Magnetic field of a long wire
Magnetic fields arise from charges, like electric fields; but they are different in that the charges must be moving.
A long straight wire carrying a current
is the simplest example of moving charge generating a magnetic field.
The magnetic field lines form circles around the wire. The strength
of the field depends only on the current
and the distance from the wire
and is given by
x 10 -7
is called the magnetic permeability of vacuum. [The fact that it is an
exact constant is tied up with the definition of the unit of charge
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