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Question

What is a magnetic field?

Answer

Fields fill the space between matter and they determine how it is that bits of matter can exert forces on other bits of matter at a distance. There are several different fields in nature, and their reality is demonstrated by our observation of the forces with which they are associated. So, for example, gravitational fields determine how it is that objects with mass are attracted together by a gravitational force. Electric fields determine how it is that objects with electric charge are attracted together by an electric force, if they have opposite electric charge, or repelled from each other, if they have the same electric charge. Interestingly, unlike an electric field, a magnetic field only comes into play when electric charges are moving. Magnetic fields determine how it is that electric currents, composed of moving electric charges, exert forces on other electric currents. Consider, then, two parallel wires, each with an electric current flowing in the same direction. By virtue of the magnetic field, they will be pulled toward each other, they experience an attractive force. If the currents are flowing in the opposite direction, then there will be a repulsive force between the wires. More generally, magnetic fields are generated by electric currents, the motion of electric charges, and, conversely, electric currents and the motion of electric charges can be induced by time-dependent magnetic fields. In fact, an electric generator works by the motion of magnetic fields.

Question

What is a permanent magnet?

Answer

Most material is non-magnetic. It is composed of molecules made of atoms, each of which have electrons orbiting nuclear protons, but where the motion of one electron, essentially a tiny electric current, generates a magnetic field that is cancelled by the magnetic field generated by the motion of another electron. In magnetic materials this cancellation is incomplete, and so the atoms of the material have small net electric currents and they thus generate small magnetic fields. For various reasons having to do with the intricacies of atomic physics, this tends to happen for certain substances, like cobalt, nickel, and, of course, iron. Within these magnetic materials, the magnetic fields of the various atoms exert torques on the electric currents of their neighboring atoms, causing the atoms to align and their magnetic fields to add together constructively. As a result, the material exhibits a magnetic field. It is a ‘magnet’. Most magnets are like the bar magnet shown in the illustration, having a simple 'dipole' arrangement of a 'north' pole, where the field diverges, and a 'south' pole, where the field converges.

Question

What is declination?

Answer

At most places on the Earth's surface, the compass doesn’t point exactly toward geographic north. The deviation of the compass from true north is an angle called 'declination'. It is a quantity that has been a nuisance to navigators for centuries, especially since it varies with both geographic location and time. It might surprise you to know that at very high latitudes the compass can even point south! Declination is simply a manifestation of the complexity of the geomagnetic field. The field is not perfectly symmetrical, it has non-dipolar ‘ingredients’, and the dipole itself is not perfectly aligned with the rotational axis of the Earth. Interestingly, if you were to stand at the north geomagnetic pole, your compass, held horizontally as usual, would not have a preference to point in any particular direction, and the same would be true if you were standing at the south geomagnetic pole. Moreover, if you were to hold your compass on its side the north-pointing end of the compass would point down at the north geomagnetic pole, and it would point up at the south geomagnetic pole. Maps of declination, such as that shown below (contours of 10 degrees east), as well as other field components, and a program for determining the magnetic field at any geographic location, are given in the Models, Charts, and Movies pages of this website

Question

Is the Earth a magnet?

Answer

In a sense, yes. You probably know that the Earth is stratified; a section is pictured here. In radius it is composed of layers having different chemical composition and different physical properties. The crust of the Earth has some permanent magnetization, and the core of the Earth, the outer part of which is liquid iron and the inner of which is solid iron, generates its own magnetic field, sustaining the main part of the field we measure at the surface. So we could say that the Earth is, therefore, a ‘magnet’. But there is no giant bar magnet near the Earth’s center, despite the depictions you may have seen in elementary textbooks on geology and geophysics. Permanent magnetization, such as discussed in question 2, cannot occur at high temperatures, like temperatures above 650 degrees centigrade or so, when the thermal motion of atoms becomes sufficiently vigorous to destroy the ordered orientations needed to establish permanent magnetization. The core of the Earth has a temperature of several thousand degrees, and so, even though the core is the source of most of the geomagnetic field, it is not, itself, permanently magnetized.

Question

How does the core generate a magnetic field?

Answer

This is explained, in general terms, in the Introduction to Geomagnetism page given on this website. Briefly, then, as the result of radioactive heating and chemical differentiation, the outer core is in a state of turbulent convection. This sets up a process that is a bit like a naturally occurring electrical generator, where the convective kinetic energy is converted to electrical and magnetic energy. Basically, the motion of the electrically conducting iron in the presence of the Earth's magnetic field induces electric currents. Those electric currents generate their own magnetic field, and, as the result of this internal feedback, the process is self-sustaining, so long as there is an energy source sufficient to maintain convection. The depiction of the geodynamo shown here is only schematic; in fact, the fluid motion and the form of the magnetic field inside the core are still the subject of intensive research.