Magnetic permeability and the definition of the amp, Electrical Engineering

The ampere has been defined (by the International Committee of Weights and Measures in 1946) as the current flowing in two long parallel wires 1 metre apart that produces a force of 2 x 10-7 Newtonsper metre length between the wires.

1884_magnetic permeability.png

So with the same current flowing in both wires spaced 1 metre apart, it is easy to increase the current until a force of 2 x 10-7 Newtons is measured between them. We then know that the current flowing is exactly 1 amp (by definition). This definition also gives the value of k = 2 x 10-7  Newtons.metre/amp

If we express one of the currents in terms of the magnetic field that it produces, then we obtain an equation for the force per metre lengthexperienced by a current carrying conductor in a magnetic field. This is by far the most useful form of the equation in practice:

897_magnetic permeability1.png

Since   k = 2 x 10-7  then we write the new constant as  µwhere

µ0 =  4π x 10-7 Henrys/metre  

(note the Henry is a unit of measure of inductance

µ0 is known as the magnetic permeability of free space

Posted Date: 8/27/2012 4:17:42 AM | Location : United States

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