It's called a magnetic field. A magnetic field

A magnetic field this is the matter that arises around sources electric current, as well as around permanent magnets. In space, the magnetic field is displayed as a combination of forces that can influence magnetized bodies. This action is explained by the presence of driving discharges at the molecular level.

A magnetic field is formed only around electric charges that are in motion. That is why the magnetic and electric fields are integral and together form electromagnetic field. The components of the magnetic field are interconnected and influence each other, changing their properties.

Properties of magnetic field:
1. A magnetic field arises under the influence of driving charges of electric current.
2. At any point, the magnetic field is characterized by the vector physical quantity entitled magnetic induction, which is the strength characteristic of the magnetic field.
3. A magnetic field can only affect magnets, current-carrying conductors and moving charges.
4. The magnetic field can be constant or alternating type
5. The magnetic field is measured only by special instruments and cannot be perceived by human senses.
6. The magnetic field is electrodynamic, since it is generated only by the movement of charged particles and affects only charges that are in motion.
7. Charged particles move along a perpendicular trajectory.

The size of the magnetic field depends on the rate of change of the magnetic field. According to this feature, there are two types of magnetic fields: dynamic magnetic field And gravitational magnetic field. Gravitational magnetic field appears only near elementary particles and is formed depending on the structural features of these particles.

Magnetic moment occurs when a magnetic field acts on a conductive frame. In other words, the magnetic moment is a vector that is located on the line that runs perpendicular to the frame.

The magnetic field can be represented graphically using magnetic lines of force. These lines are drawn in such a direction that the direction of the field forces coincides with the direction of the field line itself. Magnetic lines of force are continuous and closed at the same time.

The direction of the magnetic field is determined using a magnetic needle. The lines of force also determine the polarity of the magnet, the end with the output of the force lines is the north pole, and the end with the input of these lines is the south pole.

It is very convenient to visually evaluate the magnetic field using ordinary iron filings and a piece of paper.
If we put a sheet of paper on a permanent magnet and sprinkle sawdust on top, then the iron particles will line up according to the magnetic field lines.

The direction of the power lines for a conductor is conveniently determined by the famous gimlet rule or rule right hand . If we wrap our hand around the conductor so that thumb looked in the direction of the current (from minus to plus), then the 4 remaining fingers will show us the direction of the magnetic field lines.

And the direction of the Lorentz force is the force with which the magnetic field acts on a charged particle or conductor with current, according to left hand rule.
If we place left hand in a magnetic field so that 4 fingers look in the direction of the current in the conductor, and the lines of force enter the palm, then the thumb will indicate the direction of the Lorentz force, the force acting on a conductor placed in a magnetic field.

That's all. Be sure to ask any questions you have in the comments.

Just as a stationary electric charge acts on another charge through electric field, an electric current acts on another current through magnetic field. The effect of a magnetic field on permanent magnets is reduced to its effect on charges moving in the atoms of a substance and creating microscopic circular currents.

The doctrine of electromagnetism based on two provisions:

• the magnetic field acts on moving charges and currents;
• a magnetic field arises around currents and moving charges.

Magnet interaction

Permanent magnet(or magnetic needle) is oriented along the Earth's magnetic meridian. The end that points north is called north pole(N), and the opposite end is south pole(S). Bringing two magnets closer to each other, we note that their like poles repel, and their unlike poles attract ( rice. 1 ).

If we separate the poles by cutting a permanent magnet into two parts, we will find that each of them will also have two poles, i.e. will be a permanent magnet ( rice. 2 ). Both poles - north and south - are inseparable from each other and have equal rights.

The magnetic field created by the Earth or permanent magnets is represented, like an electric field, by magnetic lines of force. A picture of the magnetic field lines of a magnet can be obtained by placing a sheet of paper over it, on which iron filings are sprinkled in an even layer. When exposed to a magnetic field, the sawdust becomes magnetized - each of them has north and south poles. The opposite poles tend to move closer to each other, but this is prevented by the friction of the sawdust on the paper. If you tap the paper with your finger, the friction will decrease and the filings will be attracted to each other, forming chains depicting magnetic field lines.

On rice. 3 shows the location of sawdust and small magnetic arrows in the field of a direct magnet, indicating the direction of the magnetic field lines. This direction is taken to be the direction of the north pole of the magnetic needle.

Oersted's experience. Magnetic field of current

IN early XIX V. Danish scientist Ørsted did important discovery, having discovered action of electric current on permanent magnets . He placed a long wire near a magnetic needle. When current was passed through the wire, the arrow rotated, trying to position itself perpendicular to it ( rice. 4 ). This could be explained by the emergence of a magnetic field around the conductor.

The magnetic field lines created by a straight conductor carrying current are concentric circles located in a plane perpendicular to it, with centers at the point through which the current passes ( rice. 5 ). The direction of the lines is determined by the right screw rule:

If the screw is rotated in the direction of the field lines, it will move in the direction of the current in the conductor .

The strength characteristic of the magnetic field is magnetic induction vector B . At each point it is directed tangentially to the field line. Electric field lines begin on positive charges and end on negative ones, and the force acting on the charge in this field is directed tangentially to the line at each point. Unlike the electric field, the magnetic field lines are closed, which is due to the absence of “magnetic charges” in nature.

The magnetic field of a current is fundamentally no different from the field created by a permanent magnet. In this sense, an analogue of a flat magnet is a long solenoid - a coil of wire, the length of which is significantly greater than its diameter. The diagram of the lines of the magnetic field created by him, shown in rice. 6 , is similar to that for a flat magnet ( rice. 3 ). The circles indicate the cross sections of the wire forming the solenoid winding. Currents flowing through the wire away from the observer are indicated by crosses, and currents in the opposite direction - towards the observer - are indicated by dots. The same designations are also accepted for magnetic field lines when they are perpendicular to the plane of the drawing ( rice. 7 a, b).

The direction of the current in the solenoid winding and the direction of the magnetic field lines inside it are also related by the rule of the right screw, which in this case is formulated as follows:

If you look along the axis of the solenoid, the current flowing in a clockwise direction creates a magnetic field in it, the direction of which coincides with the direction of movement of the right screw ( rice. 8 )

Based on this rule, it is easy to understand that the solenoid shown in rice. 6 , the north pole is its right end, and the south pole is its left.

The magnetic field inside the solenoid is uniform - the magnetic induction vector has a constant value there (B = const). In this respect, the solenoid is similar to a parallel-plate capacitor, within which a uniform electric field is created.

Force acting in a magnetic field on a current-carrying conductor

It was experimentally established that a force acts on a current-carrying conductor in a magnetic field. In a uniform field, a straight conductor of length l, through which a current I flows, located perpendicular to the field vector B, experiences the force: F = I l B .

The direction of the force is determined left hand rule:

If the four outstretched fingers of the left hand are placed in the direction of the current in the conductor, and the palm is perpendicular to vector B, then the extended thumb will indicate the direction of the force acting on the conductor (rice. 9 ).

It should be noted that the force acting on a conductor with current in a magnetic field is not directed tangentially to its lines of force, like an electric force, but perpendicular to them. A conductor located along the lines of force is not affected by magnetic force.

The equation F = IlB allows you to give a quantitative characteristic of the magnetic field induction.

Attitude does not depend on the properties of the conductor and characterizes the magnetic field itself.

The magnitude of the magnetic induction vector B is numerically equal to the force acting on a conductor of unit length located perpendicular to it, through which a current of one ampere flows.

In the SI system, the unit of magnetic field induction is the tesla (T):

A magnetic field. Tables, diagrams, formulas

(Interaction of magnets, Oersted's experiment, magnetic induction vector, vector direction, superposition principle. Graphic image magnetic fields, magnetic induction lines. Magnetic flux, energy characteristics of the field. Magnetic forces, Ampere force, Lorentz force. Movement of charged particles in a magnetic field. Magnetic properties of matter, Ampere's hypothesis)

In the last century, various scientists put forward several assumptions about the Earth's magnetic field. According to one of them, the field appears as a result of the rotation of the planet around its axis.

It is based on the curious Barnett-Einstein effect, which is that when any body rotates, a magnetic field arises. Atoms in this effect have their own magnetic moment as they rotate around their axis. This is how the Earth's magnetic field appears. However, this hypothesis did not stand up to experimental testing. It turned out that the magnetic field obtained in such a non-trivial way is several million times weaker than the real one.

Another hypothesis is based on the appearance of a magnetic field due to the circular motion of charged particles (electrons) on the surface of the planet. She also turned out to be insolvent. The movement of electrons can cause the appearance of a very weak field, and this hypothesis does not explain the inversion of the Earth's magnetic field. It is known that the north magnetic pole does not coincide with the north geographic pole.

Solar wind and mantle currents

The mechanism of formation of the magnetic field of the Earth and other planets solar system has not been fully studied and still remains a mystery to scientists. However, one proposed hypothesis explains the inversion and the magnitude of the real field induction quite well. It is based on the work of the internal currents of the Earth and the solar wind.

The Earth's internal currents flow in the mantle, which consists of substances with very good conductivity. The source of current is the core. Energy from the core to the surface of the earth is transferred by convection. Thus, in the mantle there is a constant movement of matter, which forms a magnetic field according to the well-known law of motion of charged particles. If we associate its appearance only with internal currents, it turns out that all planets whose direction of rotation coincides with the direction of rotation of the Earth should have an identical magnetic field. However, it is not. Jupiter's north geographic pole coincides with its north magnetic pole.

Not only internal currents participate in the formation of the Earth's magnetic field. It has long been known that it responds to the solar wind, a stream of high-energy particles coming from the Sun as a result of reactions occurring on its surface.

The solar wind is, by its nature, an electric current (the movement of charged particles). Carried away by the rotation of the Earth, it creates a circular current, which leads to the appearance of the Earth's magnetic field.

Magnetic fields occur in nature and can be created artificially. The man noticed them useful characteristics, which I learned to use in Everyday life. What is the source of the magnetic field?

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Earth's magnetic field

How the doctrine of the magnetic field developed

The magnetic properties of some substances were noticed in ancient times, but their study really began in medieval Europe. Using small steel needles, a scientist from France, Peregrine, discovered the intersection of magnetic force lines at certain points - the poles. Only three centuries later, guided by this discovery, Gilbert continued to study it and subsequently defended his hypothesis that the Earth has its own magnetic field.

The rapid development of the theory of magnetism began at the beginning of the 19th century, when Ampere discovered and described the influence of the electric field on the emergence of a magnetic field, and Faraday’s discovery of electromagnetic induction established an inverse relationship.

What is a magnetic field

A magnetic field manifests itself in a force effect on electric charges that are in motion, or on bodies that have a magnetic moment.

Magnetic field sources:

1. Conductors through which electric current passes;
2. Permanent magnets;
3. Changing electric field.

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Magnetic field sources

The root cause of the appearance of a magnetic field is identical for all sources: electrical microcharges - electrons, ions or protons - have their own magnetic moment or are in directional motion.

Important! Electric and magnetic fields mutually generate each other, changing over time. This relationship is determined by Maxwell's equations.

Characteristics of the magnetic field

The characteristics of the magnetic field are:

1. Magnetic flux, a scalar quantity that determines how many magnetic field lines pass through a given cross section. Denoted by the letter F. Calculated using the formula:

F = B x S x cos α,

where B is the magnetic induction vector, S is the section, α is the angle of inclination of the vector to the perpendicular drawn to the section plane. Unit of measurement – ​​weber (Wb);

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Magnetic flux

1. The magnetic induction vector (B) shows the force acting on the charge carriers. It is directed towards the north pole, where the usual magnetic needle points. Magnetic induction is measured quantitatively in Tesla (T);
2. MF tension (N). Determined by the magnetic permeability of various media. In a vacuum, permeability is taken as unity. The direction of the tension vector coincides with the direction of magnetic induction. Unit of measurement – ​​A/m.

How to represent a magnetic field

It is easy to see the manifestations of a magnetic field using the example of a permanent magnet. It has two poles and depending on the orientation the two magnets attract or repel. The magnetic field characterizes the processes occurring during this:

1. The MP is mathematically described as a vector field. It can be constructed by means of many vectors of magnetic induction B, each of which is directed towards the north pole of the compass needle and has a length depending on the magnetic force;
2. An alternative way of representing this is to use field lines. These lines never intersect, do not start or stop anywhere, forming closed loops. The MF lines are combined into areas with a more frequent location, where the magnetic field is the strongest.

Important! The density of the field lines indicates the strength of the magnetic field.

Although the MF cannot actually be seen, field lines are easy to visualize in real world, placing iron filings in the MP. Each particle behaves like a tiny magnet with a north and south pole. The result is a pattern similar to lines of force. A person is not able to feel the impact of MP.

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Magnetic field lines

Magnetic field measurement

Since this is a vector quantity, there are two parameters for measuring MF: force and direction. The direction can be easily measured using a compass connected to the field. An example is a compass placed in the Earth's magnetic field.

Measuring other characteristics is much more difficult. Practical magnetometers did not appear until the 19th century. Most of them work by using the force that the electron feels as it moves along the MP.

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Magnetometer

Very precise measurement low magnetic fields have become practically feasible since the discovery in 1988 of giant magnetoresistance in layered materials. This discovery in fundamental physics was quickly applied to magnetic technology hard drive for storing data on computers, leading to a thousandfold increase in storage capacity in just a few years.

In generally accepted measurement systems, MP is measured in tests (T) or gauss (G). 1 T = 10000 Gs. Gauss is often used because Tesla is too large a field.

Interesting. A small magnet on a refrigerator creates a magnetic field equal to 0.001 Tesla, and the Earth's magnetic field on average is 0.00005 Tesla.

The nature of the magnetic field

Magnetism and magnetic fields are manifestations of electromagnetic force. There are two possible ways, how to organize the energy charge in motion and, consequently, the magnetic field.

The first is to connect the wire to a current source, an MF is formed around it.

Important! As the current (the number of charges in motion) increases, the MP increases proportionally. As you move away from the wire, the field decreases depending on the distance. This is described by Ampere's law.

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Ampere's law

Some materials that have higher magnetic permeability are capable of concentrating magnetic fields.

Since the magnetic field is a vector, it is necessary to determine its direction. For ordinary current flowing through a straight wire, the direction can be found using the right hand rule.

To use the rule, you need to imagine that the wire is grasped with your right hand, and your thumb indicates the direction of the current. Then the four remaining fingers will show the direction of the magnetic induction vector around the conductor.

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Right hand rule

The second way to create a magnetic field is to use the fact that in some substances electrons appear that have their own magnetic moment. This is how permanent magnets work:

1. Although atoms often have many electrons, they mostly bond so that the total magnetic field of the pair cancels out. Two electrons paired in this way are said to have opposite spin. Therefore, in order to magnetize something, you need atoms that have one or more electrons with the same spin. For example, iron has four such electrons and is suitable for making magnets;
2. The billions of electrons found in atoms can be randomly oriented, and there will be no overall MF, no matter how many unpaired electrons the material has. It must be stable at low temperatures to provide an overall preferred orientation of electrons. High magnetic permeability causes the magnetization of such substances under certain conditions outside the influence of magnetic fields. These are ferromagnetic;
3. Other materials may exhibit magnetic properties in the presence of an external magnetic field. The external field serves to align all electron spins, which disappears after the MF is removed. These substances are paramagnetic. The metal of a refrigerator door is an example of a paramagnetic material.

Earth's magnetic field

The earth can be represented in the form of capacitor plates, the charge of which has the opposite sign: “minus” at the earth’s surface and “plus” in the ionosphere. Between them is atmospheric air as an insulating gasket. The giant capacitor maintains a constant charge due to the influence of the earth's MF. Using this knowledge, you can create a scheme for obtaining electrical energy from the Earth's magnetic field. True, the result will be low voltage values.

Have to take:

• grounding device;
• the wire;
• Tesla transformer capable of generating high-frequency oscillations and creating a corona discharge, ionizing the air.

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Tesla Coil

The Tesla coil will act as an electron emitter. The entire structure is connected together, and to ensure a sufficient potential difference, the transformer must be raised to a considerable height. Thus, an electrical circuit will be created through which a small current will flow. Get a large number of electricity is not possible using this device.

Electricity and magnetism dominate many of the worlds around us, from the most fundamental processes in nature to cutting-edge electronic devices.

Video

To understand what is a characteristic of a magnetic field, many phenomena must be defined. At the same time, you need to remember in advance how and why it appears. Find out what is the strength characteristic of a magnetic field. It is important that such a field can occur not only in magnets. In this regard, it would not hurt to mention the characteristics of the earth’s magnetic field.

Emergence of the field

First we need to describe the emergence of the field. Then you can describe the magnetic field and its characteristics. It appears during the movement of charged particles. May affect in particular live conductors. The interaction between a magnetic field and moving charges, or conductors through which current flows, occurs due to forces called electromagnetic.

The intensity or strength characteristic of a magnetic field at a certain spatial point is determined using magnetic induction. The latter is designated by the symbol B.

Graphical representation of the field

The magnetic field and its characteristics can be represented in graphical form using induction lines. This definition refers to lines whose tangents at any point will coincide with the direction of the magnetic induction vector.

These lines are included in the characteristics of the magnetic field and are used to determine its direction and intensity. The higher the intensity of the magnetic field, the more of these lines will be drawn.

What are magnetic lines

Magnetic lines in straight current-carrying conductors have the shape of a concentric circle, the center of which is located on the axis of the given conductor. The direction of magnetic lines near current-carrying conductors is determined by the gimlet rule, which sounds like this: if the gimlet is positioned so that it is screwed into the conductor in the direction of the current, then the direction of rotation of the handle corresponds to the direction of the magnetic lines.

In a coil with current, the direction of the magnetic field will also be determined by the gimlet rule. It is also required to rotate the handle in the direction of the current in the solenoid turns. The direction of the magnetic induction lines will correspond to the direction of the translational movement of the gimlet.

It is the main characteristic of a magnetic field.

Created by one current, under equal conditions, the field will vary in intensity in different environments due to different magnetic properties in these substances. The magnetic properties of the medium are characterized by absolute magnetic permeability. It is measured in henry per meter (g/m).

The characteristics of the magnetic field include the absolute magnetic permeability of the vacuum, called the magnetic constant. The value that determines how many times the absolute magnetic permeability of the medium will differ from the constant is called relative magnetic permeability.

Magnetic permeability of substances

This is a dimensionless quantity. Substances with a permeability value less than one are called diamagnetic. In these substances the field will be weaker than in a vacuum. These properties are present in hydrogen, water, quartz, silver, etc.

Media with a magnetic permeability exceeding unity are called paramagnetic. In these substances the field will be stronger than in a vacuum. These environments and substances include air, aluminum, oxygen, and platinum.

In the case of paramagnetic and diamagnetic substances, the value of magnetic permeability will not depend on the voltage of the external, magnetizing field. This means that the quantity is constant for a certain substance.

Ferromagnets belong to a special group. For these substances, the magnetic permeability will reach several thousand or more. These substances, which have the property of being magnetized and enhancing a magnetic field, are widely used in electrical engineering.

Field strength

To determine the characteristics of a magnetic field, a value called magnetic field strength can be used along with the magnetic induction vector. This term is determining the intensity of the external magnetic field. The direction of the magnetic field in a medium with identical properties in all directions, the intensity vector will coincide with the magnetic induction vector at the field point.

The strength of ferromagnets is explained by the presence in them of arbitrarily magnetized small parts, which can be represented in the form of small magnets.

With no magnetic field, a ferromagnetic substance may not have pronounced magnetic properties, since the fields of the domains acquire different orientations, and their total magnetic field is zero.

According to the main characteristic of the magnetic field, if a ferromagnet is placed in an external magnetic field, for example, in a coil with current, then under the influence of the external field the domains will turn in the direction of the external field. Moreover, the magnetic field at the coil will increase, and the magnetic induction will increase. If the external field is weak enough, then only a part of all domains will turn over, the magnetic fields of which are close in direction to the direction of the external field. As the strength of the external field increases, the number of rotated domains will increase, and at a certain value of the external field voltage, almost all parts will be rotated so that the magnetic fields are located in the direction of the external field. This condition called magnetic saturation.

Relationship between magnetic induction and tension

The relationship between the magnetic induction of a ferromagnetic substance and the external field strength can be depicted using a graph called a magnetization curve. At the point where the curve graph bends, the rate of increase in magnetic induction decreases. After bending, where the tension reaches a certain value, saturation occurs, and the curve rises slightly, gradually taking on the shape of a straight line. In this area, the induction is still growing, but rather slowly and only due to an increase in the external field strength.

The graphical dependence of the indicator data is not direct, which means that their ratio is not constant, and the magnetic permeability of the material is not a constant indicator, but depends on the external field.

Changes in the magnetic properties of materials

When the current strength is increased to complete saturation in a coil with a ferromagnetic core and then decreased, the magnetization curve will not coincide with the demagnetization curve. With zero intensity, the magnetic induction will not have the same value, but will acquire a certain indicator called residual magnetic induction. The situation where magnetic induction lags behind the magnetizing force is called hysteresis.

To completely demagnetize the ferromagnetic core in the coil, it is necessary to give a reverse current, which will create the necessary voltage. Different ferromagnetic substances require a piece of different lengths. The larger it is, the greater the amount of energy required for demagnetization. The value at which complete demagnetization of the material occurs is called coercive force.

With a further increase in the current in the coil, the induction will again increase to saturation, but with a different direction of the magnetic lines. When demagnetizing in the opposite direction, residual induction will be obtained. The phenomenon of residual magnetism is used when creating permanent magnets from substances with a high index of residual magnetism. Cores are created from substances that have the ability to remagnetize electric machines and instruments.

Left hand rule

The force influencing a current-carrying conductor has a direction determined by the left-hand rule: when the palm of the left hand is positioned in such a way that magnetic lines enter it, and four fingers are extended in the direction of the current in the conductor, the bent thumb will indicate the direction of the force. This power perpendicular to the induction vector and current.

A current-carrying conductor moving in a magnetic field is considered a prototype of an electric motor that changes electrical energy into mechanical energy.

Right hand rule

When a conductor moves in a magnetic field, an electromotive force is induced within it, which has a value proportional to the magnetic induction, the length of the conductor involved and the speed of its movement. This dependence is called electromagnetic induction. When determining the direction of the induced EMF in a conductor, the rule of the right hand is used: when the right hand is positioned in the same way as in the example with the left, the magnetic lines enter the palm, and the thumb indicates the direction of movement of the conductor, extended fingers will indicate the direction of the induced EMF. Moving in a magnetic flux under the influence of an external mechanical force a conductor is the simplest example of an electrical generator in which mechanical energy is converted into electrical energy.

It can be formulated differently: in a closed loop, an EMF is induced; with any change in the magnetic flux covered by this loop, the EMF in the loop is numerically equal to the rate of change of the magnetic flux that covers this loop.

This form provides an average EMF indicator and indicates the dependence of the EMF not on the magnetic flux, but on the rate of its change.

Lenz's law

You also need to remember Lenz's law: the current induced when the magnetic field passing through the circuit changes, its magnetic field prevents this change. If the turns of a coil are penetrated by magnetic fluxes of different magnitudes, then the EMF induced throughout the whole coil is equal to the sum of the EDE in different turns. The sum of the magnetic fluxes of different turns of the coil is called flux linkage. The unit of measurement for this quantity, as well as for magnetic flux, is Weber.

When the electric current in the circuit changes, the magnetic flux it creates also changes. In this case, according to the law of electromagnetic induction, an emf is induced inside the conductor. It appears due to a change in current in the conductor, because this phenomenon is called self-induction, and the emf induced in a conductor is called self-induction emf.

Flux linkage and magnetic flux depend not only on current strength, but also on the size and shape of a given conductor, and the magnetic permeability of the surrounding substance.

Conductor inductance

The proportionality factor is called the inductance of the conductor. It refers to the ability of a conductor to create flux linkage when electricity passes through it. This is one of the main parameters of electrical circuits. For certain circuits, inductance is a constant value. It will depend on the size of the circuit, its configuration and the magnetic permeability of the medium. In this case, the current strength in the circuit and the magnetic flux will not matter.

The above definitions and phenomena provide an explanation of what a magnetic field is. The main characteristics of the magnetic field are also given, with the help of which this phenomenon can be defined.