Magnetic Induction vs. Magnetic Field
Magnetic induction and magnetic field are often used synonymously. In many cases it is
easy to conclude from magnetic induction to magnetic field and vice versa.
The magnetic field H describes the field generated by a free current only, the magnetic
induction B describes the field generated by a current plus the effect of magnetization of
a material. Materials can decrease or increase the magnetic induction. They are then
called paramagnetic or diamagnetic materials.
| Unit System | Magnetic Induction B | Magnetic Field H |
| SI units | Tesla: 1 T = 1 Vs/Am2 | A/m |
| Older units | Gauss: 1 G = 10–4 T | Oersted: 1 Oe = 103/4Π A/m |
Conversion Table for Common Magnetic Units
| mT (Tesla) | G (Gauss) | kA/m | Oe (Oersted) | gamma |
| 1 mT | = 1.0000 | = 10.000 | = 0.7958* | = 10.000* | = 10^6 |
| 1 G | = 0.1000 | = 1.0000 | = 0.0796* | = 1.0000* | = 10^5 |
| 1 kA/m | = 1.2560* | = 12.560* | = 1.0000 | = 12.560 | = 1.2560*10^6 |
| 1 Oe | = 0.1000* | = 1.0000* | = 0.0796 | = 1.0000 | = 10^5 |
* in free air
Earth's Magnetic Field - some facts
Earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis
of the earth. Charged particles become trapped on these field lines, forming the magnetosphere.
The Curie temperature of iron is about 770 C - Earth's core is hotter than that and therefore not magnetic.
So why is there a magnetic field ?
Earth's magnetic field is attributed to a dynamo effect of circulating electric current in the
molten metalic core, but it is not constant in direction. Rock specimens of different age in similar
locations have different directions of permanent magnetization. A total of 171 magnetic field
reversals during the past 71 million years are reported.
Although the details of the dynamo effect are not known in detail, the rotation of the Earth plays a part
in generating the currents which are presumed to be the source of the magnetic field. Mariner 2 found
that Venus does not have such a magnetic field although its core iron content must be similar to that
of the Earth. Venus's rotation period of 243 Earth days is just too slow to produce the dynamo effect.
Interaction of the terrestrial magnetic field with particles from the solar wind sets up the conditions
for the aurora phenomena near the poles.
Earth's magnetic field lines are not as symmetrical as those of the bar magnet. The impact of the
solar wind causes the lines facing sunward to compress, while the field lines facing away from the
Sun stream back to form Earth's magnetotail. The magnetosphere extends into the vacuum of space from
approximately 80 to 60,000 kilometers (50 to 37,280 miles) on the side toward the Sun, and trails out
more than 300,000 kilometers (186,500 miles) away from the Sun.
Maximum and minimum field strength:
Bmax = 70μT Hmax=55.7A/m
Bmin = 20μT Hmin=15.9A/m
Last-Modified: Mon, 07 May 2007 18:00:29 GMT
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