A moving electric charge creates a magnetic field in the surrounding space. A stream of electrons passing through a conductor creates a magnetic field around the conductor. If you wind a metal wire in rings on a rod, you get a coil. It turns out that the magnetic field created by such a coil has interesting and, most importantly, useful properties.
Why does a magnetic field arise?
The magnetic properties of some substances, which allow them to attract metal objects, have been known for a long time. But it was possible to come closer to understanding the essence of this phenomenon only at the beginning of the 19th century. By analogy with electric charges, there have been attempts to explain magnetic effects in terms of certain magnetic charges (dipoles). In 1820, Danish physicist Hans Oersted discovered that a magnetic needle deflects when an electric current is passed through a conductor near it.
At the same time, the French researcher André Ampere found that two conductors located parallel to each other cause mutual attraction when an electric current is passed through them in one direction and repulsion if currents are directed in different directions.
Rice. 1. Ampere's experiment with wires with current. Compass arrow near a wire with current
Based on these observations, Ampere came to the conclusion that the interaction of the current with the arrow, the attraction (and repulsion) of wires and permanent magnets among themselves can be explained if we assume that the magnetic field is created by moving electric charges. Additionally, Ampere put forward a bold hypothesis, according to which there are persistent molecular currents inside the substance, which are the cause of the appearance of a constant magnetic field. Then all magnetic phenomena can be explained by the interaction of moving electric charges, and no special magnetic charges exist.
The mathematical model (theory), with the help of which it became possible to calculate the magnitude of the magnetic field and the force of interaction, was developed by the English physicist James Maxwell. From Maxwell's equations, which combined electrical and magnetic phenomena, it followed that:
- The magnetic field arises only as a result of the movement of electric charges;
- A constant magnetic field exists in natural magnetic bodies, but in this case, the cause of the appearance of the field is the continuous movement of molecular currents (vortices) in the mass of the substance;
- The magnetic field can also be created using an alternating electric field, but this topic will be discussed in our next articles.
Magnetic field of the coil with current
A metal wire wound in rings on any cylindrical rod (wood, plastic, etc.) is an electromagnetic coil. The wire must be insulated, that is, covered with some kind of insulator (varnish or plastic sheath) in order to avoid short-circuiting adjacent turns. As a result of the current flow, the magnetic fields of all turns are added and it turns out that the total magnetic field of the coil with current is identical (completely similar) to the magnetic field of a permanent magnet.
Rice. 2. The magnetic field of the coil and permanent magnet.
Inside the coil, the magnetic field will be uniform, like in a permanent magnet. Outside, the magnetic field lines of the current coil can be detected with fine metal filings. The magnetic field lines are closed. By analogy with the magnetic needle of a compass, a coil with a current has two poles - south and north. The lines of force go out from the north pole and end at the south pole.
For current coils, there are additional, separate names that are used depending on the field of application:
- Inductor, or simply - inductance... The term is used in radio engineering;
- Throttle(drossel - regulator, limiter). Used in electrical engineering;
- Solenoid... This compound word comes from two Greek words: solen - channel, pipe and eidos - similar). This is the name of special coils with cores made of special magnetic alloys (ferromagnets), which are used as electromechanical mechanisms. For example, in automobile starters, the solenoid is a solenoid.
Rice. 3. Inductors, choke, solenoid
Magnetic field energy
In a coil with a current, energy is stored from a power source (battery, accumulator), which is the greater, the greater the current I and the value L, which is called inductance. The magnetic field energy of a coil with current W is calculated using the formula:
$$ W = ((L * I ^ 2) \ over 2) $$
This formula resembles the formula for the kinetic energy of a body. Inductance is similar to body weight, and current is similar to body speed. Magnetic energy is proportional to the square of the current, just as kinetic energy is proportional to the square of speed.
To calculate the value of the inductance of a coil, there is the following formula:
$$ L = μ * ((N ^ 2 * S) \ over l_k) $$
N is the number of turns of the coil;
S is the cross-sectional area of the coil;
l to - the length of the coil;
μ is the magnetic permeability of the core material - a reference value. The core is a metal rod placed inside the coil. It allows you to significantly increase the magnitude of the magnetic field.
What have we learned?
So, we learned that a magnetic field arises only as a result of the movement of electric charges. The magnetic field of a current coil is similar to the magnetic field of a permanent magnet. The energy of the magnetic field of the coil can be calculated by knowing the current I and the inductance L.
Test by topic
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Physics test Magnetic field of a coil with current, Electromagnets for grade 8 students with answers. The test includes 11 multiple choice questions.
1. The current coil is
1) turns of wire included in the electrical circuit
2) a device consisting of turns of wire included in an electrical circuit
3) a frame in the form of a coil, on which a wire is wound, connected to the terminals connected to the current source
2. How is a coil with a current located, hanging on flexible conductors and able to freely rotate in a horizontal plane?
1) Arbitrarily, i.e. in any direction
2) Perpendicular to the north-south direction
3) Like a compass: its axis acquires direction to the south and north poles of the Earth
3. What poles does the current coil have? Where are they located?
1) North and South; at the ends of the coil
2) North and South; in the middle of the coil
3) Western and Eastern; at the ends of the coil
4. What is the shape of the magnetic lines of the magnetic field of the current coil? What is their direction?
1) Curves covering the coil from the outside; from the north pole to the south
2) Closed curves covering all turns of the coil and passing through its holes; from the north pole to the south
3) Closed curves passing inside and outside the coil; from the south pole to the north
5. What determines the magnetic effect of a coil with current?
1) From the number of turns, current strength and voltage at its ends
2) From current strength, wire resistance and the presence or absence of an iron core inside the coil
3) From the number of turns, current strength and the presence or absence of an iron core
6. In the diagrams, conventional signs show coils that differ from each other only in the number of turns. Which of them will have the least magnetic effect at equal current strengths in them?
1) №1
2) №2
3) №3
7. The current in the coil was reduced. How has its magnetic action changed?
1) Increased
2) Decreased
3) Has not changed
8. An electromagnet is
1) coil with iron core inside
2) any coil with current
3) a coil in which you can change the current
9. What device should be included in the electromagnet circuit in order to regulate its magnetic action?
1) Galvanometer
2) Ammeter
3) Rheostat
10. The electromagnet connected to the circuit formed the poles indicated in the figure, to which iron nails were attracted. What should be done so that it has the North Pole on the left and the South Pole on the right? After that, will the carnation be attracted to the poles?
1) Change the direction of the electric current; Yes
2) Change the direction of the electric current; No
3) Change the voltage in the circuit; Yes
11. What action must be performed so that the electromagnet stops attracting iron bodies to itself?
1) Reverse current direction
2) Open the electrical circuit
3) Reduce the amperage
Answers to the physics test Magnetic field of a coil with a current, Electromagnets
1-3
2-3
3-1
4-2
5-3
6-2
7-2
8-1
9-3
10-1
11-2
Of greatest practical interest is the magnetic field of the current coil. Figure 97 shows a coil consisting of a large number of turns of wire wound on a wooden frame. When there is a current in the coil, the iron filings are attracted to its ends; when the current is turned off, they fall off.
Rice. 97. Attraction of iron filings by a coil with current
If the coil with the current is suspended on thin and flexible conductors, then it will be installed in the same way as the magnetic needle of a compass. One end of the coil will face north and the other toward the south. This means that the coil with the current, like the magnetic needle, has two poles - north and south (Fig. 98).
Rice. 98. Poles of the coil with current
There is a magnetic field around the current coil. It, like the forward current field, can be detected using sawdust (Fig. 99). The magnetic lines of the magnetic field of the current coil are also closed curves. It is generally accepted that outside the coil they are directed from the north pole of the coil to the south (see Fig. 99).
Rice. 99. Magnetic lines of the coil with current
Current coils are widely used in technology as magnets. They are convenient in that their magnetic action can be changed (strengthened or weakened) over a wide range. Consider the ways in which you can do this.
Figure 97 shows an experiment in which the action of the magnetic field of a coil with current is observed. If you replace the coil with another, with a large number of turns of wire, then with the same current strength it will attract more iron objects. Means, the magnetic effect of a coil with a current is the stronger, the greater the number of turns in it.
We will include a rheostat in the circuit containing the coil (Fig. 100) and with the help of it we will change the current in the coil. With an increase in the current strength, the effect of the magnetic field of the coil with current increases, with a decrease, it weakens.
Rice. 100. Action of the magnetic field of the coil
It also turns out that the magnetic effect of a coil with a current can be significantly enhanced without changing the number of its turns and the strength of the current in it. To do this, you need to insert an iron rod (core) inside the coil. Iron inserted inside the coil enhances the magnetic action of the coil(fig. 101).
Rice. 101. The action of the magnetic field of the coil with an iron core
A coil with an iron core inside is called an electromagnet..
An electromagnet is one of the main parts of many technical devices. Figure 102 shows an arc-shaped electromagnet holding an anchor (iron plate) with a suspended load.
Rice. 102. Arc-shaped electromagnet
Electromagnets are widely used in technology due to their remarkable properties. They quickly demagnetize when the current is turned off, depending on the purpose, they can be made the most different sizes, during the operation of the electromagnet, you can adjust its magnetic action by changing the current in the coil.
Electromagnets with a high lifting force are used in factories to carry products made of steel or cast iron, as well as steel and cast iron shavings, ingots (Fig. 103).
Rice. 103. The use of electromagnets
Figure 104 shows a sectional view of a magnetic grain separator. Very fine iron filings are mixed into the grain. These sawdust does not stick to smooth grains of healthy grains, but sticks to grains of weeds. Grains 1 are poured out of the hopper onto a rotating drum 2. There is a strong electromagnet 5 inside the drum 5. By attracting iron particles 4, it extracts weed grains from the grain stream 3 and in this way cleans the grain of weeds and accidentally caught iron objects.
Rice. 104. Magnetic separator
Electromagnets are used in telegraph, telephone sets and in many other devices.
Questions
- In which direction is the current coil suspended from long thin conductors? How does it resemble a magnetic needle?
- What methods can be used to enhance the magnetic effect of the current coil?
- What is called an electromagnet?
- For what purposes are electromagnets used in factories?
- How does a magnetic grain separator work?
Exercise 41
- It is necessary to build an electromagnet, the lifting force of which can be regulated without changing the structure. How to do it?
- What should be done to reverse the magnetic poles of the current coil?
- How to build a strong electromagnet if the designer is given the condition that the current in the electromagnet is relatively small?
- The electromagnets used in the crane have tremendous power. Electromagnets, which remove iron filings from the eyes by accident, are very weak. In what ways is this distinction achieved?
Exercise
We continue to study the issues of electromagnetic phenomena. And in today's lesson, we will consider the magnetic field of a coil with a current and an electromagnet.
Of greatest practical interest is the magnetic field of the current coil. To get a coil, you need to take an insulated conductor and wind it around a frame. Such a coil contains a large number of turns of wire. Please note: these wires are wound on a plastic frame and this wire has two leads (Fig. 1).
Rice. 1. Coil
The study of the magnetic field of the coil was carried out by two famous scientists: André-Marie Ampere and François Arago. They found that the magnetic field of the coil is fully consistent with the magnetic field of the permanent magnet (Fig. 2).
Rice. 2. Magnetic field of coil and permanent magnet
Why do the magnetic lines of the coil look like this?
If a direct current flows through a straight conductor, a magnetic field arises around it. The direction of the magnetic field can be determined by the "gimbal rule" (Fig. 3).
Rice. 3. The magnetic field of the conductor
We bend this conductor in a spiral. The direction of the current remains the same, the magnetic field of the conductor also exists around the conductor, the field of different sections of the conductor is added. Inside the coil, the magnetic field will be concentrated. As a result, we get the following picture of the magnetic field of the coil (Fig. 4).
Rice. 4. Magnetic field of the coil
There is a magnetic field around the current coil. It, like the field of a straight conductor, can be detected using sawdust (Fig. 5). The lines of the magnetic field of the current coil are also closed.
Rice. 5. Arrangement of metal filings near the coil with current
If the coil with the current is suspended on thin and flexible conductors, then it will be installed in the same way as the magnetic needle of a compass. One end of the coil will face north and the other toward the south. This means that the coil with the current, like the magnetic needle, has two poles - north and south (Fig. 6).
Rice. 6. Pole coil
In electrical diagrams, the coil is indicated as follows:
Rice. 7. Designation of the coil in the diagrams
Current coils are widely used in technology as magnets. They are convenient in that their magnetic action can be varied over a wide range.
The magnetic field of the coil is large compared to the magnetic field of the conductor (at the same current strength).
When current is passed through the coil, a magnetic field is formed around it. The more current flows through the coil, the stronger the magnetic field will be.
It can be fixed with a magnetic arrow or metal shavings.
Also, the magnetic field of the coil depends on the number of turns. The magnetic field of a coil with a current is the stronger, the greater the number of turns in it. That is, we can adjust the field of the coil by changing the number of turns or the electric current flowing through the coil.
But the most interesting was the discovery of the English engineer Sturgeon. He demonstrated the following: the scientist took and put a coil on an iron core. The point is that, passing an electric current through the turns of these coils, the magnetic field increased many times over - and all the iron objects that were around began to be attracted to this device (Fig. 8). This device is called "electromagnet".
Rice. 8. Electromagnet
When we figured out how to make an iron hook and attach it to this device, we got the opportunity to drag various weights. So what is an electromagnet?
Definition
Electromagnet is a coil with a large number of winding turns, put on an iron core, which acquires the properties of a magnet when an electric current passes through the winding.
The electromagnet in the diagram is designated as a coil, and a horizontal line is located on top (Fig. 9). This line represents the iron core.
Rice. 9. Designation of the electromagnet
When we studied electrical phenomena, we said that electric current has different properties, including magnetic. And one of the experiments that we discussed was connected with the fact that we take a wire connected to a current source, wind it around an iron nail and observe how various iron objects begin to be attracted to this nail (Fig. 10). This is the simplest electromagnet. And now we understand that the simplest electromagnet provides us with the current flow in the coil, a large number of turns and, of course, a metal core.
Rice. 10. The simplest electromagnet
Today electromagnets are very widespread. Electromagnets work just about anywhere. For example, if we need to drag large enough weights, we use electromagnets. And, by adjusting the strength of the current, we will, accordingly, either increase or decrease the strength. Another example of the use of electromagnets is the electric bell.
The opening and closing of doors and the brakes of some vehicles (for example, trams) are also provided with electromagnets.
Bibliography
- Gendenshtein L.E., Kaidalov A.B., Kozhevnikov V.B. Physics 8 / Ed. Orlova V.A., Roizen I.I. - M .: Mnemosyne.
- A.V. Peryshkin Physics 8. - M .: Bustard, 2010.
- Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M .: Education.
- Internet portal "site" ()
- Internet portal "site" ()
- Internet portal "class-fizika.narod.ru" ()
Homework
- What is a coil?
- Does any coil have a magnetic field?
- Describe the simplest electromagnet.
To concentrate the magnetic field in a certain part of the space, a coil is made from the wire, through which a current is passed.
An increase in the magnetic induction of the field is achieved by increasing the number of turns of the coil and placing it on a steel core, the molecular currents of which, creating their own field, increase the resulting field of the coil.
Rice. 3-11. Ring coil.
A ring coil (Figure 3-11) has w turns evenly distributed along the non-magnetic core. The surface, bounded by a circle of radius coinciding with the middle magnetic line, is penetrated by the total current.
Due to symmetry, the field strength H at all points lying on the middle magnetic line is the same, therefore ppm.
According to the law of total current
whence the magnetic field strength on the middle magnetic line coinciding with the center line of the annular coil,
and the magnetic induction
When the magnetic induction on the center line with sufficient accuracy can be considered equal to its average value, and, therefore, the magnetic flux through the cross section of the coil
Equation (3-20) can be shaped like Ohm's law for a magnetic circuit
where Ф is the magnetic flux; - m. d. from; - the resistance of the magnetic circuit (core).
Equation (3-21) is similar to the equation of Ohm's law for an electric circuit, that is, the magnetic flux is equal to the ratio of ppm. to the magnetic resistance of the circuit.
Rice. 3-12. Cylindrical coil.
A cylindrical coil (Fig. 3-12) can be viewed as part of a ring coil with a sufficiently large radius and with a winding located only on a part of the core, the length of which is equal to the length of the coil. The field strength and magnetic induction on the center line in the center of the cylindrical coil are determined by formulas (3-18) and (3-19), which in this case are approximate and applicable only for coils in which (Fig. 3-12).