Main Difference – Permanent vs. Temporary Magnets
Magnets are an extremely important part of our daily lives. Magnets are used in door-catchers and to stick notes on to fridge doors, but often, unbeknown to us, most of the magnets that surround us are in electric circuits in the form of electromagnets. The type of magnets on fridge doors are permanent magnets while the magnets which are used to make electromagnets are typically temporary magnets. The main difference between permanent and temporary magnets is that permanent magnets do not require an external magnetic field in order to stay magnetized whereas, temporary magnets remain magnetized only as long as there is a strong external magnetic field around them.
What are Permanent Magnets
Permanent magnets are magnets that retain their magnetism over long periods of time. Permanent magnets are made of hard ferromagnetic materials. They can be magnetised by placing them inside an external magnetic field. Ferromagnetic materials display hysteresis: when the external magnetic field is gradually reversed, the material resists demagnetization over a longer range of external magnetic field strengths. This is summarized in the magnetization curve below:
The horizontal axis gives the size of the external magnetic field, the vertical axis gives the magnetization. Note that when the external magnetic field increases around (follow the bottom curve), the magnetization of the material increases rapidly. When the external magnetic field is reversed (the top curve), the magnetization does not reduce significantly over a larger range of reduced or reversed magnetic fields. This behaviour is typical for a hard ferromagnetic material. Hard ferromagnetic materials include aluminium, cobalt, and steel.
What are Temporary Magnets
Temporary magnets can be also magnetized by placing them inside an external magnetic field. The material’s magnetism lasts so long as the external magnetic field is switched on, and disappears when the external magnetic field disappears. Temporary magnetic materials include both paramagnets and soft ferromagnets.
Atoms that make up paramagnetic materials do not have their magnetic moments aligned with each other. When they are placed inside an external magnetic field, the magnetic moments of the atoms line up with the external magnetic field, magnetizing the material. Ferromagnetic materials are different from paramagnetic materials in that their magnetization is spontaneous (their magnetization increases rapidly as the external magnetic field is increased over a narrow range).
Soft ferromagnetic materials also show hysteresis: however, their hysteresis loops tend to be much narrower than those of hard ferromagnetic materials, meaning soft ferromagnetic materials lose their magnetization much more easily when the external magnetic field is reduced. However, they are still different from paramagnetic materials in that they magnetize spontaneously.
Soft ferromagnetic materials, such as soft iron, are useful for the construction of devices that do not require permanent magnets. In transformers, when current flows in coils wound around the soft iron “core”, the core becomes magnetized and conducts the magnetic field to the other coil in the transformer. In order to produce an alternating current, the direction of the magnetic field through the core needs to change periodically. This is very difficult to achieve if the core had been made of a permanent magnetic material. In some applications that utilize powerful magnetic fields (MRI machines, for example) it is important that the magnets can be switched off (see the video below!)
Difference Between Permanent and Temporary Magnets
Dependence of Magnetization on the Presence of an External Field
Permanent magnets can be magnetized by putting them in an external magnetic field. They do not lose their magnetization over a large range of reverse external magnetic fields, or when the external magnetic field is switched off.
Temporary magnets can also be magnetized by putting the material inside an external magnetic field. When the external magnetic field is switched off, the material loses its magnetization.
Type of Material
Permanent magnetic materials are so-called hard ferromagnetic materials.
Temporary magnetic materials are paramagnetic or soft ferromagnetic materials.
“The main hysteresis loop and initial magnetization curve calculated using Stoner-Wohlfarth theory for an isotropic system of identical single-domain magnets.” by RockMagnetist (Own work) [CC0 1.0], via Wikimedia Commons