Let's understand together what a magnetic field is. After all, many people live in this field all their lives and don’t even think about it. It's time to fix it!

Magnetic field

Magnetic field – special kind matter. It manifests itself in the action on moving electric charges and bodies that have their own magnetic moment (permanent magnets).

Important: the magnetic field does not affect stationary charges! A magnetic field is also created by moving electric charges, or by a time-varying electric field, or by the magnetic moments of electrons in atoms. That is, any wire through which current flows also becomes a magnet!

A body that has its own magnetic field.

A magnet has poles called north and south. The designations "north" and "south" are given for convenience only (like "plus" and "minus" in electricity).

The magnetic field is represented by magnetic power lines. Power lines continuous and closed, and their direction always coincides with the direction of action of the field forces. If around permanent magnet scatter metal shavings, the metal particles will show a clear picture of the lines of force magnetic field, leaving the north and entering the south pole. Graphic characteristic of a magnetic field - lines of force.

Characteristics of the magnetic field

The main characteristics of the magnetic field are magnetic induction, magnetic flux And magnetic permeability. But let's talk about everything in order.

Let us immediately note that all units of measurement are given in the system SI.

Magnetic induction B – vector physical quantity, which is the main force characteristic of the magnetic field. Denoted by the letter B . Unit of measurement of magnetic induction – Tesla (T).

Magnetic induction shows how strong the field is by determining the force it exerts on a charge. This force is called Lorentz force.

Here q - charge, v - its speed in a magnetic field, B - induction, F - Lorentz force with which the field acts on the charge.

F– a physical quantity equal to the product of magnetic induction by the area of ​​the circuit and the cosine between the induction vector and the normal to the plane of the circuit through which the flux passes. Magnetic flux- scalar characteristic of the magnetic field.

We can say that magnetic flux characterizes the number of magnetic induction lines penetrating a unit area. Magnetic flux is measured in Weberach (Wb).

Magnetic permeability– coefficient that determines the magnetic properties of the medium. One of the parameters on which the magnetic induction of a field depends is magnetic permeability.

Our planet has been a huge magnet for several billion years. The induction of the Earth's magnetic field varies depending on the coordinates. At the equator it is approximately 3.1 times 10 to the minus fifth power of Tesla. In addition, there are magnetic anomalies where the value and direction of the field differ significantly from neighboring areas. Some of the largest magnetic anomalies on the planet - Kursk And Brazilian magnetic anomalies.

The origin of the Earth's magnetic field still remains a mystery to scientists. It is assumed that the source of the field is the liquid metal core of the Earth. The core is moving, which means the molten iron-nickel alloy is moving, and the movement of charged particles is electric current, generating a magnetic field. The problem is that this theory ( geodynamo) does not explain how the field is kept stable.

The Earth is a huge magnetic dipole. The magnetic poles do not coincide with the geographic ones, although they are in close proximity. Moreover, the Earth's magnetic poles move. Their displacement has been recorded since 1885. For example, over the past hundred years the magnetic pole in Southern Hemisphere has shifted almost 900 kilometers and is now located in the Southern Ocean. The pole of the Arctic hemisphere is moving through the Arctic Ocean to the East Siberian magnetic anomaly; its movement speed (according to 2004 data) was about 60 kilometers per year. Now there is an acceleration of the movement of the poles - on average, the speed is growing by 3 kilometers per year.

What is the significance of the Earth's magnetic field for us? First of all, the Earth's magnetic field protects the planet from cosmic rays and solar wind. Charged particles from deep space do not fall directly to the ground, but are deflected by a giant magnet and move along its lines of force. Thus, all living things are protected from harmful radiation.

Several events have occurred over the course of Earth's history. inversions(changes) of magnetic poles. Pole inversion- this is when they change places. Last time this phenomenon occurred about 800 thousand years ago, and in total there were more than 400 geomagnetic inversions in the history of the Earth. Some scientists believe that, given the observed acceleration of the movement of the magnetic poles, the next pole inversion should be expected in the next couple of thousand years.

Fortunately, a pole change is not yet expected in our century. This means that you can think about pleasant things and enjoy life in the good old constant field of the Earth, having considered the basic properties and characteristics of the magnetic field. And so that you can do this, there are our authors, to whom you can confidently entrust some of the educational troubles with confidence! and other types of work you can order using the link.

Already in the 6th century. BC In China, it was known that some ores have the ability to attract each other and attract iron objects. Pieces of such ores were found near the city of Magnesia in Asia Minor, so they received the name magnets.

How do magnets and iron objects interact? Let's remember why electrified bodies are attracted? Because a peculiar form of matter is formed near an electric charge - an electric field. There is a magnet around similar form matter, but has a different nature of origin (after all, ore is electrically neutral), it is called magnetic field.

To study the magnetic field, straight or horseshoe magnets are used. Certain places on a magnet have the greatest attractive effect, they are called poles(north and south). Opposite magnetic poles attract, and like magnetic poles repel.

For the strength characteristics of the magnetic field, use magnetic field induction vector B. The magnetic field is graphically represented using lines of force ( magnetic induction lines). Lines are closed, have neither beginning nor end. The place from which magnetic lines emerge is the North Pole; magnetic lines enter the South Pole.

The magnetic field can be made "visible" using iron filings.

Magnetic field of a current-carrying conductor

And now about what we found Hans Christian Oersted And Andre Marie Ampere in 1820. It turns out that a magnetic field exists not only around a magnet, but also around any current-carrying conductor. Any wire, such as a lamp cord, through which electric current flows is a magnet! A wire with current interacts with a magnet (try holding a compass near it), two wires with current interact with each other.

Direct current magnetic field lines are circles around a conductor.

Magnetic induction vector direction

The direction of the magnetic field at a given point can be defined as the direction indicated by the north pole of a compass needle placed at that point.

The direction of the magnetic induction lines depends on the direction of the current in the conductor.

The direction of the induction vector is determined according to the rule gimlet or rule right hand.

Magnetic induction vector

This is a vector quantity characterizing force action fields.

Induction of the magnetic field of an infinite straight conductor with current at a distance r from it:

Magnetic field induction at the center of a thin circular coil of radius r:

Magnetic field induction solenoid(a coil whose turns are sequentially passed current in one direction):

Superposition principle

If a magnetic field at a given point in space is created by several field sources, then magnetic induction is the vector sum of the inductions of each field separately

The earth is not only a large negative charge and source electric field, but at the same time, the magnetic field of our planet is similar to the field of a direct magnet of gigantic proportions.

Geographic south is close to magnetic north, and geographic north is close to magnetic south. If a compass is placed in the Earth's magnetic field, then its north arrow is oriented along the lines of magnetic induction in the direction of the south magnetic pole, that is, it will show us where the geographic north is located.

The characteristic elements of terrestrial magnetism change very slowly over time - secular changes. However, from time to time there are magnetic storms, when the Earth's magnetic field is greatly distorted for several hours and then gradually returns to its previous values. Such a drastic change affects people's well-being.

The Earth's magnetic field is a "shield" that protects our planet from particles penetrating from space ("solar wind"). Near the magnetic poles, particle flows come much closer to the Earth's surface. During powerful solar flares, the magnetosphere is deformed, and these particles can move into the upper layers of the atmosphere, where they collide with gas molecules, forming auroras.

Iron dioxide particles on magnetic film are highly magnetized during the recording process.

Magnetic levitation trains glide over surfaces with absolutely no friction. The train is capable of reaching speeds of up to 650 km/h.

The work of the brain, the pulsation of the heart is accompanied by electrical impulses. In this case, a weak magnetic field appears in the organs.

Magnetic field lines

Magnetic fields, just like electric ones, can be represented graphically using lines of force. A magnetic field line, or magnetic field induction line, is a line whose tangent at each point coincides with the direction of the magnetic field induction vector.

A)	b)	V)

Rice. 1.2. Direct current magnetic field lines (a),

circular current (b), solenoid (c)

Magnetic lines of force, like electrical lines, do not intersect. They are drawn with such density that the number of lines crossing a unit of surface perpendicular to them is equal to (or proportional to) the magnitude of the magnetic induction of the magnetic field in a given location.

In Fig. 1.2, A the field lines of direct current are shown, which are concentric circles, the center of which is located on the current axis, and the direction is determined by the rule of the right screw (the current in the conductor is directed towards the reader).

Magnetic induction lines can be “revealed” using iron filings, which become magnetized in the field under study and behave like small magnetic needles. In Fig. 1.2, b magnetic field lines of circular current are shown. The magnetic field of the solenoid is shown in Fig. 1.2, V.

The magnetic field lines are closed. Fields with closed lines of force are called vortex fields. It is obvious that the magnetic field is a vortex field. This is the significant difference between a magnetic field and an electrostatic one.

In an electrostatic field, the lines of force are always open: they begin and end at electric charges. Magnetic lines of force have neither beginning nor end. This corresponds to the fact that there are no magnetic charges in nature.

1.4. Biot-Savart-Laplace law

French physicists J. Biot and F. Savard conducted a study in 1820 of magnetic fields created by currents flowing through thin wires various shapes. Laplace analyzed the experimental data obtained by Biot and Savart and established a relationship that was called the Biot-Savart-Laplace law.

According to this law, the magnetic field induction of any current can be calculated as a vector sum (superposition) of the magnetic field inductions created by individual elementary sections of the current. For the magnetic induction of the field created by a current element of length , Laplace obtained the formula:

, (1.3)

where is a vector, modulo equal to the length of the conductor element and coinciding in direction with the current (Fig. 1.3); – radius vector drawn from the element to the point at which it is determined; – modulus of the radius vector.

> Magnetic field lines

How to determine magnetic field lines: diagram of the strength and directions of magnetic field lines, using a compass to determine the magnetic poles, drawing.

Magnetic field lines Useful for visually displaying the strength and direction of a magnetic field.

Learning Objective

• Relate the magnetic field strengths to the density of magnetic field lines.

Main points

• Magnetic field direction displays compass needles touching magnetic field lines at any specified point.
• The strength of the B-field is inversely proportional to the distance between the lines. It is also exactly proportional to the number of lines per unit area. One line never crosses another.
• The magnetic field is unique at every point in space.
• The lines are not interrupted and create closed loops.
• The lines stretch from the north to the south pole.

Terms

• Magnetic field lines – graphic image magnitude and direction of the magnetic field.
• B-field is a synonym for magnetic field.

Magnetic field lines

It is said that as a child, Albert Einstein loved to look at a compass, thinking about how the needle sensed force without direct physical contact. Deep thinking and serious interest led to the child growing up and creating his own revolutionary theory of relativity.

Since magnetic forces affect distances, we calculate magnetic fields to represent these forces. Line graphics are useful for visualizing the strength and direction of a magnetic field. The elongation of the lines indicates the north orientation of the compass needle. Magnetic is called the B-field.

(a) – If a small compass is used to compare the magnetic field around a bar magnet, it will show the correct direction from the north pole to the south pole. (b) – Adding arrows creates continuous magnetic field lines. The strength is proportional to the proximity of the lines. (c) – If you can examine the inside of a magnet, the lines will appear as closed loops

There is nothing difficult in comparing the magnetic field of an object. First, calculate the strength and direction of the magnetic field at several locations. Mark these points with vectors pointing in the direction of the local magnetic field with a magnitude proportional to its strength. You can combine the arrows to form magnetic field lines. The direction at any point will be parallel to the direction of the nearest field lines, and the local density can be proportional to the strength.

Magnetic field lines resemble contour lines on topographic maps, because they show something continuous. Many of the laws of magnetism can be formulated using simple concepts, such as the number of field lines through a surface.

Direction of magnetic field lines represented by the alignment of iron filings on paper placed above a bar magnet

The display of lines is affected by various phenomena. For example, iron filings on a magnetic field line create lines that correspond to magnetic ones. They are also visually displayed in auroras.

A small compass sent into a field will align itself parallel to the field line, with the north pole pointing E.

Miniature compasses can be used to demonstrate fields. (a) – The magnetic field of a circular current loop resembles a magnetic one. (b) – A long and straight wire forms a field with magnetic field lines creating circular loops. (c) – When the wire is in the plane of the paper, the field protrudes perpendicular to the paper. Note which symbols are used for the box pointing in and out

A detailed study of magnetic fields helped to derive a number of important rules:

• The direction of the magnetic field touches the field line at any point in space.
• The field strength is proportional to the proximity of the line. It is also exactly proportional to the number of lines per unit area.
• Magnetic field lines never collide, which means that at any point in space the magnetic field will be unique.
• The lines remain continuous and run from the north to the south pole.

The last rule is based on the fact that the poles cannot be separated. And this is different from electric field lines, in which the end and beginning are marked by positive and negative charges.

About two and a half thousand years ago, people discovered that some natural stones have the ability to attract iron. This property was explained by the presence of a living soul in these stones, and a certain “love” for iron.

Today we already know that these stones are natural magnets, and the magnetic field, and not a special location towards iron, creates these effects. A magnetic field is a special type of matter that is different from matter and exists around magnetized bodies.

Permanent magnets

Natural magnets, or magnetites, do not have very strong magnetic properties. But man has learned to create artificial magnets with significantly greater magnetic field strength. They are made from special alloys and are magnetized by an external magnetic field. And after that they can be used independently.

Magnetic field lines

Any magnet has two poles, they are called north and south poles. At the poles the concentration of the magnetic field is maximum. But between the poles the magnetic field is also not located arbitrarily, but in the form of stripes or lines. They are called magnetic field lines. Detecting them is quite simple - just place scattered iron filings in a magnetic field and shake them slightly. They will not be located in any way, but form a kind of pattern of lines starting at one pole and ending at the other. These lines seem to come out of one pole and enter the other.

Iron filings in the field of a magnet become magnetized themselves and are placed along the magnetic lines of force. Exactly in a similar way The compass works. Our planet is big magnet. The compass needle picks up the Earth's magnetic field and, turning, is located along the lines of force, with one end pointing to the north magnetic pole, the other to the south. The Earth's magnetic poles are slightly misaligned with the geographic ones, but when traveling away from the poles, this does not matter of great importance, and they can be considered coincident.

Variable magnets

The scope of application of magnets in our time is extremely wide. They can be found inside electric motors, telephones, speakers, and radio devices. Even in medicine, for example, when a person swallows a needle or other iron object, it can be removed without surgery with a magnetic probe.