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Electric and magnetic fields experiment

  1. Jan 1, 2009 #1
    I have a simple question here that I seem not to be able to figure out.

    If you have a regular bar magnet, it emits a magnetic field around it.
    So, if you create an experiment and found out how far that field reached, you would see that the field doesn't go very far. Maybe like 3 inches (~ 7cm). (This is for a regular, bought at the grocery store bar magnet).

    I would also bet that if you found a way to make a magnet that instead emitted an electric field, it would have the same result: not very far.

    So, if electromagnetic radiation is just these two fields put together, then why do they go much farther than electric and magnetic fields individually? Also, why do they carry energy, when individual electric and magnetic fields alone are just force?

    Thank you.
  2. jcsd
  3. Jan 1, 2009 #2


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    The static magnetic field of a magnet falls off like 1/r^3.
    Radiation fields of oscillating charges or magnets fall off like 1/r, and thus extend to much larger distances.
  4. Jan 5, 2009 #3
    Electromagnetic radiation and static E-field and B-field are completely different.
    Electromagnetic radiation is caused by accelerating charge while static fields are caused by static charge and steady current.
  5. Jan 6, 2009 #4


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    Eh, is there any reason why it's decreases inversely proportionate to r^3 rather than the more familiar r^2?
  6. Jan 7, 2009 #5

    Vanadium 50

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    I'm not sure that familiarity is a terribly good guideline.

    A monopole field falls as 1/r2. A dipole field as 1/r3. Higher moments fall faster still.

    You can prove this by calculating the field from a dipole as two oppositely charged monopoles a distance apart.
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