Get a Straight Answer

1.     What is "Magnetic Flux" and what are "Flux Lines"?

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"Flux lines" is another name for magnetic field lines, originally named "magnetic lines of force" by Michael Faraday, who introduced them.

What is "flux"? You may be aware that the mathematical description of magnetic fields is very similar to that of a fluid like water, which cannot be compressed. Where magnetic fields have "field lines", flowing water has "streamlines" (we talk about the interior flow, ignore the surface) and both field lines and flux lines are closed loops.

In describing "fluid motion" the "flux" through an area--or the one carried by a bundle of streamlines--is the amount of water crossing it per second, or crossing each second any cross section of the bundle. Magnetic flux of a bundle of lines is similarly the cross sectional area of a bundle of field lines, TIMES the average field intensity on it. Mathematically, both are similar. Flux is important in designing transformers, etc. Some engineers call the flux through a cross section "the number of magnetic field lines" through it, but it's the same thing. You can see from that how the word "flux line" arose.

2.     Is the surface of the Earth expanding?

Part of our geology course was obviously the expanding and contracting of the continental plates. On average, using your knowledge, does the surface of the earth increase or decrease within terms of expanding from plate movement, and if so (I'm going to try to phrase this intelligently) does the atmosphere end up spreading itself thinner and therefore covering the new surface of the earth?

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It is difficult to reconstruct what went on millions of years ago. However, one can be pretty sure that at the present time (1) The crustal plates of the Earth are moving and (2) The Earth IS not expanding or shrinking.

Point (1) was confirmed by accurate distance measurements based on signals from radio stars, the VLBI experiment (Very Large Baseline Interferometer), and more recently by the satellite-based Global Positioning System (GPS) which (as you might know) is capable of greater accuracy than the public is allowed to use. Point (2) is derived by comparing the observed rotation period of the Earth, measured by telescopes, with very accurate clocks. Even a tiny expansion or contraction would slow down or speed up the Earth's rotation. The Earth's rotation is observed to slow down, but from all I know this is explained by loss rotational energy to ocean tides, which transfer angular momentum to the Moon.

3.     Will a compass work inside a car?

I was recently told that a compass will not work inside of a metal building. Is that true? Doesn't the field still propagate through the concrete floor, even though surrounded by a metal roof and walls? Also, I'm told that compasses that are available in automobiles must be specifically calibrated to compensate for their metal enclosure.

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It is true that iron channels magnetic fields. Magnetic field lines of the Earth crowd into any iron oriented north-south, and since they are denser inside the iron, in the surrounding space they are more spread out, that is, the magnetic intensity there is weaker. Iron walls (if sufficiently thick) can therefore shield out magnetic fields, as described in http://www.phy6.org/earthmag/magmeter.htm , "About Electronic Magnetometers and about Smoking" (last section there).

However, I don't think a reinforced concrete building has enough iron to make much of a difference. Cars may be somewhat different, and their structural iron may be the reason why a car compass is usually mounted high on the windshield, next to the rear-view mirror. The accuracy demanded of car compasses is in any case so small--they only need enough to tell a right road from a wrong one-- that small deviations may be tolerated.

It is not so with ships and airplanes, whose navigation demands precision. Thanks to today's global positioning system, and before that, to gyrocompasses and radio beacons, this may be a moot point, but at least up to WW II the magnetic correction to a magnetic compass on a ship was quite important. The compass was placed on a pedestal called the binnacle, under an ellipsoidal cover of brass (brass has no magnetic influence) with spheres and bars of soft iron attached on the side, their positions calibrated to cancel the ship's effect. The binnacle stood in the open, since the enclosed bridge was surrounded by iron, but an electric repeater provided a helmsman inside the bridge with the compass heading.

I hope this tells you all you wanted to know.

4.     Pole Shifts? What Pole Shifts?

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I am not sure what you mean. The poles of the Earth wobble in a small circle, a fraction of a mile-- due, I believe, to the attraction of the Moon on the equatorial bulge (I may be wrong here, have not checked). However, that is apparently not what you have in mind.

Before 1965, there existed a theory of "polar wandering" by which the poles wandered all over the Earth's surface--or to be more accurate, the crust of the Earth slid around the interior which, containing most of the angular momentum, rotated more or less without change. That theory was motivated by the magnetic record in lavas: when lava from a volcano hardens, it records the direction of the Earth's magnetism at that time. Geologists, examining ancient lavas, found that at times north and south were interchanged, and for India, it was 90 degrees from either direction.

5.     What was it that Ned Benton did?

6.     Four questions about Magnetic Reversals       ***

I am an Industrial Technology teacher at a middle school and one of my students is dreaming of a space propulsion system based on magnetic repulsion of the earth's magnetic field. Could you possibly squeeze in a moment for us and provide some information on the strength of this field and how it has been measured and maybe a relative comparison? Tyson, my student, is really excited about the Internet and will be enthralled to have an answer from a NASA scientist. Perhaps you could steer him to other references as I certainly will explain to him how busy a schedule you must have. Thank you.

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But there is a more fundamental reason. Magnetic poles always come in pairs, equal and opposite: if a field attracts an N pole, it repels the attached S pole. Similarly, if we generate the field by a current in a loop of wire --say, shaped like a rectangle--for each side in which the current flows in one direction, there exists a side where it flows in the opposite direction, and the magnetic field exerts opposite forces of equal strength on the two sides.

From the preceding one would guess that magnetic forces always cancel, and no net force is exerted. So how come magnets are attracted to each other, or pins to a magnet (same thing, really, since each pin in the magnetic field turns into a small magnet)?

The answer is that the forces on the N and S poles (or on the opposing currents) are not exactly equal, if one pole, or one wire, is closer to the source of the field than the other. This can be put into a mathematical formulation and the bottom line is that a suitably oriented magnet may be attracted by a magnetic field, moving towards the greatest strength of that field. But the force is proportional to the rate at which the field changes with distance, which in the case of the Earth, is very small.

The idea of magnetism as anti-gravity has come up before. Your student may look up "Gulliver's Travels" by Swift, where in the third voyage, in a spoof on science and learned societies, Gulliver arrives at an island floating in the air, held there by the repulsion of a large magnet. Swift even gives an explanation, except it's all gibberish gobbledygook, as befits a book of satire

8.     Reversal of the Sun's Magnetic Poles       ***

I have a question about the sun that I was hoping you might be able to answer for me. A friend of mine recently returned from a new-age conference where it was presented that the magnetic poles of the sun were about to reverse, and cause a number of changes.

The idea of the sun having magnetic poles seemed counter to what I remember learning about the sun, and your web page seems to dispel the idea that the sun has actual poles. My guess is that the presenter was taking a dose of creative license with the 11 year cycle of sunspot activity.

Is it true then, that:

1.) There are no magnetic poles on the sun.
2.) Is the change in sunspots related at all to a reverse of polarity of magnetic fields?

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The Sun's most concentrated magnetic fields are of course in sunspots, but people have long suspected there might also exist polar fields, because during a total eclipse of the Sun one often sees streamers coming out from the polar regions, looking very much like the pattern of iron filings near the poles of a magnet.

But there was no good way of measuring such diffuse magnetic fields: the field of sunspots affects the light emitted from them ("Zeeman splitting") but the effect elsewhere is very weak. Then in the 1950s (if memory serves me) the Babcocks pushed the technique to its limits and found the polar field. This revealed the reversal of the polar magnetic field and suggested this field was somehow coupled to that of sunspots (which also reverse each cycle--they come in pairs, and the leading spot, in the direction of the Sun's rotation, has north or south polarity, in alternate cycles), a sort of a cumulative effect of the distant field of many spots. Theories exist by Horace Babcock and Robert Leighton, though they are somewhat qualitative.

The fact the magnetic field lines at the poles stick straight out means they do not hinder the escape of the solar wind in any way, and indeed the Ulysses spacecraft which recently passed above the Sun's poles confirmed (as was predicted) that the solar wind there is faster. There seems to exist no great effect of the reversal on Earth, though one might expect a bit more magnetic storminess when the polarity is opposite to that of the Earth.

For more on the Sun, see:

I was wondering if there was a practical method for measuring the strength and direction of the Earth's magnetic field at different geographical locations. Any help or inspiration would be greatly appreciated.

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In any case, the electronic gizmos nowadays used in space are too complicated for a quick discussion, so let me instead describe earlier, simpler methods.

The direction of the magnetic field is of course given by the compass needle: but that is just the horizontal part of the force, Actually the magnetic force also points i n t o the Earth (or out of it, in the southern hemisphere).

To find the angle at which the force points down ("dip angle") people used a needle similar to a compass needle, but on a horizontal axis, allowing it to swing in the various directions to which the hands of a wall clock might point.

That is a bit harder to arrange than a compass needle: if one end of such a needle points at an angle downwards, how is one to know whether the magnetic force is responsible, and not, say, that the needle is not quite balanced on its pivot, but that one end is slightly heavier and therefore points downwards? To avoid this problem one starts with an unmagnetized needle, balances it very carefully, and only then magnetizes it. When in 1831 the expedition of John Ross searched for the north magnetic pole, it carried along a dip needle, and when it pointed straight down (while the regular magnetic needle showed no preference for any direction), that was it .

Measuring the strength of the field is harder. Take a thin long bar magnet and hang it by a thin thread, then wait until it points north-south. After it does, push one tip slightly left or right and let go: it will swing back to north-south, but will overshoot to the other side, then turn back to the right direction, swinging back and forth like a pendulum, gradually quieting down to point steadily. The average length of each swing depends on two things: the strength of the bar magnet and the strength of the magnetic force. With a stopwatch, measure 20 swings or so and figure out how long each swing takes.

Then put a small compass needle on a table, and put the small magnet nearby, in such a position that it tries to line up the compass to point east-west.The small magnet and the Earth's magnetic force obviously compete fordetermining which way the needle points, and by looking at the actual angle of the needle, and its distance from the small magnet, we again get an observation that depends on how strong are (1) the small magnet and (2) the magnetic attraction of the Earth. Using these two observations and some calculation, the physicist can find both these unknown quantities.

This method was proposed by Carl Friedrich Gauss in Germany around 1835. It obviously won't work on an orbiting satellite--but how measurements are made there is another story altogether.

10.     The Strength of the Earth's Magnetic Field       ***

Any information would be greatly appreciated.

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11.     Magnetic Shielding       ***

I understand that we can screen out magnetic fields from a region by wrapping a piece of soft iron around the region. However, I also understand that soft iron can easily receive induced magnetism when placed near a permanent magnet.

So now my question is that: How is it possible to shield a region that near a permanent magnet by using a piece of soft iron? Won't this piece of soft iron eventually get induced magnetization and have the ability to attract any magnetic material that is nearby.?

Ong

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Soft iron--especially the kind used in shielding (mumetal, etc.) does not take permanent magnetization. Steel does, but even there, the magnetic intensity must be high enough for that to occur.

In shielding (e.g. a video tube) you wrap a sheet of soft iron around the shielded object, and the magnetic field lines which would have closed through the interior are diverted and close through the shield instead. Therefore any magnetic field that existed in the interior is greatly weakened. The field inside the iron sheet is stronger, but that is no problem--in fact, that is what we wanted to do, take the magnetic field from the inside volume and put it elsewhere (you can't just get rid of it, for all magnetic field lines have to close somewhere).

I hope this answers your question

12.     Building an electromagnet       **

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I don't know what your invention is, what the magnet is supposed to do. If you want it to close an electric circuit, you are essentially building a device known as a relay. You can probably get old relays from a radio repair shop, or any place which has junked electric devices (cars have relays, too). Or ask your science teacher for help.

Building electromagnets without calculating and measuring is not simple: you must match the size of the wire and its length to the source of current (manufacturers of relays do so, of course). In particular be cautious about using house current (you call it "real electricity", but anything you use is real electricity). A small battery is limited in what it can do--usually, not much. House current is backed by big power stations, which can pour a LOT of "juice" into whatever you attach. If your wire is short and thick, it will try to draw a big electric current: a battery will be unable to provide it, but the power station can and will, enough electricity to perhaps melt a wire and cause a fire, or at least blow the fuses or trip the circuit breakers which are meant to protect houses against just this.

Also, house current is backed by a relatively high "electric pressure" (voltage) and can cause a nasty shock. Finally, even if you got the magnet working on this, it would hum and jitter, because houses have an "alternating current", which goes down to zero and up again more than 100 times each second. If you ever heard an electric device humming (old fluorescent lights sometimes do), that is the reason.

So my advice--stick to batteries, get a relay (you can also disassemble it and use just its magnet, if that's what you want), and most important, read and learn. You are just at the very beginning of an interesting adventure.

David Stern

13.     Effect of magnetic reversals on animal migrations       ***

Thank you..... Janet

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I have no idea how to answer your question. How could one find out? The last reversal was 700,000 years ago!

I heard that some bacteria, suspended in water, find the "down" direction using magnetic materials embedded in their bodies. When they are moved to the opposite hemisphere, they tend to orient themselves in the opposite direction. That's as far as I know

14.     Which is the "True" North Magnetic Pole?       ***

I'm a teacher from Sweden. I'm also studying science and I have a question that I would like to ask you, about magnetism. I found your e-mail at http://www.phy6.org/Education/ The Earth's geographic north pole is near the magnetic north pole. But if the Earth's magnetic north pole is up north, why does a compass point up north? Then the magnetic north pole has to be a magnetic south pole, because south and north attract each other. So, my question is: Why isn't the Earth's magnetic north pole a "true" magnetic north pole?

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Your question has come up before and it is not really about science, but about language. The needle of the compass--or of any bar magnet-- has two ends, the N end tends to point to the north magnetic pole of Earth and the S end tends to point to the south magnetic pole.

So, if the source of the Earth's magnetism were a very powerful bar magnet somewhere deep inside, where would its N pole be and where its S pole? The answer, of course, is--the S pole would be at the northern end and the N pole at the southern end. How confusing!

Teachers and students have struggled with this ambiguity since times immemorial. One popular solution has been to call the N pole of a bar magnet, not its "north pole" but its "north-seeking pole", and the other end its "south-seeking pole," marking them N and S for short. You might try doing so with your students, too.

Sincerely

David

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   The horizontal intensity at Singapore is about 0.4 gauss, among the highest along the equator. The lowest is about 0.27 gauss in southern Venezuela.

    One reason for the variation is that if we represent the Earth's magnetism by a bar magnet of small size but strong intensity ("dipole"), the best description is obtained by placing that magnet NOT at the center of Earth but some distance away from it, more or less in the direction of Singapore. As a result, over South America the field is relatively weak, and if you ever heard about the "Brazilian Anomaly" (or "South Atlantic Anomaly") where trapped particles of the radiation belt are most likely to hit the upper atmosphere--the weakness of the field there is the reason.

   Of course, this is just approximate. The field near Earth has other sources of uneven structure. As one moves upwards, into space, these become smaller and smaller and the field appears smoother.

David

    Do you think it's possible to determine whether or not the earth's inner core is rotating faster than the rest of the earth by investigating the magnetic field? It seems like the strength of the magnetic field would be greater than we expect if the inner core is rotating faster, but perhaps it cannot be so easily calculated because we haven't accurately measured the amount of current that flows through the iron core.

Thanks,                 Sarah, Caltech undergrad

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    What you suggest has been proposed before, and there is even an echo back to Halley (although the inner core as we know it was only discovered in 1936, by Inge Lehmann). Evidence exists for the separate rotation of the entire core, from the gradual shift of the pattern of magnetization observed at the surface of the Earth. Acording to Halley--and some more recent evidence--it displays, in addition to its irregular variation, an average westward drift. To save rewriting, I clip below a section from a review submitted by me to Rev. Geophys.:

    "In 1634 Henry Gellibrand showed [Malin and Bullard, 1981] that the magnetic declination observed near London had undergone a systematic shift. Subsequent observations confirmed such variations and also showed them to be world-wide and without any clear-cut pattern. If the Earth was permanently magnetized (and in the 1600s no other magnetization was known), how could its magnetism vary?

    The only solution left, proposed ingeniously by Edmond Halley [Bullard, 1956; Evans, 1988; Chapman, 1941, 1943; Bauer 1896 (1990), 1913 (1990), Clark, 2000] was that the interior of the Earth consisted of concentric spherical shells, each magnetized differently, and that some rotated differently from others. Halley was so proud of his theory that when at age 80 he had his portrait painted, he appeared in it next to a model of his spherical shells. He thought he could locate 4 distinct magnetic poles, belonging to two different layers. The modern theory of the Earth's field actually suggests that the solid inner core of the Earth might rotate at a slightly different rate [Buffett and Glatzmaier, 2000] but this is a completely different process and permanent magnetism is not involved."

    The reference: Buffett, Bruce A. and Gary A. Glatzmaier, Gravitational braking of inner core rotation in geodynamo simulations, Geophys. Res. Lett., 27 (19) 3125-8, 1 October 2000.

   I am doing a project on the Earth's Magnetic Field. My question is: How does Earth's magnetic field vary with latitude, with longitude, with its distance from Earth and in time? How do people who rely on compasses account for those differences? if you couldn't give me some information on this topic, i would be very grateful. Thank you for your time and consideration

    Sincerly,                 -Ava-

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   Some of your questions are answered in "The Great Magnet, the Earth" We have magnetic charts, based on ground surveys and on satellites, and the direction of magnetic north is plotted there. You might even find such charts on the internet. The extension of the field into space near Earth follows the formulas of Gauss (see again on the web site). Far away the field of electric currents in space becomes predominant and for those regions we have models of average fields based on satellite data. On the surface, actually, the field varies more in intensity than in direction--25-42,000 nT on the equator, up to 60,000 nT at the magnetic poles.

    "Short electromagnetic waves carry enough energy to eject electrons from matter, in particular ultra-violet light and x-rays. A near-vacuum is necessary for any such procedure to be effective, because in ordinary air free electrons collide with molecules, lose their energy and are recaptured. In most of space however matter is so rarefied and encounters are so few that free electrons persist for a long time."

and

    "When one or more electrons are torn off an atom, the remaining atom becomes positively charged and is known as a positive ion. Positive ions carry most of the energy and electrical current in the magnetosphere, and are the main component of both the inner and the outer radiation belts. Fast ions are also produced by the Sun as a continuous outflow in all directions, known as the solar wind, which initiates and powers magnetic storms and similar phenomena.

    The simplest atom is the one of hydrogen, with just one electron. Tearing off that electron gives the simplest ion, the proton. The proton has a close relative, the neutron--nearly the same mass, but no electric charge--and together these two form the basic building blocks from which the nuclei of all atoms are constructed.

    Most of the fast ions in the magnetosphere and in the solar wind are protons. In the ionosphere one would expect to see ions of oxygen or nitrogen, the main atmospheric gases, and in fact most ions there are O+, oxygen atoms which have lost one electron (out of eight). Some O+ ions end up in the radiation belt, greatly energized by magnetic storms."

Which leads to my question. . .

    What effect would a magnet have on H₂O, e.g. if you put a magnet in a glass of water, what happens to the water molecule, does it lose any electrons? What about any minerals that might be in the water? Thanks in advance.

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    If you put a magnet in a glass of water ... it will get wet, for sure. Electomagnetic effects are a bit more elusive, because magnetism affects only (1) magnets or (2) electric currents. Electric charge by itself responds to electric fields (voltage differences), which is something else. Let me stick here to magnetic forces.

    Each proton is a tiny magnet, as is an electron, so a magnet will exert a small force on them. It will be like that of the Earth's field on a compass needle, namely it will try to turn them around to some preferred direction, but will not attract them anywhere.