Get a Straight Answer

    Your question is not a simple one, and has in the past confused quite a few people. A rough outline of the answer follows below; it is not exactly simple, and I can only hope that you and your son will have the patience to follow it to the end.

    I take it you refer to the axially symmetric parts of the field. The observed field also has many irregularities, and these certainly rotate with the Earth. For instance, the north magnetic pole (and the south one, too) is separated by something like 1000 kilometers from the geographic pole, the pole around which the Earth rotates. And indeed, the magnetic pole rotates every day a full 360° around the geographic one. What you have in mind--what your experiment involves--is something else: a symmetric field rotating around it s axis of symmetry.

    If you have a source of magnetic field in empty space--iron magnet, coil--and you rotate it around an axis of symmetry, there is no extra effect, and therefore, nearby objects will not feel any force to make them share the rotation.

    For instance, if you have a bar magnet and twirl it around its length, around the line connecting the poles, you get no observable change. At any point in the surrounding space, the magnetic force sensed (say, by a compass needle) is not changed by the rotation.

    All that is true in empty space. And to a very good approximation, it also holds if the space contains substances which do not conduct electric currents--air, wood, paper, glass etc. In all these cases, just having the source of magnetism rotate has no measurable effect.

    But if space is filled with a substance which can conduct electricity, and the rotating magnet also conducts electricity, the situation is different. Under certain conditions, electric currents may then be produced, and in that case, two effects are added:

• First electric currents are SOURCES of magnetic fields, and therefore the magnetic field may be modified.
• Secondly--more important here--magnetic fields exert a FORCE on the carrier of electric current, and in this case, in general, that force tends to make it share the rotation.

    Space around Earth--except for the lower atmosphere--does conduct electricity. In the ionosphere--say 120 kilometers up (70 miles) and higher, sunlight rips off electrons from atoms, creating a "plasma," a mixture of free floating electrons and positive ions (left-over atoms, which miss an electron or more), and a plasma conducts electricity. If a free electron collides with an ion, the two may recombine again--but densities are so low that in most regions recombination does not catch up with "ionization" (the break up of atoms), even during the night, when the ripping-off of electrons stops. Also, higher up in space additional sources of plasma exist, e.g the "solar wind" blowing from the Sun.

    Such magnetic forces try to make the plasma "co-rotate" with the Earth. Their effect turns out to be the same as would occur, if magnetic field lines which thread the plasma are viewed as attached to it. Then everything--Earth, plasma and field lines attached to it--rotate together. Once more--be aware that this only happens when space is filled with a good conductor of electricity. Without electric conductivity, the magnetic field does not transmit rotation. Iron does conduct electricity, but iron filings scattered on paper are not enough, they do not provide a good enough path for electric currents.

    I guess this is about as far as one can go with an 8-year old. As for the "simple experiments on magnetism" you conduct with him, I would recommend three more, described in
            http://www.phy6.org/earthmag/MagTeach.htm

    For your own interest--if you want to know more about the way rotation CAN produce electric currents--look up the sections on dynamos in "The Great Magnet, the Earth", home page
            http://www.phy6.org/earthmag/demagint.htm

    The reason, of course, is that the two are quite different. The bullets are chunks of matter, and to let them through, the matter of the armor has to give way, something it strenuously resists. The sound is a wave: the bullets make the armor oscillate and propagate the wave through it, without breaking its integrity.

    Similarly with particles and electromagnetic radiation from the Sun (or from any source): they are completely different. The issue is confused by the fact that streams of high-energy particles are also popularly called "radiation," as in "radiation belts" and "alpha-radiation" (or "beta radiation") from radioactive substances. There are historical reasons for this usage, but unfortunately, it is now too deeply rooted to be easily changed.

    Particles found in space carry an electric charge, and a moving electric charge can be viewed as equivalent to an electric current. Electric currents react to magnetic forces--in fact, magnetism may be viewed as interactions between electric currents--so it's no wonder charged particles get deflected, sometimes even trapped.

    Magnetic forces are more complicated. Imagine you have a bar magnet. A good approximation to its behavior is obtained by regarding it as a pair of "magnetic poles", an "N-pole" and an "S-pole." I won't use the words "north" and "south," because they confuse people--see

    Most flames are yellow. The main reason is that flames contain little bits of burned material--bits which later form its smoke--and they get hot enough to glow. Hot solid materials always glow--for instance, the filament in a lightbulb. However ,glowing in yellow light does not require a very hot temperature.

    To create a plasma takes more energy, and requires a higher temperature than the flame provides. The collisions between atoms need to be energetic enough to kick an electron completely out of the atom.

    An electric arc welder drives a huge current across a narrow juncture where two pieces of metal touch, and that creates a temperature high enough to create a plasma. The surrounding air is also hot enough. After touching the two pieces can be separated, and the air continues to carry the electric current, and to heat enough to create the plasma. The metal tip glow so brightly (white light, with a lot of eye-damaging ultra-violet) that the welder can only view the work through a thick dark screen.

For more about this, look up one of two files (they are identical)

• http://www.phy6.org/Education/wimfproj.htm
• http://www.phy6.org/stargaze/Simfproj.htm

    Whether the magnetic field dominates the flow of a plasma, or the flow dominates the magnetic field, always depends on one question--what dominates the energy density, the plasma or the magnetic field?

    If the field does, its structure is relatively rigid and it determines where particles can and cannot go. The inner radiation belt of Earth is a good example--dominated by the dipole structure of the Earth's field.

    If the plasma has a higher energy density than the magnetic field, its flow is relatively undisturbed, and instead, it deforms the magnetic field to suit its motion. This is the case in the solar wind.

(What if the two are comparable? We then may get complex physics, as in the Earth's outer magnetosphere!)

    Because the solar wind dominates, it drags out solar magnetic field lines (as the above web site shows), in a spiral due to the Sun's rotation, which gets more and more circular. The wind itself does not follow field lines but continues to move radially. (At the end of the above web page, however, is a story of a different population of particles, high energy particles from an eruption on the Sun, which--because their number is few--do follow the field lines.)