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The magnetic field exerts a force on electric charges that are in motion and on bodies that have a magnetic moment (permanent magnets). Together with the electric magnetic field, it forms a single electromagnetic field; By analogy with other force fields (electric and gravitational), a visual representation of the nature of the field is given by its force lines. The main quantitative characteristic of a magnetic field is magnetic induction B, therefore magnetic field lines and magnetic induction lines have the same meaning, that is, both terms can be used equally with each other.
What are power lines
The outstanding English physicist Michael Faraday (1791-1867), who studied the nature of the electromagnetic field, was the first to formulate the concept of lines of force for electric and magnetic fields.
Magnetic field lines have the following basic properties:
- Lines of force are a graphical visualization (“picture”) of an image of a force field;
- Lines of force fill space in such a way that the tangents to them at each point in space coincide in direction with the magnetic induction vector;
- Only one line of force passes through each point;
- The density (thickness) of field lines penetrating a unit perpendicular area is proportional to the magnetic induction module B in this area;
- The magnetic field lines are always closed, since the magnetic field is a vortex type field. Vortex fields are any fields that have closed lines of force.
M. Faraday is rightfully considered one of the discoverers of the nature of electromagnetic phenomena. In 1845, he was the first to clearly formulate the concept of the electromagnetic field. In addition, he discovered a fundamental law, named after him, which states that in a closed conducting circuit through which a time-varying magnetic flux passes, a potential difference arises, that is, an electromotive force proportional to the rate of change of the magnetic flux.
Earth's magnetic field
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 are the 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 the electric current that generates the magnetic field. The problem is that this theory ( geodynamo ) does not explain how the field is kept stable.
Earth's magnetic field
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 the 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.
Earth's magnetic field
Over the history of the Earth, several reversals (changes) of magnetic poles have occurred. A pole reversal is when the poles change places. The last time this phenomenon occurred was 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.
Examples of power lines
A visual representation of the magnetic field lines can be obtained if fine iron filings or filings from another ferromagnetic material (nickel, cobalt, etc.) are spread evenly (in one layer) on a flat glass sheet through which a current-carrying conductor is passed. Turning on the current leads to the appearance of a magnetic field in which the filings are magnetized, that is, they become “magnetic arrows” and line up along the field lines.
Rice. 1. Demonstration of magnetic field lines from a straight wire carrying current using iron filings.
It can be seen that the lines of force are concentric circles that are located in a plane perpendicular to the conductor. The centers of all circles lie on the axis of the conductor.
The next example is the magnetic field lines that are created by a conventional permanent strip magnet.
Rice. 2. Demonstration of magnetic field lines from a strip magnet using iron filings.
The direction of the magnetic induction vector is considered to be the direction from the south pole S to the north pole N. It is clearly seen that the field lines have a maximum concentration near the poles N and S. The directions of the magnetic field lines have a complex geometric shape, but all lines are continuous and closed. Inside the magnet, the density (thickness) of the field lines is maximum, and the field is uniform. The magnetic field is uniform when the magnetic induction is constant, that is = const.
Another example is a solenoid, which is a coil made by winding a flexible conductor that maintains its shape (for example, copper wire).
Rice. 3. Demonstration of magnetic field lines from the solenoid.
It turns out that the pattern of the solenoid's field lines is very similar to the field lines that are created by a permanent strip magnet. It can be seen that inside the coil the magnetic field is close to uniform.
To determine the direction of the vector, you need to use the “gimlet rule,” which sounds like this: the vector is directed in the direction where the handle of the gimlet (with a right-hand thread) would move if it were screwed in in the direction of the current in the wire (or in the frame).
Spin
The electron was discovered to have a magnetic field, the same as it would have if it were a ball rotating around its axis.
This magnetic field was called spin (from English to spin - to rotate). In addition, the electron also has an orbital magnetic moment. After all, the electron not only “rotates”, but moves in an orbit around the nucleus of the atom. And the movement of a charged body generates a magnetic field. Since the electron is negatively charged, the magnetic field caused by its orbital motion will look like this:
If the direction of the magnetic field caused by the electron's orbital motion coincides with the direction of the magnetic field of the electron itself (its spin), these fields add up and are amplified. If these magnetic fields are directed in different directions, they subtract and weaken each other.
In addition, the magnetic fields of other electrons in the atom can be added or subtracted from each other. This explains the presence or absence of magnetism (reaction to an external magnetic field or the presence of its own magnetic field) of some substances.
This article is an excerpt from a book about the basics of chemistry. The book itself is here: sites.google.com/site/kontrudar13/himia
UPD: The material is intended primarily for middle school students. Perhaps Habr is not the place for such things, but where is the place? He is not here.
Features of magnetic induction lines
The lines along which the magnetic field propagates have a number of properties:
- Closedness - has no beginning and end.
- The length of the magnetic line increases with distance from the center of the conductor.
- Their greatest concentration is observed at the poles.
- The direction and strength of the field - induction - changes at each point.
- The distance between the lines increases with distance from the conductor.
- The force acting on the magnetic needle at any point in a uniform field is the same in magnitude.
- Only one curve passes through any point.
- The lines of a uniform magnetic field are equidistant from each other.
Basic features and properties of magnetic lines
A magnetic field exists around permanent magnets (bar, arc, or other shapes) and around a metal wire carrying an electric current.
The magnetic field is depicted in the form of magnetic lines or magnetic induction lines. A magnetic induction line is a kind of geometric curve, at any point of which the vector (direction) of magnetic induction is directed tangentially to it.
can be identified :
- The magnetic lines are continuous;
- Magnetic lines are always closed. This means that in nature there are no separate magnetic charges by analogy with electric charges. Researchers have long tried to find this charge by reducing (crushing) the size of permanent magnets. But even the most microscopic magnet always has two poles: north and south;
- The direction of magnetic lines depends on the direction of electric current;
- The thickness (density) of the lines corresponds to the field value: the denser the lines are, the greater the field value.
Application
The magnetic field is used when recording on magnetic tape, a magnetic levitation train reaches speeds of over 600 km/h and moves over the surface without friction, when the brain and heart work, electrical impulses and a magnetic field arise.
Depending on the strength of magnetic properties, substances are classified into several groups. Let's look at some of them.
Weakly magnetic substances are divided into:
- Paramagnetic substances – when exposed to an external magnetic field, the substance’s own magnetic field begins to coincide with the direction of the external one.
- Diamagnets - against the direction of the external field.
Ferromagnets - have the ability to be strongly magnetized under the influence of an external magnetic field. Ferrites – ceramic ferromagnets – are often used in technology.
Every ferromagnetic substance has a Curie point - a temperature above which the ferromagnetic properties disappear and the substance becomes paramagnetic.
Ferromagnetic materials are divided into:
- Magnetic-soft - demagnetizes completely when the influence of the external magnetic field ceases. They are used in AC instruments and devices (transformers, electric motors).
- Magnetic-hard – retain magnetization when the external magnetic field ceases. Used for the production of permanent magnets.
Antiferromagnets - under the action of an external field, the magnetic moments of atoms are oriented along it, the substances are magnetized. Prominent representatives are chromium and manganese.
Planet Earth also has a magnetic field that protects all living things from streams of fast particles (charged) from space, mainly coming from the Sun. Particles are trapped by a magnetic field, forming radiation belts that act as magnetic traps. In the belts, particles move in fractions of seconds between the north and south poles, making spiral movements. In the region of the poles, some small part of charged particles penetrates into the upper layers of the atmosphere, then the effect of the northern lights is observed. The south magnetic pole is located near the north geographic pole of the Earth.
Using iron filings to detect magnetic field
A magnetic field arises around conductors through which current flows. There are many ways to detect it, some of which we discussed in the last lesson.
Now we will look at another method - using small iron filings.
Why can iron filings be used to study the magnetic field? The answer is very simple: these small pieces of iron, once in a magnetic field, become magnetized - they become small magnetic needles .
Oersted's experiment has already shown us that the magnetic needle deviates from its original position when there is a nearby conductor through which current flows. Now we will have not one such arrow, but a large number of them. We can observe how the axis of each such arrow will be established under the action of magnetic field forces.
Magnetic lines of straight wire with current
We use the same experimental scheme for a straight wire through which electric current flows. In this case, you can replace the transparent plate with a piece of cardboard or plywood.
Rice. 3. Magnetic lines of a straight wire with current.
The sawdust can be seen to line up in concentric circles, showing the shape of the magnetic lines. When the direction of the current changes, the filings rotate by 1800. Consequently, the direction of the magnetic lines in this case is related to the direction of the current in the conductor.
It is known that the Earth is a huge “strip” magnet. Thanks to this, with the help of a magnetic compass needle we can navigate in space. But we must keep in mind that there are places with large deposits of magnetites (iron ores), which create a strong “background” magnetic field, which turns the compass needle along its magnetic lines. One of these places is the Kursk magnetic anomaly, located in the Kursk region of our country.
