**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
*
I
*
and the distance from the wire

**
B =
***m*_{0
}
I / (2
*p**
r)
*

The constant
m_{0}
= 4
*p**
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
(Coulomb).]

*
Examples
Magnetic field sources index
Lecture index*