We explain what a magnetic field is, how it is produced and what its characteristics are. We also tell you about the Earth's magnetic field.
What is a magnetic field?
A magnetic field is the mathematical representation of the way magnetic forces are distributed in the space surrounding a magnetic source. This source can be a magnet, a moving charge, or an electric current (many moving charges). Whenever any of these elements exists, there will be a magnetic field around it, that is, a field of magnetic forces. Outside this field there are no magnetic effects.
A fundamental characteristic of magnetic fields is that are dipolar: they have a North pole and a South pole which are also called positive pole and negative pole. Unlike electric fields that can be generated by electric charges (such as an electron), there are no “magnetic charges” that generate magnetic fields. Magnetic fields always have two poles associated with them. As a consequence, the magnetic field lines are always closed, as in the case of the magnet: they leave the north pole and arrive at the south pole.
See also: Gravitational field
Origin of a magnetic field
For a magnetic field to exist there must be a source of magnetic energy (like a magnet), a moving charge, or an electric current. These elements are the only ones capable of creating a magnetic field and the only ones that can be affected by it.
An electric charge (such as an electron moving in space) generates a magnetic field around it that will exert a force on another moving charge. The same thing happens with electrical currents.
The case of magnets is particular because there are no moving charges involved, but rather these materials generate a magnetic field due to certain microscopic phenomena of a certain complexity.
As described by Ampère's Law and Maxwell's equations, magnetic fields and electric fields often exist together in nature. Certain changes over time in a magnetic field produce electric fields. A good example of the coexistence of these two fields is electromagnetic radiation, such as light.
The presence of magnetic fields can be checked using a device known as a magnetometer.
Types of magnetic field
Magnetic fields can be classified according to their source of creation:
- Magnetic fields coming from a magnet Magnets are materials that have the peculiarity of having a permanent magnetic field, created by what in physics is known as the spin of the electrons (it can be understood by thinking of it as a spin on themselves). On the other hand, there are metals that can “become” magnets when they are magnetized by an external magnetic field.
- Magnetic fields coming from current. Every moving charge produces a magnetic field. Therefore, an electric current also produces a magnetic field. For example: electromagnets (like the one in the figure above) are devices in which, by means of a battery, current is circulated through a cable wound in metal. This current generates a magnetic field around it that magnetizes the metal and generating another magnetic field. Thus, electromagnets are used to generate variable magnetic fields since changing the current changes the magnetic field.
Direction of a magnetic field
The direction of a magnetic field can be described using lines or vectors responsible for pointing out the direction where the magnetic forces point. In the figure above you can clearly see the lines of the magnetic field generated by the magnet, which indicate the direction of the force with which the magnet interacts with the metal particles.
The fact that the magnetic field has direction implies that it is a vector. Any force is a vector quantity, that is, it represents a magnitude that has a direction and a meaning, such as speed. Since the magnetic field is proportional to the magnetic force, then it is also a vector quantity. In fact, it is interesting to note that the magnetic force felt by a moving particle immersed in a magnetic field is always perpendicular to the direction of said field and its own speed.
Earth's magnetic field
Our planet has a natural magnetic field also called geomagnetic field. In the center of the Earth there is molten iron (due to high temperatures). Due to the Earth's rotation, this metallic liquid is in constant motion, forming an electric current (as the metal moves, the electrons that compose it move). This current is what produces the Earth's magnetic field, which is so intense that it escapes from the Earth's surface.
The Earth's magnetic field plays a very important role since it deflects radiation that is very dangerous for living beings from the Sun. Without Earth's magnetic field, the atmosphere would be destroyed by cosmic rays. The compasses that we use for navigation interact with this field: their magnetized needle always aligns with the Earth's magnetic field, indicating north. In addition, many migratory animals use the planet's magnetic field to orient themselves and always move in the same directions during specific periods of the year.
References
- “Magnetic field” in Wikipedia, The Free Encyclopedia.
- “What are magnetic fields?” at Khan Academy.
- “Magnetic field” in Practical Physics.
- “Magnetic field” in Natural Sciences.
- “What is the magnetic field?” (video) at The Tabletop Explainer.
- “Magnetic Field” in Encyclopaedia Britannica.
