Magnetic field of the coil with current. Electromagnets and their application

Electromagnetism is a set of phenomena caused by the connection of electric currents and magnetic fields. Sometimes this connection leads to undesirable effects. For example, the current flowing through electrical cables on the ship causes an unnecessary deviation of the ship's compass. However, often electricity is deliberately used to create high-intensity magnetic fields. An example is electromagnets. We'll talk about them today.

Electric current and magnetic flux

The intensity of the magnetic field can be determined by the number of magnetic flux lines, which per unit area. A magnetic field arises everywhere where an electric current flows, and the magnetic flux in the air is proportional to the latter. A straight wire carrying a current can be bent into a coil. With a sufficiently small radius of the turn, this leads to an increase in the magnetic flux. The current does not increase.

The effect of the concentration of magnetic flux can be further strengthened by increasing the number of turns, i.e., twisting the wire into the coil. The reverse is also true. The magnetic field of the coil with current can be weakened if the number of turns is reduced.

We derive an important relation. At the point of maximum magnetic flux density (the greatest number of flow lines per unit area), the ratio between the electric current I, the number of turns of the wire n and the magnetic flux B is expressed as follows: In is proportional to B. Current in 12 A, current in the coil of 3 turns , Creates exactly the same magnetic field as the current of 3 A, flowing through a coil of 12 turns. It is important to know this by solving practical problems.

Solenoid

A coil of wound wire that creates a magnetic field is called a solenoid. The wires can be wound on iron (iron core). A non-magnetic base (for example, an air core) is also suitable. As you can see, you can use not only iron to create a magnetic field of a current coil. From the point of view of the magnitude of the flow, any non-magnetic core is equivalent to air. That is, the above relation relating the current, the number of turns, and the flow, in this case is carried out quite accurately. Thus, the magnetic field of a coil with a current can be weakened by applying this regularity.

The use of iron in a solenoid

Why is the iron used in the solenoid? Its presence affects the magnetic field of the current coil in two respects. It increases the magnetic action of the current, often thousands of times and more. However, one important proportional relationship may be violated. This is the one that exists between the magnetic flux and the current in the coils with an air core.

Microscopic regions in the gland, domains (more precisely, their magnetic moments), under the action of the magnetic field, which is created by current, are built in one direction. As a result, in the presence of an iron core, this current creates a larger magnetic flux per unit cross-section of the wire. Thus, the flux density increases substantially. When all the domains are aligned in one direction, a further increase in the current (or number of turns in the coil) only slightly increases the density of the magnetic flux.

Let's talk a little about induction. This is an important part of the topic that interests us.

Induction of the magnetic field of a coil with a current

Although the magnetic field of a solenoid with an iron core is much stronger than the magnetic field of a solenoid with an air core, its magnitude is limited by the properties of iron. The size of the one that is created by the coil with the air core, theoretically has no limit. However, as a rule, it is very difficult and expensive to obtain the huge currents needed to create a field comparable in magnitude to the field of a solenoid with an iron core. Do not always follow this path.

What happens if you change the magnetic field of the current coil? This action can generate an electric current in the same way that a current produces a magnetic field. As the magnet approaches the conductor, the magnetic lines of force that cross the conductor induce a voltage in it. The polarity of the induced voltage depends on the polarity and the direction of the change in the magnetic flux. This effect is much more pronounced in the coil than in a separate coil: it is proportional to the number of turns in the winding. In the presence of an iron core, the induced voltage in the solenoid increases. With this method, it is necessary to move the conductor relative to the magnetic flux. If the conductor does not cross the magnetic flux lines, there will be no voltage.

How to get energy

Electric generators produce current on the basis of the same principles. Usually the magnet rotates between the coils. The magnitude of the induced voltage depends on the magnitude of the field of the magnet and the speed of its rotation (they determine the rate of change of the magnetic flux). The voltage in the conductor is directly proportional to the speed of the magnetic flux in it.

In many generators, the magnet is replaced by a solenoid. In order to create a magnetic field coil with a current, the solenoid is connected to a current source. What, in this case, will be the electrical power produced by the generator? It is equal to the product of the voltage at the current. On the other hand, the interconnection of the current in the conductor and the magnetic flux makes it possible to use a current generated by an electric current in a magnetic field to obtain mechanical motion. This principle is followed by electric motors and some electrical appliances. However, to create movement in them, it is necessary to expend additional electrical power.

Strong magnetic fields

At present, using the phenomenon of superconductivity, it is possible to obtain an unprecedented intensity of the magnetic field of the coil with current. Electromagnets can be very powerful. In this case, the current flows without loss, i.e., does not cause heating of the material. This makes it possible to use a large voltage in the solenoids with an air core and to avoid the constraints caused by the saturation effect. Very great prospects open such a powerful magnetic field coil with current. Electromagnets and their use are not in vain interested in many scientists. After all, strong fields can be used to move on a magnetic "cushion" and create new types of electric motors and generators. They are capable of high power at low cost.

The energy of the magnetic field of the coil with current is actively used by mankind. It has been widely used for many years, in particular on the railways. About how the magnetic lines of the coil current field are used to regulate the movement of trains, we will now talk.

Magnets on the railways

Railways usually use systems in which, for greater safety, electromagnets and permanent magnets complement each other. How do these systems operate? A strong permanent magnet is attached close to the rail at a certain distance from the traffic lights. During the passage of the train over the magnet, the axis of the permanent flat magnet in the driver's cab rotates a small angle, after which the magnet remains in the new position.

Regulation of traffic on the railway

The motion of the flat magnet includes a signal bell or siren. Then the following happens. After a couple of seconds, the driver's cab passes over the electromagnet, which is connected to a traffic light. If he gives the train a green street, the electromagnet turns on and the axis of the permanent magnet in the car turns to its original position, turning off the signal in the cab. When the red or yellow light is on at the traffic light, the electromagnet is turned off, and then after some delay the brake automatically turns on, if, of course, the operator forgot to do it. The brake circuit (as well as the sound signal) is connected to the network from the moment of turning the magnet axis. If the magnet returns to its original position during the delay, the brake will not turn on.