Magnetism
The basics of magnetism relies on the principal that magnetic fields
are created by moving charged particles. Even "permanent" magnets are
caused by "circulating" charges inside the magnet. Because the force
from magnets act at a distance we will again consider the concept of
fields to describe the source of the force. Magnets behave in a manner
very similar to charged particles in that opposites attract, like repel
and that the force is proportional to the inverse square of the
distance. Magnets have a NORTH and a SOUTH instead of positive and
negative. The north pole of the magnet is the one that will point to
the north magnetic pole of the earth.
Magnetic field lines go out of the north and into the south. One
fundamental difference between static electricity and magnetism is that
all known magnets are dipoles. This means that the number of field
lines drawn out of the north must equal the number of lines going into
the south. In terms of flux (the number of field lines passing through
a surface), the flux out of the magnet is ballanced by the flux into
the magnet so the total flux = 0. This fact is called Gauss's Law of
Magnetism.
Because magnetism is caused by moving charged particles, magnetic
fields will create forces on charged particles. To determine the
direction of these we use the right hand rule. The right hand rule that
I will describe is different from the vector
cross product rule that is done with the math equations, but it
works well to predict the direction of the components. To begin, each
of the three parts are at right angles to each other. The thumb of the
right hand represents the direction of the velocity (thumb a ride), the
index finger represents the direction of the magnetic field (point
north) and the other three fingers represent the direction of the force
(with considerable strain they can form the letter F). We use the right
hand for positive charges and the left hand for negative charges.
If moving charged particles create a magnetic field. The question "can
a magnetic field move charged particles?" The answer is yes - ONLY IF
the magnetic field is changing. This changing magnetic field induces a
voltage in a circuit. The rules for the directions are the same as
above where the force becomes the electromotive force (EMF) or voltage,
the same direction as the current that is produced.
The ability to "induce" a current brings in the concepts of Lens' Law,
a special consequence of the law of conservation of energy, and the use
of transformers in AC circuits. Students should know how transformers
work and the advantages of being able to change the voltage in long
distance power transmition lines.
Links
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