What is the meaning of the ampere hypothesis. Ampere's theory explaining magnetic properties

Let us consider an isolated atom not subject to the action of an external magnetic field. According to ideas classical physics, electrons in atoms move in certain closed orbits. This movement of each electron is equivalent to a closed current loop. Therefore, any atom or molecule, from the point of view of their magnetic properties, can be considered as a certain set of electronic microcurrents. This is Ampere's hypothesis about the nature of magnetism.

The magnetic moment p m of the electric current caused by the motion of an electron in orbit is called the orbital magnetic moment of the electron. Let us assume for simplicity that an electron in an atom moves at a speed v in a circular orbit of radius r(rice.).

According to the definition of the magnetic moment of the current, the orbital magnetic moment electron is numerically equal

where S is the area of ​​the electron orbit. Vector p m is directed in the same direction as the magnetic field at the center of the circular current.

The properties that substances exhibit in a magnetic field are called magnetic, and the substances themselves are called magnets. The magnetic properties of substances are determined by the presence of magnetic moments in their atoms. For most elements, in the absence of an external magnetic field, the magnetic moments of the electrons entering the atoms are equal to zero, since they have different directions and completely compensate each other. The application of an external magnetic field leads to a reorientation of the moments of magnetic atoms and the appearance of a non-zero magnetic moment. In this case, the non-zero total magnetic moment changes the magnetic field.

When studying the magnetic field in a substance (magnet), two types of currents are distinguished - macrocurrents and microcurrents. Under macrocurrents understand electric currents conductivity, as well as convection currents associated with the movement of charged macroscopic bodies. Microcurrents or molecular currents are currents caused by the movement of electrons in atoms, ions and molecules.

In a substance, an additional magnetic field of microcurrents (respectively called internal) is superimposed on the magnetic field of macrocurrents (it is often called external). The magnetic induction vector B characterizes the resulting magnetic field in the substance, i.e. it is equal to geometric sum magnetic induction external (Во) and internal (В int) fields:

Those. vector B must depend on the magnetic properties of the magnet. The magnetic field of microcurrents arises as a result of magnetization of a magnet when it is placed in an external magnetic field. Therefore, the primary source of the magnetic field in matter is macrocurrents.

Since in a vacuum the field is created only by macrocurrents, and in matter - by macrocurrents and microcurrents, then for the field in matter total current law looks like

(13.1.1)

where I macro and I micro - algebraic sums respectively, macro- and microcurrents covered by a closed loop L, i.e., the resulting macro- and microcurrents through the surface formed by the loop L.

The quantity H, which depends on the magnetic properties of the medium, is called magnetic field strength.

The unit of measurement for magnetic field strength is A/m. If the directions of the magnetization vectors and magnetic field strength coincide, then the substances are called isotropic magnets. If the direction of the magnetization vector depends on the direction of the field relative to the crystallographic axes, then the substances are anisotropic magnets. Graphically, the magnetic field strength is depicted using lines, the tangent to which at each point coincides with the direction of the strength at that point. The density of these lines is proportional to the magnitude of the tension vector. Unlike the magnetic induction vector, the lines of the H vector begin and end at the interface between two substances with different magnetic properties.

Diamagnets are substances whose magnetic moments of atoms or molecules are equal to zero in the absence of an external magnetic field, i.e. in atoms or molecules of diamagnetic substances, the vector sum of the orbital magnetic moments of all electrons is zero. Diamagnets are inert gases, majority organic compounds, many metals (bismuth, zinc, gold, copper, silver, mercury, etc.), resins, water, glass, marble.

When a diamagnetic substance is introduced into a magnetic field, a magnetic moment ΔР m is induced in each of its atoms, directed opposite to the vector B of the magnetic field induction.

To characterize the magnetization of a substance, we introduce physical quantity, called the magnetization intensity.

Magnetization vector or magnetization intensity J is the ratio of the magnetic moment of a small volume ΔV of a substance to this volume

where P mi is the magnetic moment i th molecule, n - total number molecules in volume ΔV. The volume ΔV must be so small that within its limits the magnetic field can be considered uniform. IN International system units (SI) the magnetization vector is measured in amperes per meter (A/m).

If a certain body is introduced into a uniform magnetic field of strength H 0 in a medium with permeability μ 1, then the magnetic field strength inside this body H will be equal to the sum of the strengths of the external (initial) field H 0 and the field H m created by the molecular currents of the body:

Н= Н 0 + Н m,

where N m is called demagnetization field. This field depends on the coordinates of the considered point of the body, its shape and orientation relative to external field.

Magnetic induction B in a magnet determined by the sum of the field created external sources, and the fields of magnetic moments of the magnet itself:

Where does the magnetic field strength come from?

Magnetic permeability in contrast to dielectric constant can be either greater or less than one. For diamagnetic materials μ<1, а у парамагнетиков μ>1.

If the vector sum of the orbital magnetic moments of all electrons of an atom (or molecule) is not equal to zero, then the atom as a whole has a certain magnetic moment Р m. Such atoms (molecules) are called paramagnetic, and the substances consisting of them are called paramagnetic. Paramagnetic materials include oxygen, nitric oxide, aluminum, platinum, and other substances.

In paramagnets, the magnetization vector is directed along the applied field. In this case, the magnetic moments of atoms and molecules are different from zero, but are directed randomly. When an external magnetic field is applied, their directions are redistributed. The number of magnetic moments approaching the magnetic field turns out to be predominant. This leads to the appearance of a nonzero magnetization directed along the field induction vector.

Unlike diamagnetic materials, paramagnetic materials have magnetic susceptibility that strongly depends on temperature.

For many paramagnetic substances, the change in magnetic susceptibility with temperature obeys the law established by Curie:

where T – thermodynamic temperature, C is the Curie constant, depending on the type of substance.

The classical theory of paramagnetism was developed by P. Langevin in 1905. He considered statistical problem on the behavior of molecular currents (and the corresponding magnetic moments P m) in a uniform magnetic field. The orienting effect of a magnetic field on an atom depends on the magnetic moment of the atom and the magnetic induction B of the field.

The discoveries of Oersted and Ampere led to a new and deeper understanding of the nature of magnetic phenomena. Based on the identity established in these experiments magnetic actions magnets and appropriately selected currents, Ampere decisively abandoned the idea of ​​the existence of special magnetic charges. From Ampere's point of view, an elementary magnet is a circular current circulating inside a small particle of matter: an atom, a molecule or a group of them. When magnetized, more or less of these currents are set parallel to each other, as shown in (ampere currents).

We have seen that, in its magnetic properties, a circular current is quite similar to a short magnet, the axis of which is perpendicular to the plane of the current. Therefore, shown conventionally in Fig. 209 the system of oriented molecular currents is completely equivalent to the chains of elementary magnets in the Coulomb hypothesis.

Thus, Ampere's theory made the assumption of the existence of special magnetic charges unnecessary, making it possible to explain all magnetic phenomena using elementary electric currents. Further deeper study of the properties of magnetizable bodies showed not only that the hypothesis of magnetic charges or elementary magnets is unnecessary, but that it is incorrect and cannot be reconciled with some experimental facts. We will get to know these facts later. From the point of view of Ampere's theory, the inseparability of the north and south poles, which we discussed in the previous paragraph, becomes completely clear. Each elementary magnet represents circular turn current We have already seen that one side of this coil corresponds to the north pole, the other to the south pole. That is why it is impossible to separate the north and south poles from each other, just as it is impossible to separate one side of the plane from the other.

Thus, we arrived at the following main result.

There are no magnetic charges. Each atom of a substance can be considered in relation to its magnetic properties as a circular current. The magnetic field of a magnetized body is composed of the magnetic fields of these circular currents.

In a non-magnetized body, all elementary currents are located chaotically, and therefore we do not observe any magnetic field in external space.

The process of magnetization of a body consists in the fact that, under the influence of an external magnetic field, its elementary currents, to a greater or lesser extent, become parallel to each other and create a resulting magnetic field.

Magnetic moment

Magnetic moment, magnetic dipole moment- the main quantity characterizing magnetic properties substances. The source of magnetism, according to classical theory electromagnetic phenomena, are electric macro- and microcurrents. Elementary source Magnetism is considered to be a closed current. Elementary particles have a magnetic moment, atomic nuclei, electronic shells atoms and molecules. Magnetic moment elementary particles(electrons, protons, neutrons and others), as quantum mechanics has shown, is due to the existence of their own mechanical moment - spin.

The magnetic moment is measured in A⋅m 2 or J/T (SI), or erg/Gs (SGS), 1 erg/Gs = 10 -3 J/T. The specific unit of elementary magnetic moment is the Bohr magneton.

In the case of a flat circuit with electric current, the magnetic moment is calculated as

Where I- current strength in the circuit, S- contour area, - unit vector normal to the contour plane. The direction of the magnetic moment is usually found according to the gimlet rule: if you rotate the handle of the gimlet in the direction of the current, then the direction of the magnetic moment will coincide with the direction forward motion gimlet.

Probably each of you has seen magnets and even studied their properties. If you bring a magnet to a pile small items, some of them (nails, buttons, paper clips) will be attracted to the magnet, but some (pieces of chalk, copper and aluminum coins, lumps of earth) will not react at all. Why is this so? Does a magnetic field really have no effect on some substances? This is exactly what will be discussed in this paragraph.

Rice. 5.1. As a result of the action of the electric field of a negatively charged rod, the part of the conducting sphere closest to it acquires a positive charge

Rice. 5.2. Samples from a diamagnetic (a) and paramagnetic (b) in an external magnetic field: red lines - lines of the magnetic field created by the sample; blue - magnetic lines external magnetic field; green - lines of the resulting magnetic field

Comparing the effects of electric and magnetic fields on matter

Studying in 8th grade electrical phenomena, you learned that under the influence of an external electric field a redistribution occurs electric charges inside uncharged body(Fig. 5.1). As a result, the body forms its own electric field, directed opposite to the external one, and that is why the electric field in the substance is always weakened.

The substance also changes the magnetic field. There are substances that (as in the case of electric field) weaken the magnetic field inside themselves. Such substances are called diamagnetic. Many substances, on the contrary, enhance the magnetic field - these are paramagnets and ferromagnets.

The fact is that any substance placed in a magnetic field is magnetized, that is, it creates its own magnetic field, the magnetic induction of which is different for different substances.

learn about weakly magnetic substances

Substances that become magnetized by creating a weak magnetic field, the magnetic induction of which is much less than the magnetic induction of the external magnetic field (that is, the field that caused the magnetization), are called weakly magnetic substances. These substances include diamagnetic and paramagnetic substances.

Diamagnets (from the Greek dia - divergence) are magnetized, creating a weak magnetic field directed opposite to the external magnetic field (Fig. 5.2, a). This is why diamagnetic materials slightly weaken the external magnetic field: the magnetic induction of the magnetic field inside

diamagnetic material (V d) is slightly less than the magnetic induction of the external magnetic field (V 0):

If a diamagnetic material is placed in a magnetic field, it will be pushed out of it (Fig. 5.3).

Rice. 5.4. An iron nail is magnetized in a magnetic field so that the end of the nail located near north pole magnet becomes south pole, so the nail is attracted to the magnet

Rice. 5.5. Ferromagnets create a strong magnetic field directed in the same direction as the external magnetic field (a); magnetic induction lines seem to be drawn into the ferromagnetic sample (b)

Why is a diamagnetic substance pushed out of a magnetic field (Fig. 5.2, a)?

Diamagnets include inert gases (helium, neon, etc.), many metals (gold, copper, mercury, silver, etc.), molecular nitrogen, water, etc. The human body is diamagnetic, since it is on average 78 % consists of water.

Paramagnets (from the Greek para - near) are magnetized, creating a weak magnetic field directed in the same direction as the external magnetic field (Fig. 5.2, b). Paramagnets slightly enhance the external field: the magnetic induction of the magnetic field inside the paramagnet (B p) is slightly greater than the magnetic induction of the external magnetic field (B 0):

Paramagnetic materials include oxygen, platinum, aluminum, alkaline and alkaline earth metals and other substances. If a paramagnetic substance is placed in a magnetic field, it will be drawn into this field.

Studying ferromagnets

If weakly magnetic substances are removed from a magnetic field, their magnetization will immediately disappear. It happens differently with highly magnetic substances—ferromagnets.

Ferromagnets (from the Latin ferrum - iron) are substances or materials that remain magnetized in the absence of an external magnetic field.

Ferromagnets are magnetized, creating a strong magnetic field directed in the same direction as the external magnetic field (Fig. 5.4, 5.5, a). If a body made of a ferromagnet is placed in a magnetic field, it will be drawn into it (Fig. 5.5, b).

Why are only objects made of ferromagnetic materials held by a permanent magnet (Fig. 5.6)?

Ferromagnets include a small group of substances: iron, nickel, cobalt, rare earth substances and a number of alloys. Ferromagnets significantly enhance the external magnetic field: the magnetic induction of the magnetic field inside ferromagnets (Vf) is hundreds and thousands of times greater than the magnetic induction of the external magnetic field (B 0):

Curie temperature for some ferromagnets

Thus, cobalt enhances the magnetic field by 175 times, nickel by 1120 times, and transformer steel (96-98% consists of iron) by 8000 times.

Ferromagnetic materials are conventionally divided into two types. Materials that remain magnetized after the cessation of the external magnetic field long time, are called magnetically hard ferromagnets. They are used for making permanent magnets. Ferromagnetic materials that are easily magnetized and quickly demagnetized are called soft magnetic ferromagnets. They are used for the manufacture of cores of electromagnets, motors, transformers, that is, devices that are constantly remagnetized during operation (you will learn about the structure and operating principle of such devices later).

Pay attention! When the Curie temperature is reached (see table), the ferromagnetic properties of magnetically soft and magnetically hard materials disappear - the materials become paramagnetic.

Let's get acquainted with Ampere's hypothesis

Observing the action of a current-carrying conductor on a magnetic needle (see Fig. 1.1) and finding out that current-carrying coils behave like permanent magnets (see Fig. 1.3), A. Ampere put forward a hypothesis about the magnetic properties of substances. Ampere suggested that inside substances there is huge amount undamped small circular currents and each of them, like a small coil, is a magnet. A permanent magnet consists of many such elementary magnets oriented in a certain direction.

Ampere explained the mechanism of magnetization of substances as follows. If the body is not magnetized, the circular currents are oriented randomly (Fig. 5.7, a). An external magnetic field tries to orient these currents so that the direction of the magnetic field of each current coincides with the direction of the external

Rice. 5.7. The mechanism of magnetization of bodies according to Ampere’s hypothesis: a—circular currents are randomly oriented, the body is not magnetized; b - circular currents are oriented in a certain direction, the body is magnetized

magnetic field (Fig. 5.7, b). For some substances, this orientation of currents (magnetization) remains even after the cessation of the external magnetic field. Thus, Ampere explained all magnetic phenomena by the interaction of moving charged particles.

Ampere's hypothesis served as the impetus for the creation of the theory of magnetism. Based on this hypothesis, it was explained known properties ferromagnets, but she could not explain the nature of dia- and paramagnetism, as well as why only small quantity substances have ferromagnetic properties. Modern theory magnetism is based on the laws quantum mechanics and A. Einstein's theory of relativity.

Let's sum it up

Any substance placed in a magnetic field becomes magnetized, that is, it creates its own magnetic field.

Security questions

1. Why does a substance change its magnetic field? 2. Give examples of diamagnetic materials; paramagnetic materials; ferromagnets. What is the direction of the own magnetic field of each of these substances? 3. How does a body made of a diamagnetic material behave in an external magnetic field? paramagnetic? ferromagnetic? 4. Why are ferromagnetic materials considered highly magnetic?

5. Where are soft magnetic materials used? magnetically hard materials?

6. How did A. Ampere explain the magnetization of ferromagnets?

Exercise No. 5

1. Which steel—soft magnetic or hard magnetic—is more suitable for making permanent magnets?

2. What magnetic properties will have: a) iron at 900 °C? b) cobalt at 900 °C?

3. A copper cylinder was suspended from a spring and placed in a strong magnetic field (Fig. 1). How did the spring elongation change?

4. Why can a permanent magnet hold a chain of iron objects (Fig. 2)?

5. A vessel under high pressure contains a mixture of gases (nitrogen and oxygen). Suggest a way to separate this mixture into individual components.

6. Taking advantage additional sources information, learn about magnetic levitation. What are the prospects for its application?

Experimental task

Explore the interaction enough strong magnet with bodies made from different materials(for example, copper, aluminum, iron).

This is textbook material

Any substance in the world has certain magnetic properties. They are measured by magnetic permeability. In this article we will look at the magnetic properties of matter.

Ampere's hypothesis

Magnetic permeability shows how many times the magnetic field induction in a given environment is less or greater than the magnetic field induction in a vacuum.

A substance that creates its own magnetic field is called magnetized. Magnetization occurs when a substance is placed in an external magnetic field.

The French scientist Ampere established the reason, the consequence of which is the possession of magnetic properties by bodies. Ampere's hypothesis states that there are microscopic electric currents inside a substance (an electron has its own magnetic moment, which has quantum nature, orbital motion electrons in atoms). It is they that determine the magnetic properties of a substance. If the currents have disordered directions, then the magnetic fields they generate cancel each other out. The body is not magnetized. An external magnetic field regulates these currents. As a result, the substance develops its own magnetic field. This is the magnetization of the substance.

It is precisely by the reaction of substances to an external magnetic field and by their ordering internal structure, determine the magnetic properties of a substance. In accordance with these parameters, they are divided into the following groups:

• Paramagnets
• Diamagnets
• Ferromagnets
• Antiferromagnets

Diamagnets and paramagnets

• Substances that have negative magnetic susceptibility, independent of the magnetic field strength, are called diamagnetic materials. Let's figure out what magnetic properties of a substance are called negative magnetic susceptibility. This is when a magnet is brought to a body, and it is repelled rather than attracted. Diamagnets include, for example, inert gases, hydrogen, phosphorus, zinc, gold, nitrogen, silicon, bismuth, copper, and silver. That is, these are substances that are in a superconducting state or have covalent bonds.
• Paramagnetic materials. For these substances, the magnetic susceptibility also does not depend on what field strength exists. She is positive though. That is, when a paramagnetic approaches a constant working magnet, an attractive force arises. These include aluminum, platinum, oxygen, manganese, iron.

Ferromagnets

Substances that have high positive magnetic susceptibility are called ferromagnets. For these substances, unlike diamagnetic and paramagnetic materials, the magnetic susceptibility depends on temperature and magnetic field strength, and to a significant extent. These include nickel and cobalt crystals.

Antiferromagnets and ferrimagnets

• Substances that undergo heating when heated phase transition of this substance, accompanied by the appearance of paramagnetic properties, are called antiferromagnets. If the temperature becomes lower than a certain one, these properties of the substance will not be observed. Examples of these substances would be manganese and chromium.
• Ferrimagnets are characterized by the presence of uncompensated antiferromagnetism in them. Their magnetic susceptibility also depends on temperatures and magnetic field strength. But they still have differences. These substances include various oxides.

All of the above magnets can be further divided into 2 categories:

• Hard magnetic materials. These are materials from high value coercive force. To remagnetize them, it is necessary to create a powerful magnetic field. These materials are used in the manufacture of permanent magnets.
• Soft magnetic materials, on the contrary, have a low coercive force. In weak magnetic fields they are able to enter saturation. They have low losses due to magnetization reversal. Because of this, these materials are used to make cores for electrical machines that operate on alternating current. This is, for example, a current and voltage transformer, or a generator, or an asynchronous motor.

We looked at all the basic magnetic properties of matter and figured out what types of magnets exist.

To explain the magnetization of bodies, Ampere suggested that circular currents circulate in the molecules of the substance ( molecular currents). Each such current has a magnetic moment and creates a magnetic field in the surrounding space. In the absence of an external field, molecular currents are randomly oriented, as a result of which the resulting field due to them is equal to zero. Due to the chaotic orientation of the magnetic moments of individual molecules, the total magnetic moment of the body is also equal to zero. Under the influence of a field, the magnetic moments of molecules acquire a predominant orientation in one direction, as a result of which the magnet is magnetized - its total magnetic moment becomes non-zero. The magnetic fields of individual molecular currents in this case no longer compensate each other and field B appears

At the beginning of the study of magnetism, to explain the properties of permanent magnets, Ampere put forward a bold hypothesis at that time about the existence of so-called “molecular currents,” the totality of which explains the magnetic properties of matter. At present, Ampere's hypothesis seems almost obvious; the physical mechanisms responsible for the magnetic properties of substances have been studied much more deeply than was possible in Ampere's time