The magnetism or magnetic energy is a force associated with the movement nature and capable of producing electrical attraction or repulsion of certain substances loads. Magnets are well known sources of magnetism.
Within these interactions, there is the presence of magnetic fields, which influence small pieces of iron or nickel, for example.
The magnetic field of a magnet becomes visible when placed under paper on which iron filings are scattered. Archives are oriented immediately along the field lines, creating a two-dimensional image of it.
Another well-known source is wires that carry electrical current; but unlike permanent magnets, magnetism disappears when the current ceases.
Whenever a magnetic field is produced somewhere, some agent had to work. The energy invested in this process is stored in the created magnetic field and can be considered magnetic energy.
The calculation of how much magnetic energy is stored in the field depends on this and the geometry of the device or the region in which it was created.
Inductors or coils are good places for this, creating magnetic energy similar to the way electrical energy is stored between the plates of a capacitor.
history and discovery
old applications
The legends told by Pliny about ancient Greece tell 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, this has been attributed to supernatural facts. Though not for that reason, they stopped finding interesting apps for it like the compass.
The compass invented by the Chinese uses 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 split in two, the new magnets also have both poles.
However, it was in the beginning of the 19th century that scientists realized the existence of the relationship between electric current and magnetism.
Hans Christian Oersted (1777 – 1851), born in Denmark, had in 1820 the occurrence of passing an electric current through a conductor and observing the effect this had on a compass. The compass deviated, and when the current stopped flowing, the compass pointed again, as usual, to the north.
This phenomenon can be verified by bringing the compass closer to one of the cables that come out of the car battery while the starter motor is operated.
At the moment of closing the circuit, the needle must undergo an observable deflection, as car batteries can supply currents high enough for the compass to deviate.
In this way, it became clear that moving charges are what give rise to magnetism.
modern search
A few years after Oersted’s experiments, British researcher Michael Faraday (1791 – 1867) marked another milestone when he discovered that changing magnetic fields, in turn, give rise to electrical currents.
Both electrical and magnetic phenomena are closely linked, each of which can give rise to the other. Unifying them was commissioned by Faraday’s disciple James Clerk Maxwell (1831 – 1879) in the equations that bear his name.
These equations contain and summarize 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, within the magnet there must be invisible electrical currents that generate magnetism.
All matter contains electrons that orbit the atomic nucleus. The electron can be compared to the Earth, which has a translational movement around the Sun and also a rotation 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 small spiral that creates a magnetic field.
It is the spin of the electron that contributes most to the atom’s magnetic field. In atoms with many electrons, these are grouped in pairs and with opposite rotations. Thus, their magnetic fields cancel each other out. This is what happens in most materials.
However, there are some minerals and compounds in which there is an electron missing. In this way, the net magnetic field is not null. This creates a magnetic moment , a vector whose magnitude is the product of the current in 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 intense.
Ferromagnetism, paramagnetism and diamagnetism
Materials that have this quality are called ferromagnetics . There are a few: iron, nickel, cobalt, gadolinium and some alloys thereof.
The rest of the elements on the periodic table do not have these pronounced magnetic effects. They fall under 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 pronounced diamagnetism.
Paramagnetism, in turn, consists of a magnetic response less intense than ferromagnetism, but equally attractive. Paramagnetic substances are, for example, aluminium, 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 to the rest of life on the planet. For example, some animals use the Earth’s magnetic field to be geographically oriented.
In fact, birds are believed to make their long migrations because in their brains they have a kind of organic compass that allows them to sense and use the geomagnetic field.
While humans don’t have a compass like this, they do have the ability to modify the environment in more ways than the rest of the animal kingdom. So members of our species have used magnetism to their advantage from the moment the first Greek shepherd discovered the stone of the magnet.
Some applications of magnetic energy
Since then, there have been many applications of magnetism. Here are a few:
– The aforementioned compass, which makes use of the Earth’s geomagnetic field to be geographically oriented.
– Old television screens, computers and oscilloscopes, based on the cathode ray tube, which use coils that generate magnetic fields. They are responsible for deflecting the electron beam in order to impact certain locations on the screen, 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 use electric and magnetic fields to deflect charged particles in trajectories that depend on their speed.
– Magnetohydrodynamic propulsion, in which a magnetic force pushes a jet of seawater (good conductor) back, so that, by Newton’s third law, a vehicle or vessel receives a forward thrust.
– 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 hydrogen nuclei (protons) present in tissues, which have the aforementioned spin property.
These applications are already established, but in the future it is believed that magnetism can also fight diseases such as breast cancer, using hyperthermal techniques , which produce magnetically induced heat.
The idea is to inject fluid magnetite directly into the tumor. Thanks to the heat produced by magnetically induced currents, the iron particles get hot enough to destroy the malignant cells.
Advantages and disadvantages
When thinking about the use of a certain type of energy, its conversion is necessary in some type of movement, such as that of a turbine, an elevator or a vehicle, for example; or that it is turned into electrical energy that turns on some device: telephones, televisions, an ATM and things like that.
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 continually moves more than a few coins?
To be usable, energy must be powerful and come from a very abundant source.
Primary and secondary energies
In nature there are such energies, from which other types are produced. They are known as primary energies:
– Solar energy.
– atomic energy.
– Geothermal energy.
– Wind energy.
– biomass energy.
– Fossil energy and mineral fuel.
Secondary energies, such as electricity and heat, are produced from them. Where is the magnetic energy here?
Electricity and magnetism are not two separate phenomena. In fact, the two together are known as electromagnetic phenomena. Whenever one of them exists, the other exists.
Where there is electrical energy, magnetic energy will exist 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 type of energy are established according to many criteria. This includes how easy and cheap it is to produce and also how much it is able to negatively influence the process on the environment and people.
An important thing to keep in mind is that energies are transformed many times before they can be used.
How many transformations must have taken place to create the magnet that will leave the shopping list attached to the refrigerator door? How many to build an electric car? Certainly enough.
And how clean is 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 the study of the influence of these fields on health and the environment, but, according to prestigious international organizations, there is no conclusive evidence so far 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 formed by winding copper wire with enough bends and is useful in many circuits to restrict current and prevent it from changing drastically.
By circulating a current through the loops of a coil, a magnetic field is created inside.
If the current changes, so do the magnetic field lines. These changes induce a current in the curves that opposes them, according to the Faraday-Lenz induction law.
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
In the magnetic field created in the volume delimited by the coil turns magnetic energy is stored, which will be called U B and it depends on:
– The strength of the magnetic field B.
– The cross-sectional area of coil A.
– the length of the coil l.
– Vacuum permeability 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 that region, its permeability and the field strength are known, it is possible to calculate how much magnetic energy it has.
Exercise solved
The magnetic field inside an air-filled coil 2.0 cm in diameter and 26 cm in length is 0.70 T. How much energy is stored in this field?
Data : The vacuum permeability is μ o = 4π. 10 -7 Tm / A
Solution
The numerical values in the above equation are substituted, taking care to convert the values to International System units.
- Giancoli, D. 2006. Physics: Principles with Applications. Sixth Edition Prentice Hall. 606-607.
- Wilson, JD 2011. Physics 12. Pearson. 135-146.
