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Topic 8 (Magnetism)

and how moving magnets push electric charge

moving charge is encircled by magnetic field ... magnetic field deflects moving charge

Magnetic Field exits a magnet's "North" pole , and enters a magnetic "South" pole

Opposite poles attract ; like poles repel
. . . a compass is a small bar magnet on a pivot;
      its magnetic North pole is labeled with "N" ... attracted to Magnetic South poles
. . . compass "N" points geographically Northward - toward Canada
=> "Magnetic South" pole is underneath Canada (magnetic North is below AntArtica)

Electric current flowing ccw around a coil makes a magnetic field
. . . magnetic field is most intense up thru the inside of the coil Area
      same number of loops down around the coil outside, but spread farther
. . . single current loop has one North face and one South face
      field looks just like a "button magnet" field
. . . second current loop in the same sense (cw/ccw) adds to that field
      (current loop in opposite sense cancels it)
. . . coil wrapped around iron core can be 100's of times as strong.

magnetic material contains atomic-scale current loops

If you break a magnet, and pull the 2 pieces apart,
. . . a new "South pole" and a new "North pole" appear in the middle.
. . . the "ends" of a magnet are not really the source for the Magnetic field
=> the entire magnetic material is the source for the Magnetic Field

each atom is a tiny magnet ... the valence electron orbits are most important
      but each proton is a magnet, each neutron is a magnet, each electron is a magnet)
. . . the electron's orbit axis can re-orient ... in a magnet, the orbits align along their neighbors' orbits
. . . their magnetic fields re-inforce one another ... add together constructively
      (c.f: when electric dipoles align, their Electric fields tend to cancel)
=> there is no such thing as an "isolated magnetic pole" ... always magnetic field pointing along an axis

Magnetic field intensity decreases with distance
      (as their field spreads, of course!)
. . . an object magnet will tend to turn ... when a subject magnet approaches
      so that its opposite pole is nearest the subject's approaching pole
. . . so magnets turn each other in a way that they will attract one another
      but the source dipole partly cancels, and the Force partly cancels
=> magnet-to-magnet Force goes as ~ 1/distance to the fourth
    so   magnets aligned oppositely will attract each other from the side
    and   magnets aligned alike will attract each other from either end.

electromagnet & motors & generators & transformers

generator: mechanical Energy input, turns a permanent magnet
. . . this pushes the (conduction) charges along a (wire) coil
      which "seem to be moving" , relative to the turning magnet
. . . while the magnetic pole tip sweeps across the coil material
      (that is, when their torque is maximum)
. . . the current flows from the coil carrying electric Power
      as (−) charge at (−) electric potential.
. . . some motors can also be used as a generator
. . . some generators can be used as a motor
=> "hybrid" cars can "brake" by having their magnets push charge into their batteries

as we push a magnet end (N pole) into a coil, the magnet-in-coil acts as a generator
. . . the outward magnet field sweeps across the coil wire,
      and pushes charge around the coil
      ... (N end in ⇒ counter-clockwise current ... S in ⇒ cw current)
. . . as the magnet is removed, the current is pushed oppositely.

transformer: a "generator" that uses an electro-magnet input, instead of a permanent magnet
. . . many-loop coil (input) makes intense magnetic field even with small current
      (its long wire impedes current, even if the voltage is high)
. . . few-loop coil at output has lots of current, even if not much voltage
      it is the number of loops that determines the voltage
. . . Power in ≈ Power out ; they are often over 90% efficient
      wall : 0.2 Amp · 120 Volt = 4 Amp · 6 Volt , at battery

a transformer requires changing current ... so they do not work with DC ...
. . . some DC-DC converters make their input current change
      by "switching" the input circuit on and off

here's a map of where the Magnetic South Pole has been . . . wandering around the North geographic (spin) pole       it has been moving much faster than usual, recently . . . it has been getting less intense, also recently       mostly because it seems to be fragmenting South pole and North pole "switch" routinely in the geologic record . . . on average, every half million years it is over-due to reverse again . . . since the last reversal was ¾ million years ago.