MAGNETISM: MAGNETIC PROPERTIES OF MATERIALS, USES - PHYSICAL - 2025

The magnetism or magnetic energy is a force associated nature movement and capable of producing electrical attraction or repulsion in certain substances loads. Magnets are well known sources of magnetism.

Inside these there are interactions that are translated into the presence of magnetic fields, which exert their influence on small pieces of iron or nickel, for example.

The beautiful colors of the Northern Lights are due to cosmic particles emitting energy as they are deflected by the Earth's magnetic field. Source: Pixabay.

The magnetic field of a magnet becomes visible when it is placed under a paper on which iron filings are spread. The filings are immediately oriented along the field lines, creating a two-dimensional image of the field.

Another well-known source is wires that carry electrical current; But unlike permanent magnets, the magnetism disappears when the current stops.

Whenever a magnetic field occurs somewhere, some agent had to do work. The energy invested in this process is stored in the created magnetic field and can then be considered as magnetic energy.

The calculation of how much magnetic energy is stored in the field depends on the field and the geometry of the device or the region where it was created.

Inductors or coils are good places to do this, creating magnetic energy in much the same way that electrical energy is stored between the plates of a capacitor.

History and discovery

Old apps

The legends told by Pliny about ancient Greece speak of the shepherd Magnes, who more than 2000 years ago found a mysterious mineral capable of attracting pieces of iron, but not other materials. It was magnetite, an iron oxide with strong magnetic properties.

The reason for the magnetic attraction remained hidden for hundreds of years. At best, it was attributed to supernatural events. Although not for this reason, interesting applications were found for it, such as the compass.

The compass invented by the Chinese makes use of the Earth's own magnetism to guide the user during navigation.

First scientific studies

The study of magnetic phenomena had a great advance thanks to William Gilbert (1544 - 1603). This English scientist of the Elizabethan era studied the magnetic field of a spherical magnet and concluded that the Earth must have its own magnetic field.

From his study of magnets, he also realized that he could not get separate magnetic poles. When a magnet is sectioned in two, the new magnets also have both poles.

However, it was at the beginning of the 19th century when scientists realized the existence of the relationship between electric current and magnetism.

Hans Christian Oersted (1777 - 1851), born in Denmark, had in 1820 the idea of ​​passing an electric current through a conductor and observing the effect that this had on a compass. The compass would deviate, and when the current stopped flowing, the compass would again point north as usual.

This phenomenon can be verified by bringing the compass closer to one of the cables coming out of the car battery, while the starter is being operated.

At the time of closing the circuit the needle should experience an observable deflection, since the batteries of the cars can supply currents high enough so that the compass deviates.

In this way it became clear that moving charges are what give rise to magnetism.

Modern investigations

A few years after the Oersted experiments, British researcher Michael Faraday (1791 - 1867) marked another milestone by discovering that varying magnetic fields in turn give rise to electric currents.

Both phenomena, electric and magnetic, are closely related to each other, with each one giving rise to the other. They were brought together by Faraday's disciple, James Clerk Maxwell (1831 - 1879), in the equations that bear his name.

These equations contain and summarize the electromagnetic theory and are valid even within relativistic physics.

Magnetic properties of materials

Why do some materials exhibit magnetic properties or acquire magnetism easily? We know that the magnetic field is due to moving charges, therefore inside the magnet there must be invisible electric currents that give rise to magnetism.

All matter contains electrons orbiting the atomic nucleus. The electron can be compared to the Earth, which has a translational motion around the Sun and also a rotational motion on its own axis.

Classical physics attributes similar motions to the electron, although the analogy is not entirely accurate. However, the point is that both properties of the electron make it behave like a tiny loop that creates a magnetic field.

It is the spin of the electron that contributes the most to the magnetic field of the atom. In atoms with many electrons, they are grouped in pairs and with opposite spins. Thus, their magnetic fields cancel each other out. This is what happens in most of the materials.

However, there are some minerals and compounds in which there is an unpaired electron. In this way, the net magnetic field is not zero. This creates a magnetic moment, a vector whose magnitude is the product of the current and the area of ​​the circuit.

Adjacent magnetic moments interact with each other and form regions called magnetic domains, in which many spins are aligned in the same direction. The resulting magnetic field is very strong.

Ferromagnetism, paramagnetism and diamagnetism

Materials that possess this quality are called ferromagnetic. They are a few: iron, nickel, cobalt, gadolinium and some alloys of the same.

The rest of the elements in the periodic table lack these very pronounced magnetic effects. They fall into the category of paramagnetic or diamagnetic.

In fact, diamagnetism is a property of all materials, which experience a slight repulsion in the presence of an external magnetic field. Bismuth is the element with the most accentuated diamagnetism.

On the other hand, paramagnetism consists of a less intense magnetic response than ferromagnetism but equally attractive. Paramagnetic substances are for example aluminum, air and some iron oxides such as goethite.

Uses of magnetic energy

Magnetism is part of the fundamental forces of nature. As human beings are also part of it, they are adapted to the existence of magnetic phenomena, as well as the rest of life on the planet. For example, some animals use the Earth's magnetic field to orient themselves geographically.

In fact, it is believed that birds carry out their long migrations thanks to the fact that in their brains they have a kind of organic compass that allows them to perceive and use the geomagnetic field.

While humans lack a compass like this, they instead have the ability to modify the environment in many more ways than the rest of the animal kingdom. Thus, members of our species have used magnetism to their advantage from the moment the first Greek shepherd discovered the lodestone.

Some applications of magnetic energy

Since then there are many applications of magnetism. Here are a few:

- The aforementioned compass, which makes use of the Earth's geomagnetic field to orient itself geographically.

- Old screens for televisions, computers and oscilloscopes, based on the cathode ray tube, which use coils that generate magnetic fields. These are responsible for deflecting the electron beam so that it hits certain places on the screen, thus forming the image.

- Mass spectrometers, used to study various types of molecules and with many applications in biochemistry, criminology, anthropology, history and other disciplines. They make use of electric and magnetic fields to deflect charged particles in trajectories that depend on their speed.

- Magnetohydrodynamic propulsion, in which a magnetic force drives a jet of seawater (a good conductor) backwards, so that by Newton's third law, a vehicle or boat receives a forward impulse.

- Magnetic resonance imaging, a non-invasive method to obtain images of the interior of the human body. Basically, it uses a very intense magnetic field and analyzes the response of the hydrogen nuclei (protons) present in the tissues, which have the aforementioned property of spin.

These applications are already established, but in the future it is believed that magnetism can also combat diseases such as breast cancer, through hyperthermic techniques, which produce magnetically induced heat.

The idea is to inject fluid magnetite directly into the tumor. Thanks to the heat produced by the magnetically induced currents, the iron particles would become hot enough to destroy the malignant cells.

Advantages and disadvantages

When you think about the use of a certain type of energy, it requires its conversion into some type of movement such as that of a turbine, an elevator or a vehicle, for example; or that it is transformed into electrical energy that turns on some device: telephones, televisions, an ATM and the like.

Energy is a magnitude with multiple manifestations that can be modified in many ways. Can the energy of a small magnet be amplified so that it continuously moves more than a few coins?

To be usable, the energy must have a great range and come from a very abundant source.

Primary and secondary energies

Such energies are found in nature, from which the other types are produced. They are known as primary energies:

- Solar energy.

- Atomic Energy.

- Geothermal energy.

- Wind power.

- Biomass energy.

- Energy from fossil fuels and minerals.

Secondary energies, such as electricity and heat, are produced from these. Where is the magnetic energy here?

Electricity and magnetism are not two separate phenomena. In fact, the two together are known as electromagnetic phenomena. As long as one of them exists, the other will exist.

Where there is electrical energy, there will be magnetic energy in some form. But this is a secondary energy, which requires the prior transformation of some of the primary energies.

Characteristics of primary and secondary energies

The advantages or disadvantages of using some kind of energy are established according to many criteria. These include how easy and cheap its production is, and also how much the process is capable of negatively influencing the environment and people.

Something important to keep in mind is that energies transform many times before they can be used.

How many transformations must have occurred to make the magnet that will stick the shopping list to the refrigerator door? How many to build an electric car? Surely enough.

And how clean is the magnetic or electromagnetic energy? There are those who believe that constant exposure to electromagnetic fields of human origin causes health and environmental problems.

Currently there are numerous lines of research dedicated to studying the influence of these fields on health and the environment, but according to prestigious international organizations, there is so far no conclusive evidence that they are harmful.

Examples of magnetic energy

A device that serves to contain magnetic energy is known as an inductor. It is a coil that is formed by winding copper wire with a sufficient number of turns, and it is useful in many circuits to restrict the current and prevent it from changing abruptly.

Copper coil. Source: Pixabay.

By circulating a current through the turns of a coil, a magnetic field is created inside it.

If the current changes, so do the magnetic field lines. These changes induce a current in the turns that opposes them, according to the Faraday-Lenz law of induction.

When the current increases or decreases suddenly, the coil opposes it, therefore it can have protective effects on the circuit.

The magnetic energy of a coil

Magnetic energy is stored in the magnetic field created in the volume delimited by the turns of the coil, which will be denoted as U _B and which depends on:

- The intensity of the magnetic field B.

- The cross-sectional area of ​​coil A.

- The length of the coil l.

- The permeability of the vacuum μ _o.

It is calculated as follows:

This equation is valid in any region of space where there is a magnetic field. If the volume V of this region is known, its permeability and the intensity of the field, it is possible to calculate how much magnetic energy it possesses.

Exercise resolved

The magnetic field inside an air-filled coil with a diameter of 2.0 cm and a length of 26 cm is 0.70 T. How much energy is stored in this field?

Solution

The numerical values ​​are substituted in the previous equation, taking care to convert the values ​​to the units of the International System.

1. Giancoli, D. 2006. Physics: Principles with Applications. Sixth edition. Prentice Hall. 606-607.
2. Wilson, JD 2011. Physics 12. Pearson. 135-146.