Magnetic units

The magnetic field strength established by an electric current, is given the symbol B in elementary work. The unit is the Tesla. An older unit of a more convenient size, the Gauss, is equivalent to 1x10^-4 Tesla.

Strictly speaking, in more advanced work, the magnetic field strength of a current in free space is defined not as B, but as H. The symbol B, is reserved for a quantity called magnetic induction.

In free space the magnetic induction B is given by ...

B = mo H

The constant mo is called the permeability of free space, defined as ...

mo = 4p x10^-7 Vs/Am

In the presence of magnetic materials, magnetic induction B, is given by ...

B = mr mo H

... where mr is the relative permeability.

The value of mr varies from a little less than one for diamagnetic materials, to over 1000 for specially prepared soft iron, known as m-metal. (In some treatments the product mr mo is quoted simply as m for the given material.)

Note: this explanation, and the definition of H is not required for IB examination purposes. Magnetic field strength is defined simply as B in units of Newtons per Amp meter (the Tesla).