ELECTROMAGNET: COMPOSITION, PARTS, HOW IT WORKS AND APPLICATIONS - PHYSICAL - 2024

An electromagnet is a device that produces magnetism from electrical current. If the electric current ceases, then the magnetic field also disappears. In 1820 it was discovered that an electric current produces a magnetic field in its environment. Four years later the first electromagnet was invented and built.

The first electromagnet consisted of an iron horseshoe painted with insulating varnish, and eighteen turns of copper wire without electrical insulation were wound over it.

Figure 1. Electromagnet. Source: pixabay

Modern electromagnets can have various shapes depending on the final use that is going to be given to them; and it is the cable that is insulated with varnish and not the iron core. The most common shape of the iron core is the cylindrical, on which the insulated copper wire is wound.

You can make an electromagnet with just the winding producing a magnetic field, but the iron core multiplies the intensity of the field.

When electric current passes through the winding of an electromagnet, the iron core becomes magnetized. That is, the intrinsic magnetic moments of the material align and add, intensifying the total magnetic field.

Magnetism as such has been known at least since 600 BC, when the Greek Thales of Miletus spoke in detail about the magnet. Magnetite, an iron mineral, produces magnetism naturally and permanently.

Advantages of electromagnets

An undoubted advantage of electromagnets is that the magnetic field can be established, increased, decreased, or removed by controlling the electrical current. When making permanent magnets, electromagnets are necessary.

Now why is this happening? The answer is that magnetism is intrinsic to matter just as it is to electricity, but both phenomena only manifest under certain conditions.

However, it can be said that the source of the magnetic field is moving electric charges or electric current. Inside matter, at the atomic and molecular level, these currents are produced that produce magnetic fields in all directions that cancel each other out. This is why materials do not normally show magnetism.

The best way to explain it is to think that small magnets (magnetic moments) are housed inside the matter that point in all directions, so their macroscopic effect is canceled.

In ferromagnetic materials, magnetic moments can align and form regions called magnetic domains. When an external field is applied, these domains align.

When the external field is removed, these domains do not return to their original random position, but remain partially aligned. In this way the material becomes magnetized and forms a permanent magnet.

Composition and parts of an electromagnet

An electromagnet is made up of:

- A winding of cable insulated with varnish.

- An iron core (optional).

- A current source, which can be direct or alternating.

Figure 2. Parts of an electromagnet. Source: self made.

The winding is the conductor through which the current that produces the magnetic field passes and is wound in the form of a spring.

In winding, the turns or turns are usually very close together. That is why it is extremely important that the wire with which the winding is made has electrical insulation, which is achieved with a special varnish. The purpose of varnishing is that even when the turns are grouped together and touch each other, they remain electrically isolated and the current continues its spiral course.

The thicker the winding conductor, the more current the cable will withstand, but limits the total number of turns that can be wound. It is for this reason that many electromagnet coils use a thin wire.

The magnetic field produced will be proportional to the current that passes through the winding conductor and also proportional to the density of turns. This means that the more turns per unit length are placed, the greater the intensity of the field.

The tighter the winding turns are, the greater the number that will fit in a given length, increasing their density and therefore the resulting field. This is another reason why electromagnets use cable insulated with varnish instead of plastic or other material, which would add thickness.

Solenoid

In a solenoid or cylindrical electromagnet like the one shown in figure 2, the intensity of the magnetic field will be given by the following relationship:

B = μ⋅n⋅I

Where B is the magnetic field (or magnetic induction), which in units of the international system is measured in Tesla, μ is the magnetic permeability of the core, n is the density of turns or number of turns per meter and finally the current I that circulates through the winding that is measured in amps (A).

The magnetic permeability of the iron core depends on its alloy and is usually between 200 and 5000 times the permeability of air. The resulting field is multiplied by this same factor with respect to that of an electromagnet without an iron core. The permeability of air is approximately equal to that of a vacuum, which is μ ₀ = 1.26 × 10 ^-6 T * m / A.

How does it work?

To understand the operation of an electromagnet it is necessary to understand the physics of magnetism.

Let's start with a simple straight wire carrying a current I, this current produces a magnetic field B around the wire.

Figure 3. Magnetic field produced by a straight wire. Source: Wikimedia Commons

The magnetic field lines around the straight wire are concentric circles around the lead wire. The field lines comply with the right hand rule, that is, if the thumb of the right hand points in the direction of the current, the other four fingers of the right hand will indicate the direction of circulation of the magnetic field lines.

Magnetic field of a straight wire

The magnetic field due to a straight wire at a distance r from it is:

Suppose we bend the cable so that it forms a circle or loop, then the magnetic field lines on the inside of it come together pointing all in the same direction, adding and strengthening. In the inner part of the loop or circle the field is more intense than in the outer part, where the field lines separate and weaken.

Figure 4. Magnetic field produced by a wire in a circle. Source: Wikimedia Commons

The magnetic field in the center of a loop

The resulting magnetic field at the center of a loop of radius a carrying a current I is:

The effect multiplies if each time we bend the cable so that it has two, three, four,… and many turns. When we wind the cable in the form of a spring with very close coils, the magnetic field inside the spring is uniform and very intense, while on the outside it is practically zero.

Suppose we wind the cable in a spiral of 30 turns in 1 cm long and 1 cm in diameter. This gives a density of turns of 3000 turns per meter.

Ideal solenoid magnetic field

In an ideal solenoid the magnetic field inside it is given by:

Summarizing, our calculations for a cable that carries 1 ampere of current and calculating the magnetic field in microteslas, always 0.5 cm away from the cable in different configurations:

1. Straight cable: 40 microteslas.
2. Cable in a circle of 1 cm in diameter: 125 microteslas.
3. Spiral of 300 turns in 1 cm: 3770 microteslas = 0.003770 Tesla.

But if we add to the spiral an iron core with a relative permittivity of 100, then the field is multiplied 100 times, that is 0.37 Tesla.

It is also possible to calculate the force that the electromagnet in solenoid form exerts on a section of the iron core of cross section A:

Assuming a saturation magnetic field of 1.6 Tesla, the force per square meter section of iron core area exerted by the electromagnet will be 10 ^ 6 Newton equivalent to 10 ^ 5 Kilograms force, that is 0.1 tons per square meter of cross section.

This means that an electromagnet with a saturation field of 1.6 Tesla exerts a force of 10 kg on an iron core with a cross section of 1 cm ².

Electromagnet Applications

Electromagnets are part of many gadgets and devices. For example, they are present inside:

- Electric motors.

- Alternators and dynamos.

- Speakers.

- Electromechanical relays or switches.

- Electric bells.

- Solenoid valves for flow control.

- Computer hard drives.

- Scrap metal lifting cranes.

- Metal separators from urban waste.

- Electric brakes for trains and trucks.

- Nuclear magnetic resonance imaging machines.

And many more devices.

References

1. García, F. Magnetic Field. Recovered from: www.sc.ehu.es
2. Tagueña, J. and Martina, E. Magnetism. From the compass to the spin. Recovered from: Bibliotecadigital.ilce.edu.mx.
3. Sears, Zemansky. 2016. University Physics with Modern Physics. 14th. Ed. Volume 2. 921-954.
4. Wikipedia. Electromagnet. Recovered from: wikipedia.com
5. Wikipedia. Electromagnet. Recovered from: wikipedia.com
6. Wikipedia. Magnetization. Recovered from: wikipedia.com