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 Posted: Tue May 27, 2008 7:40 pm
Do magnetic fields react with each other and produce a force? What are the electrons in a magnet doing, when approached by another magnetic field?
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Posted: Wed May 28, 2008 8:43 pm
Magnetic fields themselves don't really interact (one can't say that they don't interact per se, but nothing of any interest happens).
Magnetic fields are caused by charges moving and in turn they cause moving charges to accelerate perpendicular to their direction of motion.
Electrons can be thought of as moving in tiny circles, thus causing tiny magnetic fields. Usually, the circles are all askew relative to each other, so the magnetic fields cancel out. In a magnet, the circles are aligned so the magnetic fields add, causing magnetism.
When a magnetic field from a magnet approaches another magnet, it causes the electrons in the second magnet to feel a force perpendicular to their direction of motion; since they are moving in tiny circles, the second magnet feels a force either towards or away from the first magnet.
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Posted: Mon Jun 02, 2008 1:38 pm
very interesting. now, what happens when electromagnetic radiation is passing a material? How does some radiation like radiowaves pass walls into homes?
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Posted: Mon Jun 02, 2008 8:54 pm
The electromagnetic force is carried by particles called photons; any changes in the electromagnetic field, either from a charged particle accelerating or from a magnetic field changing, causes photons to form.
Because of the nature of photons, they can pass easily through atoms if they have high enough energy. Atoms can absorb photons, but only if those photons have specific amounts of energy; different elements can absorb different energies. If bunch of photons have low energy, they might all be absorbed together if their total energy matches one of the possible absorption energies, although this is somewhat improbable.
If a photon has too high energy, then it can't be absorbed at all, and will pass through the atom completely. This is how x-rays work.
Radio waves are very low energy, so one might think that they would get stopped by the atoms of the walls; fortunately, most of the photons blocked by wall materials are those that correspond to visible light, not radio waves, which pass through because they are low energy.
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Posted: Sat Jun 07, 2008 12:50 am
Layra-chan's discussion on charges and magnetic fields is spot on, although the behavior of EM waves, although not incorrect, is a bit misleading. But classically, both your questions have a common answer. Magnetic fields are ultimately tied to moving charge; in a magnet, there is still bound currents as electrons within the magnet move about. They don't have to leave the atoms to do so--for example, shifts in the spins of unpaired electrons are responsible for paramagnetism, while changes in the electron orbits themselves are account for diamagnetism. This means that the magnetic field (B, the actual field) is different from the applied field (H, corresponding to the to the contribution of the free currents alone, i.e., sans magnetization), and their relationship is called magnetic permeability. Something analogous happens to the electric field, with the corresponding relationship being called permittivity. For 'nice' materials, this is simply a constant; for others, things are lot more complicated.
In any case, the magnetization field M of a magnetized medium is the magnetic dipole density. The force on a dipole in immersed in a prior field B is (m×∇)×B = ∇(m·B), where m is the magnetic dipole moment. This easily generalized for cases where the magnet is permanent with magnetization relatively independent of the applied field, with M·B as the magnetic potential energy density. If the magnetization is not permament and the material is close to linear (i.e., not strongly ferromagnetic), the potential energy is actually half that.
As for the behavior of EM waves in a material, classically, the critical parameters of permeability and permittivity change, which affects the propagation of the wave. Intuitively, it's analogous to a sound wave in air suddenly meeting water--the mechanics of its propagation are now different because the material properties are different. For example, the speed of the wave is 1/sqrt(εμ), where ε is permittivity and μ is permeability. From geometrical considerations alone, it is easy to show that if the speed of the wave changes, it must bend (cf. Snell's law), and so we get effects like refraction and so on. These changes may be dependent on frequency, so an object may be transparent to some frequencies but not others.
At the photon level, it's fairly true that atoms behave differently for photons of different energies, effectively absorbing only some of them and allowing others to pass. But actually, the electrons in the atoms can absorb photons of all kinds, it's just that if the photon does not have the correct energy to take them to another energy eigenstate, they must immediately re-emit it. Thus, the effect is that the material is transparent to the EM wave, but the absorption and re-emission is not instantaneous, so that the effective speed of light in the material will be slower than that of vacuum. Thus, we still get effects like refraction. [Note: in case it isn't clear, wave frequency<-->individual photon energy.]
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