your refrigerator is disconnected, its inside will gradually warm up.

We know the average temperature of the surface of the Earth is near 20 deg Centigrade or about 300 deg absolute. If we dig down, it always gets hotter, and the heat flow from the inside out has been measured. Gold miners deep in South Africa, for instance, endure great heat. Thus if the inside of the Earth were solid oxygen, it would quickly melt and evaporate. The heating is consistent with energy release by radioactivity. See
http://www.phy6.org/earthmag/ magnQ&A4.htm#q60

(2) In addition, we know the density of the Earth interior--e.g. its effect on the kinetic energy of rotation (via the moment of inertia). It is dense. In general, dense material will not rest with stability on top of less dense material: there is gravitational energy to be gained by the densest material dropping to the lowest possible level.

117.  Heating the inside of Earth

Reviewing your article on the earth's magnetic field (http://www.phy6.org/earthmag/dynamos2.htm), how much heat can be attributed to gravitational and frictional forces at work in the earth's (or other planets') core?

Heat is a form of energy. Frictional forces generate heat at the expense of motion, and the motion itself (creating the magnetic dynamo in the Earth's core) is the result of heat-generated circulation, so it is probably a small factor (motions in the Earth's core are exceedingly slow, anyway).

Gravitational forces create heat only if the Earth shrinks (=everything is falling down to lower altitude). Stars can release a lot of energy that way, but I think the Earth is done with shrinking. Gravitational squeezing heats the interior of the satellite Io of Jupiter
http://www.phy6.org/stargaze/Siomoons.htm
but the effect here seems small.

So what remains? Mainly radioactivity: it is a very diffuse source of energy, it occurs mostly in the top hundred miles of the crust (if it were to occur at deeper layers, the interior could heat past its boiling point!), and it is effective only because the Earth is so large and therefore heat has difficulty escaping. See
http://www.phy6.org/earthmag/magnQ&A4.htm#q60
It has been suggested that condensation of molten iron onto the solid inner core also contributes, but it's a secondary factor.

118.   Magnetism inside the Earth

Me myself am **** , an engineering student in India. I just want to know a simple answer to my simple question: how has the earth magnetism begun? How a simple earth turned out to be a giant magnet? How the rotating molten core about the solid inner core produced such strong magnetic field? Please reply soon.

119.   Electric field due to electromagnetic induction

Hi,

If changing magnetic fields create electric fields and earth's magnetic poles are shifting, does this mean that an electric field must exist perpendicular to earth's magnetic north and south? If so, does earth have electric poles? And if so, do we know where the poles are located? Thanks,

The strength of an induced electric field depends on the rate the magnetic field changes: the gradual change of the Earth's "main" field is so slow--5 to 7% per century--that any electric field generated by it is too weak to matter, or even to be observed.

Much more rapid variations (though of limited extent) occur in magnetic storms and substorms, and they induce secondary currents at Earth--in the ground ("earth currents"), in power grids close to the auroral zone and even in Alaska's oil pipeline. On the rare occasions when they were unusually intense, they have disrupted electric power (caused a "black-out") or damaged network equipment, but they generally only last a short time.

The Earth's atmosphere also has an appreciable electric field , but its source is thunderstorm activity (
http://www.phy6.org/stargaze/Svandgrf.htm). Because air is such an excellent electrical insulator, the currents driven by that electric field are small and unimportant. And because thunderstorm activity shifts all the time, the pattern of the Earths electric field constantly changes, too.

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Author and Curator:   Dr. David P. Stern
Mail to Dr.Stern:   earthmag("at" symbol)phy6.org

Last updated 27 May 2008