The first electromagnet consisted of an iron horseshoe painted with insulating varnish, and eighteen strands of uninsulated copper wire were wound around it.
Modern electromagnets can take many forms, depending on the end use they will receive; and it is the cable that is insulated with varnish and not the iron core. The most common shape of an iron core is a cylindrical, in which insulated copper wire is wound.
An electromagnet can be made with just the winding that produces a magnetic field, but the iron core multiplies the field strength.
When electrical current passes through the winding of an electromagnet, the iron core is magnetized. That is, the material’s intrinsic magnetic moments align and add up, intensifying the total magnetic field.
Magnetism as such has been known since at least 600 BC, when the Greek tales of Miletus speak in detail about the magnet. Magnetite, an iron ore, produces magnetism naturally and permanently.
Advantages of Electromagnets
An undisputed advantage of electromagnets is that the magnetic field can be established, increased, decreased or removed by controlling the electrical current. When manufacturing permanent magnets, electromagnets are needed.
Now why does this happen? The answer is that magnetism is intrinsic to matter and electricity, but both phenomena manifest themselves only under certain conditions.
However, it can be said that the source of the magnetic field is moving electrical charges or electrical current. Within matter, at the atomic and molecular level, these currents are produced that produce magnetic fields in all directions that cancel each other out. That’s why materials typically don’t show magnetism.
The best way to explain this is to think that within matter there are small magnets (magnetic moments) that point in all directions, so that their macroscopic effect is canceled out.
In ferromagnetic materials, magnetic moments can align and form regions called magnetic domains . When an outer field is applied, these domains are aligned.
When the outer field is removed, these domains do not return to their original random position but remain partially aligned. In this way, the material is magnetized and forms a permanent magnet.
Composition and parts of an electromagnet
An electromagnet is composed of:
– Cable winding insulated with varnish.
– An iron core (optional).
– A current source, which can be continuous or alternating.
The winding is the conductor through which the current produced by the magnetic field passes and is wound in the form of a spring.
In winding, the curves are usually very close. Therefore, it is extremely important that the cable with which the winding is manufactured has an electrical insulator, which is obtained with a special varnish. The purpose of varnishing is that, even when the spins are grouped and touched together, they are kept electrically isolated and the current continues in a spiral course.
The thicker the winding conductor, the greater the amount of current the cable can handle, but it limits the total number of turns that can be rolled. This is why many electromagnetic coils use a thin cable.
The magnetic field produced will be proportional to the current passing through the winding conductor and also proportional to the density of the winding. This means that the more loops per unit of length that are placed, the greater the field strength.
The tighter the windings, the greater the number that fits a given length, increasing their density and therefore the resulting field. This is another reason why electromagnets use insulated cables with varnish instead of plastic or other material, which would increase the thickness.
In a solenoid or cylindrical electromagnet as shown in Figure 2, the magnetic field strength 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 which circulates through the winding measured in amperes (A).
The magnetic permeability of the iron core depends on its alloy and is usually between 200 and 5000 times the permeability to air. In this same factor, the resulting field is multiplied with respect to that of an electromagnet without an iron core. The permeability to air is approximately equal to that of vacuum, which is µ= 1.26 × 10 -6 T * m / A.
How it works?
To understand how an electromagnet works, it is necessary to understand the physics of magnetism.
Let’s start with a simple straight cable that carries a current I, this current produces a magnetic field B around the cable.
The magnetic field lines around the straight wire are concentric circles around the lead wire. The field lines are in accordance 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 movement of the magnetic field lines.
Magnetic field of a straight cable
The magnetic field due to a straight wire at a distance r is:
Suppose we bend the cable so that it forms a circle or a loop, and then the magnetic field lines on the inside of the cable come together, all pointing in the same direction, adding and reinforcing. On the inside of the loop or circle, the field is more intense than on the outside, where the lines of the field separate and weaken.
The magnetic field at the center of a spiral
The resulting magnetic field at the center of a coil of radius one carrying a current I is:
The effect is multiplied if each time we bend the cable so that it has two, three, four, … and many turns. When we coil the cable in the form of a spring with very close turns, the magnetic field inside the spring is uniform and very intense, while outside it is practically zero.
Suppose we wind the cable into a 30-turn spiral 1 cm long and 1 cm in diameter. This provides a slew density of 3000 turns per meter.
Ideal solenoid magnetic field
In an ideal solenoid, the internal magnetic field is given by:
In summary, our calculations for a cable that carries 1 amp of current and calculates the magnetic field in microteslas, always 0.5 cm away from the cable in different configurations:
- Straight cable: 40 microteslas.
- Cable in a circle 1 cm in diameter: 125 microteslas.
- Spiral of 300 turns in 1 cm: 3770 microteslas = 0.003770 Tesla.
But if we add an iron core with a relative permittivity of 100 to the spiral, the field is multiplied 100 times, that is, 0.37 Tesla.
It is also possible to calculate the force that the solenoid electromagnet exerts on an iron core section of cross section A :
Assuming a magnetic saturation field of 1.6 Tesla, the force per square meter of the section of iron core area exerted by the electromagnet will be 10^6 Newton equivalent to 10^5 Kilograms of force, or 0.1 tons per square meter of cross section.
This means that a 1.6 Tesla saturation electromagnet exerts a force of 10 kg on an iron core of 1 cm 2 in cross section.
Electromagnets are part of many devices and devices. For example, they are present inside:
– Electric motors.
– Alternators and dynamos.
– Relays or electromechanical suiches.
– electric bells.
– Solenoid valves for flow control.
– Computer hard drives.
– Junk cranes.
– Metallic urban waste separators.
– electric brake station and truck s .
– Nuclear magnetic resonance machines.