Magnet
A magnet is a body or device with significant magnetism, such that it attracts other magnets or ferromagnetic metals (for example, iron, cobalt, nickel and their alloys). It can be natural or artificial and the materials can also be paramagnetic, which are weakly attracted, such as magnesium, platinum, aluminum, among others, or diamagnetic, which are not attracted, such as carbon graphite, gold, silver, etc. lead and bismuth.
Natural magnets maintain their continuous magnetic field unless they receive a shock of great magnitude or are subjected to opposite magnetic charges or high temperatures.
The magnetic field is not visible but it is responsible for magnets attracting or repelling various materials, those that are strongly attracted to a magnet have great magnetic permeability, as in the case of iron and some types of steel, and receive the denomination of ferromagnetic materials. Materials with low magnetic permeability are only weakly attracted to magnets and are called paramagnetic, an example would be liquid oxygen. Finally, there are some materials, such as water, which have such a low magnetic permeability that magnetism only manifests itself in the presence of an external magnetic field (diamagnetism). Everything has a measurable magnetic permeability.
Magnets can be permanent, if once magnetized they persistently retain their magnetic properties, or temporary, if they only retain their magnetic properties while under the influence of a magnetic field, disappearing when the field disappears. An electromagnet would be a type of temporary magnet made with a winding of electric wire through which an electric current passes, and it only behaves like a magnet while the current passes; sometimes the winding is made around a ferromagnetic material to enhance the magnetic field that is produced.
The SI unit of measurement for the magnetic field is the tesla, while the unit for magnetic flux is the weber; 1 Tesla is 1 Weber per square meter.
Etymology
From the Greek, adamas, adamantos (diamond, steel) from «a» (privative, prefix of annoyance or negation) and damao (burn). Fig. hard stone that cannot or should not be burned, heated, since the Greeks must have known that heat destroys magnetism.
- The chronicler Gonzalo Fernández de Oviedo tells his use and various names in the centuryXVI:
The tide needles are barley and made up of the virtue and a half of the "small skull" (which we commonly call "yman" stone) of which and of its properties make great mention the natural ones and name it by various names: for more than two that I have said they call it "magnete, ematite siderita and heraclion", it is of various species or genera this stone, one more is. (Written in 1535)
- In French aimant.
History
The oldest known descriptions of the properties of magnets come from ancient Egypt and ancient Greece, India, and China. In 585 B.C. C. Thales of Miletus described that magnetite attracted iron but thought that the cause was that it had a soul (at that time a movement implied life, soul or the intervention of a god). An attempt at an explanation without the intervention of the gods or the soul can be found in the work De rerum natura by Lucretius (98. C.- 54 B.C.). But it was not until 1600 with the publication of De Magnete by William Gilbert that the science of magnetism began in Europe. In China, the first known mention of magnets and their properties is from the IV century, and the first descriptions of the use of Compasses are from the early XI. The use of these devices would be common everywhere in the 12th and 13th centuries.
Physical origin of magnetism
Permanent Magnets
Any ordinary object is made up of particles such as protons, neutrons and electrons, each of which counts among its quantum properties spin, which associates a magnetic field with these particles. From this point of view, it would be expected that any material, being made up of an immense number of particles, would have magnetic properties (even antimatter particles have magnetic properties), but daily experience contradicts this possibility.
Within each atom or molecule, the arrangement of each spin strictly follows the Pauli exclusion principle, but for any diamagnetic substance there is no spin arrangement affecting a large number of particles, so there is no magnetic field, the magnetic moment of one particle cancels that of another.
On the other hand, in permanent magnets, we do find a significant degree of ordering of the spin of their particles. The highest level of ordering occurs in the so-called Weiss domains or magnetic domains: which can be considered as microscopic regions where there is a strong interaction between the particles, called exchange interactions, which generates a very ordered situation, the fatter it is. the degree of order of the domain, the stronger the magnetic field that will be generated.
A high ordering (and therefore a strong magnetic field) is one of the main characteristics of ferromagnetic materials.
A strategy that can be used to generate a very strong magnetic field consists of orienting all the Weiss domains of a ferromagnetic material with a less intense field, generated by a coil of conductive material through which an electric current passes: a electromagnet.
Magnetism generated by electrons
The electrons have an important role in the formation of the magnetic field, in an atom the electrons can be found alone or in pairs within each orbital. If they are in pairs, each electron has a spin opposite to the other (spin up and spin down), since the spins have opposite directions they cancel each other, therefore a pair of electrons cannot generate a magnetic field.
In many atoms, the number of electrons is odd, all magnetic materials have such electrons, but an atom with unpaired electrons cannot be said to be ferromagnetic. In order to be ferromagnetic, the unpaired electrons in the material also have to interact with each other on a large scale, so they all have to be oriented in the same direction. The specific electronic configuration of the atoms, as well as the distance between each atom, is the main factor driving this long-range order, which affects many particles. If the electrons have the same orientation they will be in a lower energy state.
Electromagnets
An electromagnet in its simplest form, is that of a conducting wire that has been wound one or more times, this configuration is called a spiral (one turn) or solenoid. When the electric current passes through the conductive wire of the winding, a magnetic field is generated that is concentrated near the winding (especially inside it). Its field lines are very similar to those of a magnet and its orientation follows the right-hand rule. The moment and the magnetic field of the electromagnet are proportional to the number of turns of the wire (also called turns), in the section of each turn and in the density of the current that passes through the wire.
If the conductive wire loops are made around a material without special magnetic properties, or in the air, the magnetic field that will be generated will be a weak force; but if the wire is wrapped around a ferromagnetic and paramagnetic material, such as an iron nail, the magnetic field produced will be much larger, its strength will be a few hundred times greater and can be multiplied by 1000.
The magnetic field observed around a magnet extends a considerable distance compared to the size of the magnet, and follows the inverse cube law: the field strength is inversely proportional to the cube of the distance.
If the electromagnet is based on a metal plate, the force needed to separate the two objects will be even greater, since the two surfaces will be flat and smooth, in this case there will be more points of contact and the reluctance of the magnetic circuit will be less.
Electromagnets have applications in various fields, from particle accelerators to electric motors to cranes used in car wrecks or machines that produce magnetic resonance images. There are also more complex machines in which simple magnetic dipoles are not used, but involve four (quadripoles) or more magnetic poles; An example would be the mass spectrometer where they are in charge of concentrating the bundles of particles.
Recently it has been possible to produce magnetic fields of several million Tesla using micrometer solenoids through which a current of millions of amperes was passed using a pulsed discharge produced with a capacitor bank. The intense force of the discharge caused the system to implode, destroying the experiment in a few milliseconds.
Discovery
It was Ørsted who first demonstrated in 1820 that an electric current generates a magnetic field in its surroundings. Inside the matter there are small closed currents due to the movement of the electrons that the atoms contain, each one of them originating a microscopic magnet. When these small magnets are oriented in all directions, their effects cancel each other out and the material has no magnetic properties; On the other hand, if all the magnets are aligned, they act as a single magnet. In this case, the substance has been magnetized.
The first scientist to build an electromagnet was the French physicist and politician François Arago.
Characteristics of magnets
Magnetic field
The magnetic field (usually represented as B) is what in physics is called a field because it has a value at every point in space. The magnetic field (at any given point) will be determined by two properties:
- your address (following the needle orientation of a compass)
- its magnitude (or strength) which is proportional to the force with which the compass needle is orientated in the direction of the field.
Direction and magnitude are the characteristics of a vector and therefore B is a vector field. B can also depend on time.
Magnetic moment
The magnetic moment of a magnet, also called magnetic dipole moment, and which is symbolized by the letter μ, is a vector that characterizes the magnetic properties of any body. For example, in a bar magnet the direction of the magnetic moment is from the north pole to the south pole and its magnitude depends on the strength of the poles and their distance. In SI units it is expressed in A·m².
A magnet produces its magnetic field and at the same time responds to other magnetic fields, its magnetic field at any point is proportional to the value of its magnetic moment. When the magnet is inside a magnetic field produced by another source, it is subjected to a couple of forces that will tend to orient the magnetic moment parallel to the field. The value of this couple of forces will be proportional to the magnetic moment and the external field.
A loop with a section of area A that is traversed by a current I will behave like a magnet with a magnetic moment that will have a value equal to AI.
Magnetization
The magnetization of magnetic materials is the local value of their magnetic moment per unit volume, usually represented as 'M and its units are A/m. More than a vector (which would be the case of the magnetic moment) it is a vector field because different areas of a magnet can be magnetized with different directions and strength. A good quality bar magnet can have a magnetic moment of 0.1 A m² and if we assume a volume of one cm³ (0.000001 m³) a magnetization of 100,000 A/m. Iron can have a magnetization of about a million A/m, this large value explains why magnets are so effective in producing magnetic fields.
Magnetic Poles and Atomic Currents
Gilbert's model
In all magnets, whatever their type, the maximum force of attraction is found at their ends, called the pole. A magnet has at least two poles, called the north pole and south pole. The pole is not something material but a concept used to describe magnets. Like poles repel each other and opposite poles attract. There are no isolated poles, and therefore, if a magnet breaks into two parts, two new magnets are formed, each with its north pole and its south pole. If we continue to divide there will come a time when the parts will be too small to maintain a magnetic field, they will have lost the ability to generate magnetism. In the case of some materials, you can go down to the molecular level and still see a north pole and a south pole. Some scientific theories predict the existence of a north and south magnetic monopole, but until now they have never been observed.
The model of the magnetic poles the surface of the poles of the permanent magnets is imagined covered with the so-called magnetic charge, particles of north types to the north pole and particles of south types to the south pole, which would be the source of the magnetic field lines. This model correctly describes the magnetic field outside the magnet, but it does not give the correct field inside. This model is also called the Gilbert model of a magnetic dipole.
Ampère's model
The alternative to the pole model is the Ampère model according to André-Marie Ampère, electric currents would be the cause of all magnetic phenomena, explaining both the magnetism of magnets and all the other sources of magnetism. Magnetic dipoles would be small atomic turns of current, small closed circuits of atomic current. Today, Ampère's idea continues to be the basis of the theory of magnetism, but it is considered that magnetic materials also have currents that they have to be related to the quantum property of spin, an intrinsic angular momentum associated with subatomic particles. The right-hand norm tells us the direction in which the current flows. The Ampère model gives the exact magnetic field both inside and outside the magnet.
Name of the poles of a magnet
Historically, the terms north pole and south pole of a magnet reflect knowledge of the interaction between a magnet and the earth's magnetic field: a magnet freely suspended in the air, it will be oriented along a north-south axis due to the action of the magnetic north and south poles of the Earth, the tip of the magnet that points towards the magnetic north pole of the Earth is called the north pole of the magnet, while the other end will be the south pole of the magnet.
But today, Earth's geographic north pole roughly corresponds to its magnetic south pole, and to complicate matters further, magnetized rocks on the ocean floor have been found to show that the magnetic field has reversed its polarity several times throughout Earth's history. Fortunately, using an electromagnet and the right-hand rule, we can orient any magnetic field without having to use the earth's magnetic field.[citation needed]
Types of magnets
Magnets can be natural or artificial, or permanent or temporary. A natural magnet is a mineral with magnetic properties (magnetite). An artificial magnet is a body of ferromagnetic material to which an electromagnetic field has been induced. A permanent magnet is made of magnetized steel. A temporary magnet loses its properties once the cause that causes the magnetism ceases. An electromagnet is a coil (in the minimal case, a spiral) through which electric current circulates.
- Natural magnets: the magnetite is a powerful natural magnet, has the property of attracting all magnetic substances. Its characteristic of attracting iron pieces is natural. It is composed of iron oxide. Magnetic substances are those that are attracted by magnetite.
- Permanent artificial magnets: the magnetic substances that when rub them with the magnetite, become magnets, and for a long time retain their attraction property.
- Temporary artificial magnets: those who produce a magnetic field only when they circulate an electric current. An example is electromagnet.
The calculation of magnetic force
Field of a magnet
The magnetic field created by a magnet can be roughly described as the field of a magnetic dipole, characterized by its total magnetic moment. This would hold regardless of the shape of the magnet as long as the magnetic moment is nonzero. One of the characteristics of a dipole field is that the strength of the field decreases inversely as the cube of the distance from the center of the magnet.
The closer to the magnet, the more complicated the magnetic field becomes and the more dependent on its shape and magnetization. Formally, the field can be expressed as a multipole expansion: a dipole field, plus a quadrupole field, plus an octopole field, etc.
The magnetic force
Calculating the attractive or repulsive force between two magnets is, in general, an extremely complex operation as it depends on the shape of the magnets, their magnetization, their orientation and their separation.
Force between two magnetic poles
In classical mechanics, the force between two magnetic poles will be given by the equation:
- F=μ μ qm1qm24π π r2{displaystyle F={{mu q_{m1}q_{m2}}} over {4pi r^{2}}}}}
where
- F is strength (in newtons)
- qm1 and qm2 is the magnitude of the magnetic poles (amperio-meter)
- μ is the permeability of the medium tesla metro per amperio, henry per metre or newton per square amperio)
- r is separation (meters).
Even so, this equation does not describe a real physical situation since the magnetic poles are purely theoretical entities, real magnets present a much more complex distribution of dust than just north and south poles. Below are some more complex equations that are more useful.
Magnetic force between two surfaces
The force between two nearby surfaces of area A and an equal but opposite magnetic field H will be given by:
- F=μ μ 02AH2{displaystyle F={frac {mu _{0}}{2}}}AH^{2}
where
- A is the area of each surface, in m2
- H is the magnetic field, in A/m.
- μ0 is the permeability of space, which is equal to 4π×10-7T·m/A
Force between two bar magnets
The force between two identical cylindrical magnets that touch each other at their ends will be given by:
- F=[chuckles]B02A2(L2+R2)π π μ μ 0L2][chuckles]1x2+1(x+2L)2− − 2(x+L)2]{displaystyle F=left[{frac {B_{0}{2}A^{2}{2}left(L^{2}+R^{2}right){pi mu _{0}{2}{2}}{2}{left[{frac {1}{x^{2}}}{frac {1⁄2}{x1⁄2}{x1⁄2}{x1⁄2}}{x1⁄2}}{x1⁄2}{x1⁄2}{x1⁄2}{x1⁄2}{x1⁄2}{x1⁄2}}{x1⁄2}}{x1⁄2}}{x1⁄2}}{x1⁄2}}{x1⁄2}}{x1⁄2}}{x1⁄2}{f)}{x1⁄2}{x1⁄2}{x1⁄2}}}{x1⁄2}}{x1⁄2}{x1⁄2}}}{
![{displaystyle F=left[{frac {B_{0}^{2}A^{2}left(L^{2}+R^{2}right)}{pi mu _{0}L^{2}}}right]left[{frac {1}{x^{2}}}+{frac {1}{(x+2L)^{2}}}-{frac {2}{(x+L)^{2}}}right]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8c2ccfa67f81b88d43b73ca8bc486e1a14731107)
where:
- B0 is the density of magnetic flow very close to each pole, in T,
- A is the area of each pole, in m2,
- L is the length of each magnet, in m,
- R is the radius of each magnet, in me, i
- x is the separation between the two magnets, in m
- B0=μ μ 02M{displaystyle B_{0},=,{frac {mu _{0}}{2}M} relates magnetic flow density to the pole to magnetization.
Note that these equations are based on the Gilbert model, which is usable in cases of relatively large distances. In other models (such as the Ampère model) the formulation is more complicated, so much so that sometimes it is not analytically solvable and numerical analysis methods must be used.
Current Uses
Magnets are used in many different ways: on hard drives, speakers, fridge magnets, compasses, fridge or freezer locks, magnetic walls, coded keys, credit or debit card magnetic strips, horns, motors, a basic switch, generators, metal detectors, for closing furniture. Some of these devices can be damaged if a certain amount of opposing magnetism or high temperature is applied to them.
Parts of a magnet
- Magnetic axis: line bar linking the two poles.
- Neutral line: line of the surface of the bar that separates the polarized zones.
- Polos: the two ends of the magnet where the forces of attraction are more intense. These poles are the north pole and the south pole; the same poles repel and the different ones attract.
Magnetism
It is said that they were first observed in the city of Magnesia in Asia Minor, hence the term magnetism. They knew that certain stones attracted iron and that bits of iron attracted attracted others. These were termed natural magnets. It was Oersted who showed for the first time in 1820 that an electric current generates a magnetic field around it. Inside the matter there are small currents related to the movement of the electrons that the atoms contain; each one of them originates a microscopic magnet. When these small magnets are oriented in all directions, their effects cancel each other out and the material has no magnetic properties; and instead, if all the magnets are aligned, they act as a single magnet and the substance is said to have been magnetized.
Magnetic poles
If it is both a type of magnet and another, the maximum force of attraction is at its ends, called poles. A magnet consists of two poles, called the north pole and the south pole. Like poles repel each other and unlike poles attract. There are no isolated poles (see magnetic monopole) and, therefore, if a magnet breaks into two parts, two new magnets are formed, each with its north pole and its south pole, although the attractive force of the magnet decreases.
Between both poles lines of force are created, these lines being closed, so that inside the magnet they also go from one pole to the other. As shown in the figure, they can be visualized by spreading iron filings on a piece of cardboard placed on top of a bar magnet; by tapping the cardboard, the filings are oriented in the direction of the lines of force.
Polarity of a magnet
To determine the poles of a magnet, the tendency of the magnet to orient itself according to the magnetic poles of the Earth, which is a gigantic natural magnet, is considered: the north pole of the magnet is oriented towards the south magnetic pole (located close to the pole geographic north), which in a strictly magnetic sense is a south pole. The south pole of a magnet is oriented toward the magnetic north pole (located close to the geographic south pole), which in a strictly magnetic sense is a north pole.
In a practical way, to determine which pole of a magnet is north and which is south, it is not necessary to use the Earth's magnetic field. For example, one method is to compare the magnet to an electromagnet, the poles of which can be identified using the right-hand rule. Magnetic field lines, by convention, emerge from the north pole of a magnet and enter the south pole.
The angle between local magnetic north, indicated by a compass, and true north (or geographic north) is called magnetic declination.
Magnetization
The magnetization of an object is the local value of its angular-magnetic momentum per unit volume, usually denoted M, with units A/m. It is a vector field, rather than just a vector (like magnetic moment), because different sections of a bar magnet are generally magnetized with different directions and strengths. A good bar magnet can have a magnetic moment of magnitude 0.1 A m² and a volume of 1 cm³, or 0.000001 m³; for this reason the average magnitude of magnetization is 100 000 A/m. Steel can have a magnetization of about a million A/m.
How to magnetize a substance
Placing the material in a strong magnetic field produced by a permanent magnet or by an electric current, or when the material has magnetic properties and when melted (eg steel or basaltic lava) it cools in the presence of some magnetic field.
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