Differences Between Electromagnet and Permanent Magnet

What are the differences between a permanent magnet and an electromagnet?

The difference between a permanent magnet and an electromagnet is not in the content but in the form! In other words, the main difference is in the technology used to produce the magnetic field.

The electromagnet requires electricity to operate while the permanent magnet delivers a magnetic field by itself.

The electromagnet is made up of a conductive wire, usually copper or aluminum, which is wrapped around a steel core. So we get a coil. By connecting this coil to a direct current source, a magnetic field is generated. The higher the intensity traveled and the greater the number of turns, the stronger the magnetic field!
The nucleus, whether it is a cylinder or some other shape, will polarize. One end will be named North Pole and the other South Pole. The field will come out of one pole to go towards the other. If the direction of the current is reversed, the polarities are reversed. This reversibility is used in solenoid valves, piston latches, etc.
Regardless of the direction of the current, the shape of the field and the induction it generates remain the same.

A considerable advantage of an electromagnet is that the shape and strength of its field can be varied by simply varying the intensity. The magnetic flux of a permanent magnet is constant, as is its shape. The induction it delivers is proportional to its material and its original characteristics. For example, a ferrite permanent magnet will deliver 2000 Gauss while a latest generation neodymium-iron-boron magnet will deliver up to 15000 Gauss (its Br Max). I invite you to read the article on the different materials of permanent magnets (The 5 big families).

A huge advantage of a permanent magnet is...that it is permanent! Indeed, its material was subjected to a very powerful magnetic field during its manufacture. The induction of this field, emitted by a generator, saturates matter. The consequence is the permanent modification of the orientation of the molecules and a permanent induction...for life. In reality, depending on the nuances, permanent magnets will lose 0.1 to 0.5%/year. Permanent but not eternal! However, they will have rendered many services for decades before being unusable. Some factories are still equipped with magnetic industrial machines from the 70s.

Electromagnets are also reliable over the long term. However, an electromagnet needs to be used respecting the voltage and the intensity for which it was designed. If one of the two values ​​is too high, the wire that makes up the coil will heat up, and the electrical insulation of its turns will weaken. It is therefore possible for these turns to short-circuit. The resistance of the winding is modified, the magnetic characteristics change, the electrical cabinet or the power source may be damaged. Eventually, the coil heats up to being out of service. There are therefore monitoring constraints to be adopted in the context of the use of an electromagnet. Let's not forget either that the electromagnet consumes electricity, often for a fairly low yield, which has an ecological impact. The components of an electromagnet are on the other hand recyclable, whether it is its core or its coil.

The ecological impact is also negative on permanent magnets, in particular the rare earths which are extracted from the mines by not very virtuous processes, and which are then treated by even less virtuous chemical means. New, slightly more "eco-friendly" processes are in the industrial test phase and will be used in the 2020s. Similarly, the first rare earth recycling processes are appearing.

Permanent magnets, whether ceramic or rare earth type, lose their magnetic properties if a temperature is exceeded. The curie point is the temperature beyond which the molecules (or moments) become permanently misaligned under the disturbing action of heat.

The electromagnet can, under certain conditions provided for during its manufacture, work at high temperatures. Since it is made of metals, such as steel or copper, its Curie points are very high. However, it is often the electrical insulators that lose their dielectric capacity as the temperature increases.

Permanent magnet or electromagnet, their own ability to withstand high temperatures should not be confused with the ability that magnetism may have to stick or attract a hot ferromagnetic mass. If a steel plate or a nail is hot, it will lose its magnetic permeability in proportion to the rise in temperature. For example, even if the electromagnet is at room temperature but the steel plate is at 600°C, then the passage of flux through the plate will decrease by about 40%. This is an important point for the safety of machines and people. A magnet, permanent or electro, may be unable to lift a mass depending on whether it is "cold" or "hot".

The weight is different between an electromagnet and a permanent magnet. With equal induction and magnetic field, a neodymium magnet will be much more compact and light than an electromagnet composed of metals. It is not for nothing that we find neodymium in all devices requiring miniaturization, such as smartphones and other electronic components.

Finally, the price is different but it is very difficult to generalize. In the family of permanent magnets, we mainly find ferrites (or ceramics), samarium-cobalt and neodymium-iron-boron. Between the first and the third, the price can be multiplied by 20 or more.

If the use allows it, using ferrites will be both simple to implement and inexpensive.

In both cases, electromagnet or permanent magnet, it is often the application that makes the choice of technology to adopt. Whatever your need and the magnet that will have your favour, it is with a good gaussmeter that you will have to check the induction delivered!