The earth's magnetic field It is the magnetic effect that the Earth exerts and that extends from its interior to hundreds of kilometers in space. It is very similar to that produced by a bar magnet. This idea was suggested by the English scientist William Gilbert in the 17th century, who also observed that it is not possible to separate the poles of the magnet..
Figure 1 shows the Earth's magnetic field lines. They are always closed, go through the interior and continue on the outside, forming a kind of cover.
The origin of the Earth's magnetic field is still a mystery. The outer core of the earth, made of cast iron, cannot by itself produce the field, since the temperature is such that it destroys the magnetic order. The temperature threshold for this is known as the Curie temperature. Therefore it is impossible for a large mass of magnetized material to be responsible for the field.
Having ruled out this hypothesis, we must look for the origin of the field in another phenomenon: the earth's rotation. This causes the molten core to rotate non-uniformly, creating the dynamo effect, in which a fluid spontaneously generates a magnetic field..
It is believed that the dynamo effect is the cause of the magnetism of astronomical objects, for example that of the Sun. But until now it is unknown why a fluid is able to behave in this way and how the electrical currents produced manage to stay.
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- The Earth's magnetic field is the result of three contributions: the internal field itself, the external magnetic field and that of the magnetic minerals in the crust:
- The magnetic field is polarized, presenting north and south poles, much like a bar magnet.
- As the opposite poles attract each other, the compass needle, which is its north pole, always points to the vicinity of geographic north, where the south pole of the earth's magnet is..
- The direction of the magnetic field is represented in the form of closed lines that leave the magnetic south (north pole of the magnet) and enter the magnetic north (south pole of the magnet).
- In the magnetic north - and in the magnetic south as well -, the field is perpendicular to the earth's surface, while at the equator, the field is grazing. (see figure 1)
- The field intensity is much higher at the poles than at the equator..
- The axis of the terrestrial dipole (figure 1) and the axis of rotation are not aligned. There is a displacement of 11.2º between them.
Since the magnetic field is vector, a Cartesian coordinate system XYZ with an origin O helps to establish its position.
The total intensity of the magnetic field or induction is B and its projections or components are: H horizontally and Z vertically. They are related by:
-D, the angle of magnetic declination, formed between H and geographic north (X-axis), positive toward the east and negative toward the west.
-I, the angle of magnetic inclination, between B and H, positive if B is below horizontal.
The compass needle will be oriented in the direction of H, the horizontal component of the field. The plane determined by B and H is called the magnetic meridian, while ZX is the geographic meridian.
The magnetic field vector is fully specified if three of the following quantities are known, which are called geomagnetic elements: B, H, D, I, X, Y, Z.
Here are some of the most important functions of the Earth's magnetic field:
-Humans have used it to orient themselves by compass for hundreds of years.
-It exerts a protective function of the planet, by enveloping it and deflecting the charged particles that the Sun continuously emits.
-Although the Earth's magnetic field (30 - 60 micro Tesla) is weak compared to those in the laboratory, it is strong enough that certain animals use it to orient themselves. So do migratory birds, homing pigeons, whales and some schools of fish.
-Magnetometry or magnetic field measurement is used for prospecting mineral resources.
They are known as the northern or southern lights, respectively. They appear at latitudes near the poles, where the magnetic field is almost perpendicular to the earth's surface and much more intense than at the equator..
They have their origin in the large amount of charged particles that the Sun sends continuously. Those that are trapped by the field generally drift towards the poles, because of the higher intensity. There they take advantage to ionize the atmosphere and in the process visible light is emitted.
The northern lights are visible in Alaska, Canada, and northern Europe, due to the proximity of the magnetic pole. But due to the migration of this, it is possible that over time they become more visible towards the north of Russia.
Although this does not seem to be the case for now, as the auroras do not exactly follow the erratic magnetic north..
For navigation, especially on very long trips, it is extremely important to know the magnetic declination, in order to make the necessary correction and find the true north.
This is achieved by using maps that indicate the lines of equal declination (isogonal), since the declination varies greatly depending on the geographical location. This is due to the fact that the magnetic field experiences local variations continuously..
The large numbers painted on the runways are the directions in degrees relative to magnetic north, divided by 10 and rounded..
Confusing as it may seem, there are several types of north, defined by some particular criteria. Thus, we can find:
Magnetic north, it is the point on Earth where the magnetic field is perpendicular to the surface. There the compass points, and by the way, it is not antipodal (diametrically opposite) with the magnetic south.
Geomagnetic north, it is the place where the axis of the magnetic dipole rises to the surface (see figure 1). As the Earth's magnetic field is a bit more complex than the dipole field, this point does not exactly coincide with magnetic north..
Geographic north, the axis of rotation of the earth passes through there.
North of Lambert or the grid, it is the point where the meridians of the maps converge. It does not exactly coincide with true or geographic north, since the spherical surface of the Earth is distorted when projected onto a plane.
There is a puzzling fact: magnetic poles can change position in the course of a few thousand years, and it is currently happening. In fact, it is known to have happened about 171 times before, in the last 17 million years..
The evidence is found in rocks emerging from a rift in the middle of the Atlantic Ocean. As it comes out, the rock cools and solidifies, setting the direction of the Earth's magnetization for the moment, which is conserved.
But so far there is no satisfactory explanation of why it happens, nor where does the energy necessary to invert the field come from..
As previously discussed, magnetic north is currently moving rapidly towards Siberia, and the south is also moving, albeit more slowly..
Some experts believe it is due to a high-speed flow of liquid iron, just below Canada, that weakens the field. It may also be the beginnings of a magnetic reversal. The last one that happened was 700,000 years ago.
It may be that the dynamo that gives rise to the terrestrial magnetism turns off for a time, either spontaneously or due to some external intervention, such as the approach of a comet for example, although there is no evidence of the latter..
When the dynamo restarts, the magnetic poles have switched places. But it can also happen that the inversion is not complete, but a temporary variation of the axis of the dipole, which will finally return to its original position..
It is carried out with Helmholtz coils: two identical and concentric circular coils, through which the same intensity of current passes. The magnetic field of the coils interacts with that of the Earth, giving rise to a resulting magnetic field.
An approximately uniform magnetic field is created inside the coils, the magnitude of which is:
-I is the intensity of the current
-μor is the magnetic permeability of the vacuum
-R is the radius of the coils
-With a compass placed on the axial axis of the coils, determine the direction of the earth's magnetic field BT.
-Orient the axis of the coils so that it is perpendicular to BT. In this way the field BH generated as soon as the current is passed, it will be perpendicular to BT. In this case:
-BH is proportional to the current passed through the coils, so that BH = k.I, where k it is a constant that depends on the geometry of these coils: radius and number of turns. When measuring the current, you can have the value of BH. So that:
BH = k.I = BT. tg θ
Therefore:
-Various currents are passed through the coils and the pairs (I, tg θ).
-The graph is made I vs. tg θ. Since the dependence is linear, we expect to obtain a line, whose slope m it is:
m = BT / k
-Finally, from the adjustment of the line by least squares or by visual adjustment, we proceed to determine the value of BT.
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