Magnetic Flux Density

What is Flux Density?

As shown in figure (a), the flux density is the number of magnetic fields lines per unit area of a section perpendicular to the direction of flux. Flux Density symbol is B, as a formula

B = Φ/A

Where Φ is the flux through an aria A, and the flux density is B

Flux Density, Magnetic Flux Density, Unite of Magnetic Flux Density, Flux Density Units

Flux Density Units:


In the cgs system, this unit is one line per square centimeter, or 1 Mx/cm2. As an example in figure (a), the total flux Φ is 6 lines, or 6 Mx.  At point P in this field, however, the flux density B is 2 G because there are 2 lines per cm2.  The flux density has a higher value close to the poles, where the flux lines are more crowded.

As an example of flux density, B for a 1 lb magnet would be 1000 G at the poles.  This unit is named for Karl F. Gauss (1777-1855), a German mathematician.


With a flux density of 10,000 Mx through a perpendicular area of 5 cm2, what is the flux density in Gauss?


According to Flux density formula      B = Φ/A

Then                                                    10,000 Mx/5cm2 = 2000 Mx/cm2

                                                            B = 2000 G

As a Typical value, B for the earth’s magnetic field can be about 0.2 G: a large laboratory magnet produces B of 50,000 G.  Since the Gauss is so small, it is often used in kilogauss units, where 1 KG = 103 G


In SI, the unit of flux density B is webers per square meter (Wb/m2).  One Weber per square meter Is called a Tesla, abbreviated T. This unit is named for Nikola Tesla (1857-1943), a Yugoslav-born American inventor in electricity and magnetism.

When converting between cgs and mks units note that

1 m = 100 cm or 1 × 102 m

1 m2 = 10,000 cm2 or 104 cm2

These conversions are from the larger m and m2 to smaller units of cm and cm2. To go the opposite way.

1 cm = 0.01 m or 1 × 10-2 m

1 cm2 = 0.0001 m2 or 1 × 10-4 m2

As an example, 5 cm2 is equal to 0.0005 m2 or 5 × 10-4 m2. The calculations for the conversion are

5 cm2 × 0.0001m2/cm2 = 0.0005m2



In powers of 10, the conversion is

5cm2×  1×10-4m2/cm2 = 5×10-4m

In both cases, note that units of cm2 cancel to leave m2 for the desired units.


With the flux of 4000 μWb through an area of 0.0005m2, what is the flux density B in tesla units?



B = Φ/A = 4000×10-6Wb/5×10-4m2

                  =400/5 × 10-2

            = 80 × 10-2 Wb/m2

 B = 0.80 T



The Tesla is a larger unit than the Gauss, as

1 T = 1 × 104 G                      


 For example , the flux density of 20,000 G is equal to 2 T. the calculations for this conversion are

20000 G/1×104 G/T  =  2×104T/1× 104  = 2T

Note that the G units cancel to leave T units for the desired answer.  Also, the 1/T in the denominator becomes inverted to T units in the numerator.

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