Question #13
Magnetic Shielding
My question is about induced magnetism and 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
Reply
Magnetic shielding is not my speciality and you might get a better
answer from an engineer familiar with magnetic design, but I will try.
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
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Question #14
Using the Solar Wind for Space Propulsion
I am a student working on a science fair project which deals
with possible use of the solar wind in space travel. My hypothesis is that
ions in solar wind can be gathered and used as propulsion for long-term
spaceflights. In theory, would this work?
Thank you. SEG
Reply
Dear SEG
Something like this has been considered, but using the pressure of
sunlight rather than that of the solar wind (I have not calculated it,
but it seems the former is much bigger). The idea is to spread a huge
sail, say of mylar with a reflecting coating (the kind that is used
to darken glass walls in office building) and have it face the sun,
so that sunlight bounces back. It is analogous to the way the wind
pushes a sailing ship and in fact, this has been called a "solar sail."
None has been tried so far. I believe there was even a story by
Arthur Clark on that idea.
If you wish to study this further, look up
http://www.phy6.org/stargaze/Solsail.htm which is part of a sister-site "From Stargazers to Starships." You will find additional links there.
By the way, sunlight pressure and solar wind pressure combine to
push comet tails away from the sun--sunlight pushes the part of the tail
consisting of dust, the solar wind pushes the part which consists of ions.
The two tails are sometimes distinct, and people who watched the bright comet Hale-Bopp through binoculars could see both. Good luck with your project!
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Question #15
A Working Model of the Magnetosphere?
I am an 8th grade science teacher in Texas. I am trying to find
out if anyone has ever tried to make a working model
of a sphere capable of generating a magnetosphere. I
feel that the testing of such a model in space could
provide some insight into geologically problems such
as why the earth's magnetic field periodically reverses itself. Please
let me know if anyone has ever made such a model to test in space and what
has been learned from such models. Thank you -- Jim
Reply
Dear Jim
Your short questions requires a very long answer!
Briefly: Yes, people have devised working models of the magneto-
sphere, but while such models have provided clues on magnetic fields
in space, they tell nothing about reversals of the Earth's field.
For that you have to probe the inside of the Earth, on which whatever
happens in space has little effect. However, there also exists
progress in that direction.
Back to your question: The Earth's magnetism affects surrounding space in interesting ways. But surrounding space has very little effect on it--things would be just the same if the Earth were a hunk of magnetized iron. Look up under "Terrella" (in the index here) and you might see why ideas like yours are nearly 400 years old!
Yes, it has been suggested that the astronauts on the shuttle
stick out a large magnet and see how it reacts with the ionized
gas (or plasma) through which its orbital motion carries it. It's hard to control such an experiment, hard to put measuring instruments around it, so scientists perform it instead in a lab--put a magnetized ball ("terrella") in a vacuum tank, blow a puff of plasma at it, and measure what happens. Prof. Hafez U-Rahman at the University of California, Riverside, has such a tank and has experimented with it.
But the source of field reversals is not above our heads--rather,
it is beneath our feet. Why is the Earth a magnet, you may ask? It
could in principle have a huge iron magnet somewhere in its middle,
but that does not work: any magnetic material loses its magnetism when
heated to red heat, and the interior of the Earth is much hotter than
that, in fact, earthquake waves tell us that in the middle is a molten
core (and inside that, a solid inner core, but at a temperature of many thousands of degrees).
So the Earth's magnetism is not produced by magnetized iron, but rather,
by electric currents (see "magnetism" in "Exploration"). Those currents
are produced by a "dynamo process" (see again, "Exploration") in the
flowing hot metal (we think) of the Earth's interior. The Earth's
magnetic field changes slowly, so that magnetic charts have to be
redrawn every few decades, and that is apparently because the pattern
of the currents shifts.
Recently, the process has been successfully simulated by a computer.
And yes, the poles sometimes reverse. Oh, and did you know that the
polar field of the Sun reverses every sunspot cycle, every 11 years or so?
David Stern
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Question #16
The Van Allen Belt
Dear sir, I wonder if you could tell me exactly what the VAN ALLEN BELT is
and how much radiation does it contain, i.e. how many rems of radiation are
there out there?
Plus, what protection would organic life need to be protected from this
radiation?
Reply
The radiation belts are regions of high-energy particles, mainly
protons and electrons, held captive by the magnetic influence of the
Earth. They have two main sources. A small but very intense "inner belt"
(some call it "The Van Allen Belt" because it was discovered in 1958 by
James Van Allen of the University of Iowa) is trapped within 4000 miles or
or so of the Earth's surface. It consists mainly a high-energy protons
(10-50 MeV) and is a by-product of the cosmic radiation, a thin drizzle
of very fast protons and other nuclei which apparently fill all our galaxy.
In addition there exist electrons and protons (and also oxygen particles
from the upper atmosphere) given moderate energies (say 1-100 keV; 1 MeV
= 1000 keV) by processes inside the domain of the Earth's magnetic field.
Some of these electrons produce the polar aurora ("northern lights") when
they hit the upper atmosphere, but many get trapped, and among those
protons and positive particles have most of the energy .
I looked up a typical satellite passing the radiation belts (elliptic
orbit, altitude ranging from 200 miles to 20000 miles) and the radiation dosage per year is about 2500 rem, assuming one is shielded by 1 gr/cm-square of aluminum (about 1/8" thick plate) almost all of it while passing the inner belt. But for ourselves no danger exists. The way these particles move in the magnetic field prevents most of them from hitting the atmosphere, and the few scattered into orbits that intersect the ground, are absorbed by the atmosphere before they get very far. Even the space station would be safe, because the trapped orbits usually stop above it--any particles dipping deeper down are lost much faster than they can be replenished.
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Question #17
Magnets of Different Shapes
I'm curious as to what effect the shape of a magnet has on the nature of
the magnetic field patterns generated. For example, would there be any
significant or even noticeable difference between a cylindrical bar
magnet with fillet 'radiused' edges and one without the fillets?
Instead of your run-of-the mill bar magnet or cylindrical bar magnet
with flat ends, what can be expected in a spherical magnet, an oval/oblate
spheroid ('jelly bean' shape), a cylindrical bar magnet w/hemispherical
divots scooped out of the ends?
Thanks, -- Rhamis
Reply
Dear Rhamis
The shaping of magnetic fields is a complicated art, with
formulas and computer codes. In general engineers separate the source
of the magnetic field--an electromagnet or a piece of magnetized iron--
from the "pole pieces" which shape the field, which are usually made of
soft iron or special alloys and are fitted to the ends of the magnet.
The virtue of soft iron is to confine the magnetic field lines inside it.
So if a magnet has conical pole pieces, tapering to a sharp point, the
iron will try to keep the field lines inside itself even though (as one
approaches the tip) the cross section becomes smaller and smaller. When
the lines finally emerge near the tip (the must emerge somewhere), they form a tight small bundle, and therefore the magnetic field is much stronger there--though in a much smaller area--than it would be in the absence of any pole pieces. Recording heads of audio and video tape recorders use such tapered poles to concentrate the magnetic field to the strength needed for writing a record.
I don't know about spherical and elliptical magnets, but formulas
probably exist for them. Some intricately shaped magnetic fields are
used inside research accelerators which speed up protons and electrons
to very high energies, to keep the beam confined to its vacuum tube,
to focus the beam's particles and to push them out of the machine onto
the target area. As noted above, it is a whole science unto itself.
David Stern
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Question #18
Building an electromagnet
My name is Jon and I am a 6th grader. I have an
invention using magnetism to prevent cars from being stolen and to keep them from bumping into each other. I tried making an electromagnet with a 9Volt battery, but it wasn't very strong. Can you tell me how to make a stronger
magnet? Can I use a larger battery or real electricity? Thank you,
Reply
Dear Jon
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 somethimes 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
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Question #19
Capturing the Energy of the Solar Wind
Could a large ring of wire (or concentric rings) placed in space with
current flowing through them be used to effectively focus solar energy?
Would space help toward a superconductor effect such that power
consumption is reduced?
Rings of wire are cheap, if they could form a magnetic lens it could be
directed to a point and harnessed...
Curious.... Mike
Reply
Dear Mike
It wouldn't work, for several reasons, but mainly, the ring's magnetic
field would only affect charged particles of the solar wind, and these
carry much less energy than ordinary sunlight. A mirror is a much more
efficient way of harvesting solar energy.
(Maintining the current in the ring also consumes energy, and the
particles are not deflected and concentrated the way light is focused
by a lens.)
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Question #20
About the Upcoming Solar Maximum
Date: Wed, 15 Dec 1999 13:42:48 -0600
I have a question pertaining to the forecast Solar Maximum.
I have heard about the upcoming Solar Maximum starting soon (CNN.com
article, Nov. 11, 1999). I have also heard (unofficially), that there could be a very large solar storm near the end of April.
Finally, it is relatively commonly known that there is going to be an
unusual alignment of the planets in our solar system at the beginning
of May, 2000. Has there been an in-depth study to determine effects of the
combination of these phenomena, and the potential impacts on both our
solar system, and our planet? Could this combination of phenomena:
- Promote a solar flare, or CME, significantly larger than
previously experienced in recorded history?
I have heard of Super flares emitting from G Class
Stars, and the theory describes large planets in a close orbit (Jan.
8th Article, Sun-like stars said to emit super flares, CNN). Now, I don't expect this size of phenomena to happen here, but with the unusual planetary alignment, I do believe that this could create larger effects than normal, like a significant Solar-Magnetic Ejection, especially with the excitation of the Solar Phenomena. I'm just curious as to how much larger.
- Disrupt the crust of our planet, creating a significant
amount of tectonic activity, and if so, by how much?
Now, I know our planets are very far apart, but if
the magnetic attractions are larger than normal, and these magnetic
attractions promote significant SME activity, this could promote some
strange tectonic happenstance, especially with the fragility of our
planet and its crust.
- Potentially disrupt our magnetic field severely with the
combination of solar magnetic and gravitational forces?
I am aware that during our earth's geological history the
magnetic poles have sometimes changed. Could this happen here with the combination of a large CME and gravitational forces?
I hope you don't mind this intrusion, I did receive these e-mail
addresses through a simplistic study I conducted on the internet
surrounding this conceptual theory. No calculations, or in-depth study
has occurred, but I have a hunch this should be looked at more closely,
and by qualified people.
Sean.
Reply
Dear Sean
Your message made me once more appreciate
the amount of misleading and loose information circulating on the web.
I have spend a great deal of time and thought on creating a web site
describing what is known about the magnetic field of the Earth and
the Sun's effects on it, and for a real understanding, you better
look there:
http://www.phy6.org/Education/Intro.html
To answer your questions in brief: The solar maximum is already here. It is
not an abrupt event you can date, but the crest of a wave whose width extends over at least several years. From what I have heard, the current peak is lower
than expected.
No one can predict a large solar storm months ahead of time--the best
we can say is that they are more frequent near the peak of the sunspot
cycle. Some big ones cause little disturbance near Earth--depends on
factors like the precise orientation of the interplanetary magnetic
field. Planetary alignments have no effect whatsoever. [Please look up the following item below, too!]
The large planets you read about are unlike anything in the solar system
--usually Jupiter-size or bigger, and very close to the star (this has to
do with the method of detection--it's hard to detect distant planets).
No solar eruption has ever been found to affect the solid Earth. Their
energy is too small, and almost all of it is dissipated outside the
breathable atmosphere. No earthquakes follow CMEs.
I have no control over CNN. But if you seek to understand Nature,
please look up my site and at sources linked or cited there!
Happy new century
Dr. David P. Stern
Goddard Space Flight Center
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Question #21
Lining-up of Planets
Enjoyed browsing through some of your efforts on the Web. I am hoping
you could help settle some of my thoughts before I make a fool of
myself.
In your experience, has anyone tried to correlate lineups of the sun,
earth and major planets' magnetospheres with the sunspot cycles? My
spare-time effort found some correlation between lineups and cycles in a
number of years. My wonderment centers around the possibility that
some forces of the planets when lined up, possibly relating to their
magnetospheres, impact the sun's magnetosphere causing a maximum of solar activity. I've also considered the possibility that related magnetosphere effects could be the cause of previous polar reversals on the earth. Additionally, ringing of our magnetosphere might impact charged tectonic plates...but that is again another direction. Only if you have time, please comment.
A copy of this goes to a high school physics teacher working
on translating these kinds of ideas into lesson
plans for students - and for my daughter.
Hugo
Reply
Dear Hugo
There exists a tempting closeness between the length of the solar
cycle and the orbital period of Jupiter, but I don't think the two
are related. I cannot imagine any mechanism coupling the two--
especially since the Sun rotates in about 27 days, so the relative
period of Jupiter going around the Sun is of that order. Furthermore,
the solar wind moves with supersonic speed, which means that solar
disturbances can (and do) travel downstream with it, but disturbances
from a planetary magnetosphere (whatever they might be) propagate too slowly to make it back to the Sun, upstream against the flow of the solar wind.
Above and beyond all these, there is always the question of energy--
the currency in which the price of any physical process must be paid.
The energy required by the solar cycle is much bigger than anything
planetary magnetospheres can supply.
So what causes the cycle? The Sun rotates unevenly, slower near the
poles, faster near the equator, probably because of the way gas flows
in it (Jupiter also has such a difference). In a magnetized hot gas,
this difference deforms and amplifies the magnetic field, and there
exist some general theories of the sunspot cycle based on this, although
many details remain unclear. The general idea is that as the magnetic
field gets amplified, it forms concentrated "ropes" which push out
the hot gas, and when they reach a certain strength, gas is displaced making the ropes are light enough to float to the surface, where they are seen as sunspots.
Again, the magnetosphere is a relatively weak influence on the Earth's
internal magnetism--even a big magnetic storm only reduces the surface
equatorial field by 1%. Furthermore, the time scale differs--reversals
happen on time scales of 0.5-1 million years, while magnetic storms
have a 1-day scale or faster.
Magnetic reversals seem to be connected to the currents which circulate in the Earth's core, presumably driven by flows there, which (like flows on the Sun) get their energy from heat. The magnetic field is fairly complicated--the
2-pole structure we see (north-south poles) is dominant, but not by as much as
it seems, because more complicated modes get filtered away faster by
distance. Right now the 2-pole field is declining at about 5-7% per
century, but the late Ed Benton has shown that the more complex parts
are gaining ene