We know from everyday experience that magnets attract or repel each other, depending on the relative position between them. We can understand this attraction/repulsion by imagining magnetic fields generated by these magnets, but not only magnets are capable of generate magnetic fields, electric currents can also generate magnetic fields. Thus, we can think of magnetism of the magnets as "micro currents" running inside them. Therefore, the origin of any magnetic field can be attributed to electric charges in motion.

Magnetic Field

Illustration of the magnetic field of a magnet. Note that near the magnet the field lines are closer to each other, farther from the magnet they are farther apart.

Magnetism is the basic phenomena in the operation of electric motors, generators, recording information in computer memory and various other technological applications. There are four important elements.

Magnetic Field \((\vec{B})\)
Magnetic field is a region of the space modified by the presence of a magnet, a conductor traversed by an electric current, or an electrified body in motion. The magnetic field vector \(\vec{B}\) is the intensity and direction of the magnetic field at a point in space.
The magnetic field unit in the \(IS\) is Tesla \((T)\) , where \([B] = T = \frac{N}{(A \cdot m)}\).
It is the set of phenomena associated to forces that are produced between circuits when there is an electrical current in them, or between magnets.
Pieces made of materials that have the property to attract or repel each other, and attract metallic materials. They can be natural or artificial. Magnetite is a natural element with magnetism.
Magnetic Poles
In any magnet, as small as it can be, there are always two distinct regions, where the magnetic properties are manifested. These regions are called magnetic poles. For the poles of a magnet, it is important to note that:
  • Equal poles repels each other. Different poles attract each other.
  • It is observed experimentally that it is impossible to isolate one of the poles of the magnets; that is, we can never get a magnet with only one type of pole.

Field Lines

Field lines are lines that allow a visualization of the magnetic field. They have the following characteristics:

  • Field lines begin its path on the magnet north pole, and finishes at the south pole, but, inside the magnet, field lines return from the south pole back to the north pole.
  • The lines are tangent to the magnetic field vector at each point.
  • The lines are oriented in the direction of the magnectic field vector.
  • The lines always form closed loops, i.e., has no sources or sinks.
  • The density of the field lines allows to evaluate the intensity of the magnetic field in the given region. The region, with many lines very close to each other, has a high intensity magnetic field. For regions with few lines, we have weak magnetic intensity.
  • A magnet in the presence of a magnetic field will tend to align with the field.

Influence of Temperature on the Magnetization

Generally, when warmed, a ferromagnetic material that has a noticeable magnetization will experience a decrease in magnetization as the temperature increases. The temperature where the material no longer presents spontaneous magnetization is called Curie point.

Earth's Magnetic Field

Around Earth, there is a magnetic field. When a magnet is free to rotate around its center of gravity, in a horizontal plane (paralel to Earth's surface), one of its poles always points to the North, and it is called magnetic north. The other pole, which points to the geographic South Pole is called magnetic south. A simple experiment is to put a magnetized needle on the water's surface of a recipient in rest, so that the needle can float and rotate freely.

Magnetic Substances

Paramagnetic are those that, in the presence of an external magnetic field, magnetize itself in the direction of the external field, causing an increase in the value of the resulting magnetic field in the vicinity. These substances lose their magnetization once the external field is removed.
They are those which, in the presence of an external magnetic field, magnetize itself in the opposite direction of the external field, causing a decline in the value of the magnetic field in the vicinity. These substances lose their magnetization once the external field is removed.
Iron, cobalt, nickel and its alloys, under the action of a magnetic field, magnetize themselves strongly, making the resulting magnetic field much larger than the applied field. These materials remain magnetized even when the external field is removed.
The vast majority of substances in nature are paramagnetic or diamagnetic.