Permeability of free space

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The permeability of free space, μ0, is a physical constant used often in electromagnetism. It is defined to have the exact value of 4π x 10-7 N/A2 (newtons per ampere squared).[1] It is connected to the energy stored in a magnetic field, see Hyperphysics for specific equations.

It is related to the speed of light by the equation:

[math]c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}[/math]


A magnetic field, [math]B[/math] in a region of space has field energy associated with it. That energy density is defined as:[2]

Energy [math]= \frac{B^2}{2 \mu_0} [/math]

One particular use is the magnetic force. The constant gives how strong the force is between two electric currents separated by a distance:[1]

[math]F = \frac{I_1 I_2}{2 \pi \mu_0 r} [/math]


  • [math]F[/math] is the Magnetic force,
  • [math]I_1[/math] and [math]I_2[/math] are two currents, and
  • [math]r[/math] is the separation between the wires carrying the currents.

Visit Hyperphysics for more information on this constant.

For Further Reading

For further information please see the related pages below:


  1. 1.0 1.1 Hyperphysics. (August 28, 2015). Electric field [Online], Available:
  2. Jackson, John David (1998). Classical Electrodynamics (3rd ed.). New York: Wiley. p. 213