104. Weaker global magnetic field--higher cosmic ray dosage?
The Earth's magnetic field had diminished by some 10% since the mid 1800's. Isn't there a possibility that this has caused the magnetosphere's shielding effect to become diminished as well, thereby allowing more high speed radiation to penetrate our atmosphere. When, and if you find the time, I would appreciate your perspective on this possibility.
Reply
A weaker magnetic field is slightly less effective in shielding out cosmic rays, but not of the solar wind.
The contribution of cosmic rays to global warming is negligible: the rate they add energy to Earth is comparable to the one due to starlight. For more about them
http://www.phy6.org/Education/wcosray.html
The increase in our exposure to cosmic rays due to weakening of the Earth magnetic field, is much smaller than the one due to a moderate rise in altitude--say, from sea level to that of Denver. About 30 years ago I listened to Marvin Schneiderman of the National Institutes of Health (NIH) lecture about the statistics of cancer, and asked him whether Denver experienced higher cancer rates, due to its increased radiation level. He said, "Actually, it is one of the healthiest places."
The solar wind is shielded out by the boundary of the magnetic field. If the field weakens, the boundary shrinks, but there is very little change on the Earth surface. (The boundary distance oscillates anyway, as the intensity of the solar wind varies.)
Overall, I don't think the 10% change has much effect.
105. Sound waves on the Sun?
I have a question about our sun. The sun has an atmosphere, which ought to transmit sound. What would it sound like to be on the sun? Have there been any probes to the surface to listen to the sun?
Reply
Your simple question leads to many complications, most of them going far beyond my limited knowledge, or even that of experts.
Sound waves certainly exist on the Sun, and are likely to be exceedingly LOUD. No instruments have ever landed on the Sun, nor are any likely to do so (they would boil away instantly) but all we can observe, especially the constant churning of the solar surface, suggests a high density of sound-like waves.
Furthermore, the visible "surface" of the Sun, at about 6000 degrees K, has above it a rarefied corona of about 1,000,000 deg. K. Assuming the required energy comes from the Sun below, how does it reach the corona, without heating the surface layers? Sound waves have been suggested--and to create a million degrees, they need to be pretty intense.
However... sound is not the only wave mode. Sound in our atmosphere (or in solids) is the periodic interchange of motion (kinetic energy) and compression (one form of potential energy). The surface of the Sun his hot enough to conduct electricity, and that allows modes where magnetic fields get periodically compressed (Alfvén waves), also electric oscillations, and to make things really complex, all sorts of hybrid modes between various forms of energy (including ordinary sound). Your "sound" turns into something very complicated. Many modes get reflected back from the low densities of the corona, which is why to the best of my knowledge, the heating of the corona is still unsolved.
If you want to get a taste of what's involved, ask Google about (say) "photospheric sonic waves."
106. Mapping the magnetosphere using a surface network?
I am on staff at a university and I have an idea for what could be an interesting and useful university student R & D project. It's related to mapping the earth's magnetosphere, so I believe you would be a good person to ask for advice. Here is the idea...
As I see as the problem, Earth's magnetosphere is not mapped in as much detail as scientists would like it to be, due to the vastness of the earth and the relatively small number of magnetosphere measuring instruments. One Possible Solution is as follows:
- Distribute a very large number (perhaps thousands) of fairly simple, relatively low cost magnetic field sensors at various locations around the globe. They may just give the horizontal field, or observe it 3 dimensions, even measure its strength. Cost could be a major design constraint.
- Connect the sensors to volunteers' PCs, either of private citizens or else at "controlled" locations, e.g. government and research facilities. The CPU load on the PCs would be very small.
- Network these PCs together via the Internet using existing PC "grid networking" technology. Similar projects are already operating.
- Gather the data at a central server, and analyze it, to better understand Earth's magnetic field and help us to predict things like reversal trends.
My university would like to be active in such a project. Can you please advise us, if this seems like a good project and one that would benefit the scientific community?
Reply
Unfortunately, your idea will not work, for several reasons:
- The magnetosphere is constantly varying, due to changes in the solar wind (the cause of the magnetosphere!), changes in the "tilt angle" between the Earth axis and the direction perpendicular to the solar wind (due to the Earth's rotation, since the magnetic pole is distinct from the rotation pole), and variations in the currents flowing in the magnetosphere, due to various internal sources.
- Because of the basic nature of the magnetic field, observing it just on the surface of the Earth is not enough for reconstructing the electrical currents which create it in space or in the Earth's core. We need detailed observations in the regions where those currents flow, too. We therefore cannot fully reconstruct the field in the regions of currents.
Satellites allow us observe the field in current regions in space (though we have too few of those to get more than an average structure). As for the currents in the Earth's core, which give Earth its "main" field, we would love to know their structure, too, but no way exists to make observations within the core.
Surface observations such as the ones you propose can predict the extension further out in space of the field which the core produces. However such information cannot determine the currents flowing out in space, or their contributions to the field. Even to estimate the outward extension of the core field, we need first estimate how much of the surface field originates out in space, and subtract that part from the surface data.
For more, see
http://modelweb.gsfc.nasa.gov/magnetos/data-based/modeling.html
107. History of Cosmic ray Research
Perhaps you can help me. I'm working on a history of science story on cosmic rays and I'm confused about their nature and how scientists know what they know. Historically I understand that cosmic rays were first thought to be photons and part of the electromagnetic spectrum. Then it was discovered that they carried electric charges and were deflected by Earth's magnetic field. This put them in the camp of particles. (As an aside it is mind blowing to think that a photon with an electric and magnetic field but no charge is not effected by Earth's magnetic field.)
Now historically this is where I get lost. What did Patrick Blackett's discovery do? The Nobel prize website below has a lovely description of him finding both a positive and negative electron -- confirming Anderson's discovery of the positron and Dirac's theory on antimatter. But the description goes on to talk about "transmutation of light into matter." Which are cosmic rays? The light or the matter?
http://nobelprize.org/nobel_prizes/physics/laureates/1948/press.html
Thanks so much for you help,
Reply
Here is the history, as far as I remember (you may check it, at 75 my memory may have gaps).
A long period of uncertainty about the property of atoms (especially their emitted characteristic "spectral lines" of very sharply defined color) reached a breakthrough with Schrdinger's equation and his quantum theory in 1925-6 (see here and in sections that follow for some f the history behind it), followed in 1928 by Dirac's generalization, in which he tried to reconcile Schrdinger's work with special relativity.
One result of Dirac's generalization was the prediction that every elementary particle has an "antiparticle" of opposite charge but the same mass. This was confirmed by Anderson in 1932, who saw in a cloud chamber the track of a particle--a fragment from an interaction of cosmic rays--which behaved like an electron but whose motion in a magnetic field curved in the opposite direction from that of electrons, suggesting it had a positive electric charge. He named it the positron.
What Blackett observed was the way positrons were produced--by simultaneous "pair production" of a positron and an electron by a gamma ray photon of sufficient energy, interacting with matter.
Any such process (and any other in nature!) must conserve energy, electric charge and momentum. The energy is provided by the photon: part of it goes to create the masses of the new electron and positron, at the exchange rate dictated by Einstein's famous E=mc2, and the rest is given as kinetic energy to the newly created particles. The net electric charge remains zero, like that of the photon (the positron has a charge equal to that of an electron but of opposite sign), and to conserve momentum some massive particle must also be present to absorb the recoil, which is why pair production usually happens in the strong electric field near an atomic nucleus.
The process is reversible--once a positron is slowed down by matter, it can interact with an atomic electron and "annihilate," a process where the particles convert to gamma rays. (Antiprotons also exist and can annihilate, and there exists a funny poem--see http://dnp.nscl.msu.edu/nplinks/history.html--in which Dr. Teller meets Dr. Anti-Teller, they shake hands "and the rest was gamma rays.")
As for cosmic rays, they were discovered by Victor Hess in 1912. By that time it was known that air had a slight electric conductivity, ascribed to radioactivity in soil minerals. Hess took sensitive instruments on a balloon flight and found that the effect, instead of decreasing as he rose (radiation from the soil being absorbed by the atmosphere below him), actually increased dramatically. That meant some ionizing radiation was arriving from space, and it could consist of either particles or gamma rays, he had no way of telling.
Hess had ascended only part-way across the atmosphere, and whatever was arriving from space still had to cross a fair thickness of air. Robert Millikan of Caltech, famous for measuring the electric charge of the electron, guessed they were gamma rays, since that was the most penetrating radiation he knew. He thought that gamma rays of higher energy than known at his time could have crossed that thickness.
In fact, they would not, because of pair production. We know now that the primaries actually are charged nuclei--mostly protons--and as they interact with the upper atmosphere, they produce secondaries, mainly pions (pi-mesons). Some of those are neutral and decay almost immediately to gamma rays, which produce electro-positron pairs, and these create gamma rays of lower energy by "braking radiation" (bremsstrahlung, a German word), which again produce pairs, which again... until at ground level (if energy does not run out) there arrives a mixed shower of electrons, positrons and gamma rays, sometimes covering many acres. This is how some (relatively rare) cosmic ray primaries were found to have enormous energies.
But Millikan was both famous and stubborn, so his theory of cosmic gamma rays was widely circulated. Some cosmic photons do exist, and have recently yielded important information--see
http://www.phy6.org/Education/wcosray.html
But they are a very small minority.
The realization that cosmic rays were charged came when detectors were placed on a ship--I think it was in 1927, the ship sailed from Holland to Java and back, and the cosmic ray intensity declined as the magnetic equator was approached. Charged particles are affected by magnetism, gamma rays are not. It was a small effect, because only cosmic rays with enough energy to send secondary fragments through the entire atmosphere would contribute, and those energies are not too affected by the Earth's magnetism. Lower energies, whose effects can be observed on high mountaintops, vary much more with distance from the magnetic poles.
In 1930 Bruno Rossi predicted that if the particles were positive, more would come from the west than from the east, and that was confirmed by Alvarez and Compton, and others, (including Rossi himself), using Geiger counters in directional arrays, linked by early primitive logic circuits ("coincidence circuits", now called 'and' circuits). See for instance Rossi's autobiography "Moments in the life of a Scientist." After World War II, unmanned high altitude balloons listed stacks of photographic emulsion to the top layer of the atmosphere, actually capturing the tracks of those "primary cosmic rays."
That is it, in a nutshell.
Response
Thank you for taking the time to explain the cascade of
particles through the sky. This is very helpful. The science is
fascinating. I'm having fun researching the history. Thanks for
filling in gaps in my story.
108. 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
Reply
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.
109. Why does Plasma Follow Field Lines?
These days I have been reading your info about geomagnetism and its relations with the solar wind, CMEs, etc.
I know you're an expert in the subject. But it's quite complicated for me and now I feel a bit messy and lost about it. There are many interacting processes. For instance, I still can't understand why plasmas follow magnetic field lines. I thought that their movement should be perpendicular to the magnetic lines due to its electrical charge, like the movement of those electrons trapped in the Ring Current zone, that swirl around Earth. (Abbreviated from a longer message)
Reply
Why do plasmas follow magnetic field lines... ? Presumably you refer to the bulk flow of plasma, not to individual electrons and ions, which do follow field lines while spiraling around them... but also drift at right angles due to other processes, discussed in section 10a.
Well, such drifts may or may not determine the bulk motion. It all depends on the electric field E (a vector like the magnetic field B) created (as you noted) by electric charges in the plasma itself. The plasma also tries to obey Newton's laws, e.g. its momentum tries to keep it going in its original direction, and if it has enough momentum, its charged particles re-arrange themselves to create E to enable it to do so.
If the plasma is rarefied, it does not have much momentum (per unit volume, to be accurate), and this process may be too weak. The plasma's particles will individually spiral around magnetic field lines, as you noted, and slide along them, but while electric fields may also be present, any E produced to preserve their original bulk motion will not have much effect. Motion around guiding field lines will be persist, as will motion along these lines, also magnetic drifts and mirroring, but all these are purely magnetic effects.
If on the other hand the plasma is dense--if it has considerably higher energy per cubic centimeter than the magnetic field has (not asking now how that is calculated, though some hints are given here)--then it "goes wherever it wants" and behaves more like a regular gas. It does so by moving the average density of electric charges to create an electric field E, which allows the motion "sideways" across the magnetic field. See "Exploration of the Earth's Magnetosphere", section #10a, the effect of "electric and magnetic forces together."
In that second case, instead of the magnetic field deforming the motion of the plasma, the plasma deforms the magnetic field, as discussed for the solar wind in section #18b. In that case, the solar wind expands radially, deforming the magnetic field into a spiral.
An example where part of the plasma is deforming the field and another (a burst of high energy protons) has its motion deformed by magnetic field lines is described there at the end, for the event of 17 November 1999. This sort of motion is described in "http://www.phy6.org/Education/wimfproj.html" , where some of the particles (rarefied ones at high energy) illustrate the first case and others (dense ones at low energy) illustrate the second.
110. A solar wind contribution to global warming?
My question to you is whether anyone has ever attempted to correlate the Earth's magnetic field strength with climate changes. My layman's logic wonders whether a reduction in field strength of 10% would result in some additional solar energy reaching the atmosphere, and hence cause an increase in overall global temperatures. I understand that the geologic record of magnetic field strength may span far more time than the records of global temperature in the polar ice core samples (or however those estimates are made), but has any attempt at correlation of the data been made?
Reply
The "solar energy" received by Earth presumably includes sunlight and the solar wind (with the magnetic effects it brings with it). Let us compare.
Suppose the Earth had NO magnetism at all, so that the solar wind would hit its surface directly--as it hits the Moon, most of the time. The solar wind has a density of about 6 protons per cc, and velocity about 400 km/sec, so each square centimeter facing it is hit each second by as many protons as are in a column about 400 kilometers high and of 1 square centimeter cross section:
400,000 x 100 x 6 = 2.4 x 100,000,000 protons
and each square meter (10,000 times larger) about 2.4 1012 protons
Each proton carries about 1000 electron volts, each of which is abut 1.6 10–19 joule
So that each square meter gets about 4 10–4joules per second, that is, 0.0004 watt.
The "solar constant" of sunlight energy received by a square meter on the Earth perpendicular to sunlight is about 1300 watt. It's more than a million times larger.
The changing strength of the Earth's magnetic field may have less effect than its magnitude suggests. That field diverts the solar wind around the Earth, though some energy is transmitted in other ways, through reconnected field lines. If the field were only half as strong, the obstacle would be smaller, but still, most of the solar wind would flow around. The total effect remains roughly the same as before--that sunlight is more than a million times more effective as carrier of energy.
111. Waves in the Magnetosphere
I have not fully read your article at http://www.phy6.org/Education/Intro.html yet but I am looking for an answer to whether or not there are any identifying characteristics of the earth's magnetosphere along the lines of specific frequency, with certain frequencies in Mhz, much the same way as electricity or radio waves?
If so, where would I find this information?
I am not a physics student but am just trying to learn a little about magnetism. I will continue to finish reading your article
though.
Reply
The ocean has waves, of various size and frequency. But the bulk of its energy, activity etc. is in the water itself.
The magnetosphere, similarly, supports all sorts of waves--whistlers, Alfvén waves, micropulsations, hybrid wave modes, auroral kilometric radiation etc., waves whose frequency is tied to the density of the plasma and to its magnetic field. Many of them may be viewed as electromagnetic waves modified by the presence of plasma and magnetic fields.
This is a huge field of research and to understand it takes a lot of data and math, combined. But as in the ocean, these are mostly secondary phenomena. My web pages mostly describe that "ocean." Read them!
112. What are "frozen" magnetic field lines?
Firstly, I must thank you for taking so much of your precious time in creating a superb site covering the Magnetosphere. I have enjoyed reading the entire section with great enthusiasm.
I am a 38 year old electrician from the UK with some college physics education and I have a question that I could not find an answer to in your site. My question is this:
I have been taught that all magnetic fields are maintained by electrical currents due to the movement of charged particles, including the orbit and spin of electrons in atoms and molecules, but I keep finding the phrase 'frozen magnetic field' in all sorts of literature covering astrophysics. Can you explain to me what these are?
I have searched for answers on the web but I just find explanations of 'perpetual' currents maintaining 'frozen' magnetic fields in superconductors. Is this the same phenomenon?
Reply
Indeed, there exists a sort of connection between "frozen" magnetic fields in superconductors and in space plasmas. Both media can be considered as perfect conductors of electricity, having zero resistivity and infinite conductivity, and for that reason, they cannot tolerate any electric field E. The symbols for electric field E, magnetic field B and velocity v, really need to be marked in bold face letters, since they are all vector quantity, with direction in space as well as magnitude..
If you plan to read further, better tighten your seat belt.
In an ordinary conductor at rest, E causes electric currents to flow in the direction of E, trying to short it out, and if the conductor is perfect, always E=0 because no generator can supply the infinite current that is called for.
If in addition the material is MOVING through a magnetic field B, however, the motion adds in the moving material a "dynamo" electric field v x B --where "x" will be the vector notation for the "cross product," proportional to B and to the component of v perpendicular to B. That electric field is directed PERPENDICULAR to the plane defined by v and B --and of the two possible two perpendicular directions, it is the one of the middle finger of the RIGHT hand, if v is along the thumb and B along the index (that is the definition of the cross product).
The total electric field sensed in any moving material is thus influenced by its motion (making dynamos possible, etc.), and equals E + v x B. If that material is a perfect conductor, THAT must be zero, so in such materials, the locally sensed value of E is zero, and
E = – v x B
Since v x B has (by its definition) no component along magnetic field lines (which point in the direction of B), E cannot have one either. In a perfect conductor, any electric field parallel to field lines is shorted out.
Plasma contains free charged particles, and if a magnetic field exists, those spiral around the magnetic field lines. To a very good approximation, in a plasma rarefied enough for collisions to be extremely rare (as is the case in interplanetary space), there too
E = – v x B
Any electric field along a field line will cause ions and electrons to spiral up or down the field line and cancel it. An electric field PERPENDICULAR to B can exist, but must be associated with a bulk velocity v to make the above equation hold. This can happen in different ways, but with two extremes. Either the plasma is very RAREFIED (energy density is small compared to that of the magnetic field): the electric field imposes a flow v and moves the plasma until the equation is satisfied. Or else, the plasma is DENSE and has a lot of momentum: then nature lets it move where it wants, which may require shifting electric charges so that E of the right value is created, and also perhaps modifying the magnetic field. In the end, however, the above equation is also (to a large degree) satisfied.
This is how the solar wind (outside about 10 solar radii) overpowers the Sun's magnetic field and manages to move radially outwards, no matter what the sources of magnetic field are. In the first case magnetic field lines stay fixed and constrain the motion of particles, in the second case the velocity v is more or less unchanged, and field lines bend and twist to preserve the above "MHD condition" (MHD = magneto-hydrodynamics).
Now mathematicians have analyzed this equation, and applied to is an operation known as "curl", involving vectors and partial derivatives--if you have not studied it, it will take you too long. Let me just saw that the curl of a velocity vector v (say in water, where this was first studied) is the local "vorticity," measuring the tendency to spin tornado-like, and the direction of the curl (it's a vector, remember) is the axis of that tornado. We then get, in a perfect, conductor
curl E = - curl (v x B)
(there is a mathematical symbol for the curl, too, ∇×) Now the magnetic induction law of Faraday, the principle behind transformers etc., says that a changing magnetic field creates an induced electric field. Actually, it only promises a total e.m.f.--a voltage in a closed circuit, which distributes itself according to local resistivity and motion. This e.m.f. is expressed by the curl of E
dB /dt = - curl E
which is one of Maxwell's famous equations summing up the properties of electromagnetic fields (the "d" here is a partial derivative--only with respect to time, and not other variables, such as (x,y,z)) . Put it together
dB /dt = curl (v x B)
This equation has the advantage that it deals only with the magnetic field and bulk flow; the electric field (troublesome to calculate and often to observe as well) has dropped out. And mathematicians have interpreted this equation; in terms they use:
"The magnetic field is frozen to the flow velocity distribution."
In more intuirive terms:
"If two charged particles of matter participating in the flow pattern of v are initially on the same field line, they will always be on the same field line. If they are on different field lines, they will never be on the same line."
An escape clause exists with flow involving points where B =0 ("neutral points"), where field lines can "reconnect." This however happens only at isolated points (though it may be important in nature). Overall, this is a very helpful property--incidentally, first discovered in the flow of ordinary fluids, with B replaced by the vorticity "curl v," giving a theorem about "preservation of vortex lines."
For more, see
http://www,phy6.org/Education/wimfproj.html
or an identical file
http://www.phy6.org/stargaze/Simfproj.htm
Sorry if I gave you more than you expected!
David P. Stern
Greenbelt, Maryland (where your queen visited today!)
113. Why doesn't magnetism affect electro-magnetic waves?
I'm a 6th form student in the UK and it suddenly occurred to me while I was revising what we had learnt on waves and magnetic fields that, if EM waves, are indeed electro-magnetic waves, why is it that they are not affected by magnetic fields? And why do such waves not display the properties of magnetic fields in the fact that they loop from pole to pole?
Forgive me if this is a rather naive question, I do understand the principals behind EM radiation and waves quite well, its wave-particle nature, thus allowing superposition etc., but I'm just curious as to how there is such a distinction between an electromagnetic wave and a magnetic field. If one consists of the other then how is it not affected by the other, and how is it EM radiation can travel in a straight line for nearly infinite distances (gravity aside)?
Reply
Electromagnetic waves are linear--when several are added together, each preserves its identity and can be separated again. On the radio, or on TV, many stations can send their signals through the same region of space, and yet your receiver can pick out any of them and amplify it alone.
You could regard a steady magnetic field as a signal of zero frequency (taking forever to switch to its electric signature). It does not interact with any electromagnetic waves of non-zero frequency, at least in vacuum.
In a material medium the magnetic field may modify the electromagnetic properties of the medium, affecting the propagation of waves. For instance, the Faraday effect in transparent media may rotate the plane of polarization of an electromagnetic wave.
In a plasma (gas containing freely floating ions and electrons) many different kinds of modified electromagnetic waves are possible--depending on the frequency of the wave and how far it is from some characteristic resonances, which depend on density and magnetic field in the plasma. In particular, "Whistler Waves" (in space near Earth they are produced by lightning at frequencies like 3000 Hz) are indeed guided by field lines, sometimes bouncing back and forth from one hemisphere to the other.
114. Eddy Currents
A question has been nagging me about eddy currents. They are never visualized the same way. Sometimes they are circling this direction, sometimes that, and sometimes there's just one big circle in the conductor they are being induced in.
I'm slowly beginning to suspect that they are not circling at all, or at least that they are not aligned in any way. Is it because the visualizations are just some hand-waving argument, to help people imagine what really must be expressed by equations? Or am I browsing too many scripts meant to be read by electrical engineers?
Reply
Eddy currents are indeed best visualized with the help of mathematics.
Imagine first at the flow of air in the atmosphere (or of water in a large volume, say in the ocean). There exist two basic patterns of flow: "irrotational" flow in which air or water flow from high pressure to low, and "swirling" (or "rotational," also called "solenoidal") where it just goes around in circles. You have to take my word for it that in any description of the distribution of velocities v in such a fluid (and v is a vector, with direction as well as strength), the flow can always be viewed as a combination of these two types, and these types only--some of the flow is irrotational, some rotational (and to be sure, there also exists a small range of motions which can be placed in either camp). That is known, by the way, as Helmholtz'es theorem.
Now the same holds for flows of electric currents (which you can visualize as the motion of electrons in matter--say, in a metal). "Irrotational" motions are the kind you get from batteries, where electricity flows from plus to minus (more accurately, electrons from minus to plus, a historical error in naming electric charge signs), or where it flows from some place where electrons have been separated by friction, back to where they came from.
"Rotational" flow of electricity has a different source, a changing magnetic field--say, in the secondary coil of a transformer, inside which the magnetic field changes because it is produced by an alternating current in the primary coil. The primary coil is attached to the AC power and its alternating current creates an alternating magnetic field in its iron core. This then induces a rotational electric field in the "secondary" coil wrapped around the same core--perhaps producing there a different voltage, and driving there an alternating current which can then be put to practical use.
However, the magnetic core is iron, which is also an electric conductor. What would prevent a voltage from being induced, not just in the secondary coil but also in the iron core? If a solid iron core is used, that indeed is likely to happen. Those would be eddy currents, absorbing energy and wasting it by turning it to heat, which can also damage the transformer.
If you have ever looked closely at a transformer, you know how this is avoided. The magnetic core is not solid iron, but consists of a stack of thin iron plates, glued together by some insulating gunk. The gunk breaks up any large-scale circuit in which eddy currents can flow, and as a result, only tiny harmless eddy current loops can flow. The iron in practically any AC machinery is made up of thin plates, for the same reason.
The magnetic field in a region changes if it is generated by AC, but another kind of change is also possible: the magnetic field sensed by a piece of metal also changes when it moves through a magnetic field--say, enters it or leaves it. There too eddy currents are produced, circulating in the metal itself.
In a battery ("irrotational" electric flow) you can talk about a voltage--the poles are separated by a voltage difference of, say, 1.5 volt. In a rotational electric source, all you can say is that an "electromotive force" (e.m.f.) of, say, 1.5 volts exists. The e.m.f is like a voltage distributed around the circular path, but how exactly its parts are distributed depends on the distribution of changing magnetic fields and of electric conductivity, along the path in which the induced current flows.
It is not an easy concept! Please look up for more at
http://www.phy6.org/earthmag/magnQ&A5.htm#q75
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