Get a Straight Answer

69.     Magnetic Flux

    We are currently studying Magnetic flux and EMF as far as our syllabus demands, but what it doesn't do is actually tell us what magnets really are and why they work as they do. It is all just a case of "this is how it is and this is what you need to know, so learn it," which is great for the exam, but I and the rest of my class are really fascinated and would like to know more. On searching the internet I found your site and was wondering if you would be able to help us out at all.

    The information we have is that the reason iron makes such a good magnet is because of its unpaired electrons. This makes sense to me in that the negative charge orbiting the positive nucleus produces the same kind of effect as a when done with a wire wrapped around an iron core and the EMF produced. (I think this is right anyway, my understanding is only to A level standard)

    However I do not understand what magnetic flux is, how it 'prefers' to take the shortest route through the metal, but how, when it passes through air, it will curve and 'the flux lines will repel each other.' What is it though that is actually repelling? What is it that is carrying the flux and how is it that this attracts or repels? "What is it?" that I suppose is my real question. I was told that the atoms arrange themselves the same way around so that their electrons are all orbiting the same way, but why is it that that creates a magnetic effect?

    My class and I would be very grateful of any help you can offer us on this subject.

REPLY

    It is hard to cover everything in a short letter, but luckily, similar questions have been asked in the past. Permanent magnetism ("ferromagnetism") is a complex subject, understandable only on the quantum level. You might think that iron magnets are magnetized by the electron charge orbiting the nucleus, but in fact the dominating cause is the magnetism of the electrons themselves. Think of the electron as a small charged sphere spinning around its axis, and this gives the right order of magnitude--electrons do have an angular momentum or "spin"--but the ratio between angular momentum and the strength of the source of magnetism ("magnetic moment") does not fit a spinning sphere in which mass and charge distributed the same way. If I recall it right, the electron is twice as magnetic as what you may think, a difference which has quantum reasons.

    The simple answer is, any electric current DOES consist of moving charges. If a current is generated in a wire connected to a battery, the charges move inside the wire. If they are electrons and protons in the Earth's magnetic field, they move on their own, but the effect is the same.

    On a deeper level... it gets more complicated and mathematical, like so much of physics. You have an electron moving in space, you expect it to generate a magnetic field and it does.

    But suppose you match velocities with the electron, so both of you move in the same direction with the same speed. In your frame of reference, the electron does not move. Does it create a magnetic field?

    No, it does not. It creates an electric field, due to its negative charge, but that's all.

    This is explained by recognizing that electric and magnetic fields are two facets of the same phenomenon, the electromagnetic field. By moving to a different frame of reference, the relative contributions of "electric" and "magnetic" may change, and in this case, the "magnetic" part is zero--but the electric part is also changed, so that the total force is the same (with some small corrections near the speed of light). This was recognized even in the second half of the 19th century, when the term "electromagnetic field" arose, but Einstein's theory of relativity modified it and found the proper relationship which holds up to the velocity of light.

    In Einstein's theory, electric and magnetic fields are parts of the same 4-dimensional entity (a tensor, if you need a label), and the way we observe it can be worked out for any velocity.

(1st of 2 questions)

    I assume you have a PhD. in Physics. This is rather alarming. Any truth to this?

REPLY

    I am sorry if you are alarmed by my physics degree! As for the Earth's core, you can rest easy. Still, I am curious to know what was that "science program on TV" and who prepared it, because it is rather misleading.

    One thing in it is correct: the main north-south magnetic field of the Earth is weakening, and the current trend goes to zero in about 1200 years. It is not clear if that trend will last that long--the field is changing all the time; if it does, the north-south polarity may indeed reverse, as has happened many times in the geological history of Earth. Even then, the field probably does not vanish. Other more complex magnetic components are currently growing stronger. All we would get is a smaller and different magnetosphere.

    Other claims are wrong. The core of the Earth is probably iron, certainly not uranium: there is just a little uranium around--also thorium and potassium, radioactive elements which provide heat in the interior of the Earth and should last billions of years. As for the solar wind--its particles can penetrate the atmosphere just a small bit (currently they only come down in two small patches near the poles). Maybe given billions of years it could erode the atmosphere--although it has not happened to Venus, which has a thick atmosphere but no magnetic field to protect it, and which faces a more intense solar wind, being closer to the Sun

    Yes, you can, though there is a limit to the amount of concentration. I suppose you have already read about magnetic fields in "Exploration of the Earth's Magnetosphere" and know how they can be described by magnetic field lines (lines of force).

    Where these lines are far apart, the field is weak, while where they are close together, it is strong. For instance, the field lines of the Earth are far apart where they cross the plane of the equator, then get close together as they approach the poles, where the field is strong.

    Iron and similar "ferromagnetic" substances channel magnetic field lines inside them, not letting them out. So on laboratory magnets you may see conical "pole pieces" of iron facing each other, to concentrate the magnetic field lines and the magnetic field, guiding them into a smaller volume, where the field is stronger.

Gravity and magnetism are not connected (although you are by far not the first to feel that maybe they were).

Magnetism is the effect of electric currents: see for instance
        http://www.phy6.org/Education/wmfield.htm
and the historic section following it.

    Gravity is the property of massive matter. That was shown in the lab in 1796 by Henry Cavendish--look it up on
        http://www.phy6.org/stargaze/Sgravity.htm and in more detail by Roland (or Lorand) Eotvös
        http://www.phy6.org/stargaze/Smass.htm

    After going through your article on Planetary Magnetism
        (http://www.phy6.org/earthmag/planetmg.htm),
I just had a question as to how can it be known that a particular planet has a strong or mild magnetic field? Is there any specific device by which we can measure it or its just an inferential.

REPLY

    Spacecraft that explore space around other planets usually carry magnetometers--instruments to measure the magnetic field. They are so sensitive that they are usually put at the end of long booms, to keep them away from the influence of electric instruments on the spacecraft itself. The one on the Voyager spacecraft was 13 meters long, looking like a folding tower--see
        http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1977-076A
By measuring the distribution of the strength of the magnetic field around the planet, one can calculate the total source strength ("magnetic moment") of the planetary magnetic field.

    I was examining a kilowatt hour meter removed from a home for routine maintenance. On either side of the rotating aluminum disk are permanent magnets that can be adjusted to add drag to the disk. The disk is driven by a coil who's field strength is regulated by the current being drawn by the home.

        a) How is aluminum(non ferrous) driven by an electromagnet.

        b) how do the permanent magnets (which are arranged like caliper brakes on a bicycle wheel) retard the disk from moving?

    Thank you for your time, your website is most informative.

REPLY

    Good for you--you want to know! The key phrase here is "eddy currents"

    Actually, I do not have a precise answer to your question, but I hope you know that magnetism is not always the force between iron magnets, or associated with iron at all. It is fundamentally the force between electric currents, and the involvement of iron ("ferromagnetism") is an interesting sideshow of nature. In the technology of our society, of course, it is enormously important to applications of magnetism, but out in space (which I used to study) currents are almost always carried by free electrons and ions, with no iron in sight.

    I wondered if you see or have conceived a link between geomagnetism and the changes in the earths climate.

REPLY

    The energy involved in the climate is much, much greater than what creates the Earth's magnetic field. Plus, since the atmosphere is an electric insulator, it's hard to conceive a way for it to interact with climate.

    The amount of matter contributed by humanity to the atmosphere is also small. However, molecular gases (like methane, carbon dioxide and water vapor) have a great effect on the opacity of air to infra-red radiation, out of proportion to their actual amount. So it is possible for them to have an appreciable effect.

    Computers have simulated global warming. Also, there is no question that the amount of carbon dioxide, which is constantly being monitored, is rising. Arctic sea ice in the northern polar cap is breaking up earlier and in general climate is warmer--not just this winter, which is unusually warm where I live, but also statistics of the distribution of the 10 warmest winters on record, etc.

    The situation is complicated by the fact that climate itself is subject to fluctuations, which are not sufficiently understood--say, the warming of the western Pacific, which triggers of "El Niño." When the warming trends were first noticed, some people attributed it to a fluctuation and predicted it would fade again. But it seems less and less so now. And even without the evidence of the last decade, it always pays to err on the side of caution.

Your answer depends very much on what you mean by "magnetic field."

A steady field like that of a magnet--no. A spark does not generate one.

    I am involved in the oil and gas industry in Western Canada particularly in the fracture stimulation of oil and gas wells. I am looking for an alternate method to detect the induced fractures in the formation. As a background if you are not familiar with the technique, high pressure pumpers pump a slurry down the wellbore through holes in the steel casing over the zone of interest. They pump at high enough pressures to fracture the rock and fill the fractures (1 to 10mm wide) with the sand laden slurry. We currently add a radioactive tracer to this slurry and run in the wellbore after the fracture treatment is done to "see" where the fracture has gone. Typically these fractures can extent 1 to 500 meters away from the wellbore and grow vertically 1 to 100 meters in height. The current method of fracture detection only identifies the radioactive material up to 20 inches away from the wellbore. It would be advantageous to be able to map the direction, length and width of the fractures for their full length.

    Based on this limited explanation do you see potential for some type of magnetometer to be useful in this application. Keep in mind there is steel casing in the wellbore. As well we could add magnetic particles to the slurry which could in effect make the fracture a magnetic conduit?

REPLY

2 February 2006

    I tried hard to figure out a way magnetometers could help you, but could not find any. Even with highly magnetized material, the field gets rapidly weaker with distance, and quickly gets undetectable..

    If the slurry were highly conducting electrically, you could perhaps put a high voltage pulse on the pipe and see how "echoes" of the pulse is received at various locations on the ground--a bit like an EKG. However... high voltage electricity near oil and gas is not a good idea, and electrical properties of the slurry are probably not much different from those of the surrounding rock.