# Permeability of free space

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:

where

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

Energy

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

where

• is the Magnetic force,
• and are two currents, and
• is the separation between the wires carrying the currents.