Electromagnets vs Permanent magnets
By: Sijie Ran
Both electromagnets and permanent magnets have been widely used in many applications. Permanent magnets such as bar magnets have been applied in hard drives, vehicles and magnetic hooks. Electromagnets such as coils are most frequently used in transformers and various medical devices. In some applications, both electromagnets and permanent magnets have been used together. What is the main difference between electromagnets and permanent magnets and how do we select the right one for our application?
Permanent magnets are made of strongly magnetic hard materials and they can keep their magnetic characteristics without external stimulus. Due to the strong magnetic properties in materials such as NdFeB, permanent magnets are a perfect selection to create consistent strong force to attract other objects at close distance. The force between two 8cm NdFeB cube magnets can easily up to 5K newtons when two magnets stick together. In many applications such as magnetic hooks the permanent magnets can provide long time grabbing force without any additional energy input.
On the other hand, the disadvantages of permanent magnets are also caused by its material property. Since these magnets keep their magnetic properties, they can never be turned off or control the force when their relative positions are fixed. Which makes them hard to apply to applications that require changing forces. The force of permanent magnets become inefficient for long distance as the force decay as distance cube. One may notice the force between two 8cm NdFeB magnets quick drop from 5k to 100 newton when their distance increased from 0cm to 10cm. At last, a permanent magnet field can only be created below a certain temperature, as a result, they are not suitable for hot-device applications.
The electromagnets are just like the opposite of permanent magnets. The magnetic field is created by the current flowing through a solenoid. The magnetic properties of coils depend on the current flowing through the wires. Due to this property, the electromagnets are most frequent type used in applications that require fast changing magnetic field or precise controlled field. One example is in MRI where it is often needed to create an accurate field/field gradient at a specific location in the space, by changing the current in different coils one can easily achieve the precise control of magnetic field in space while it is almost impossible to get the same result by using permanent magnets. By playing with different current input, it is possible to create complex and real time changing field arrangement by coils.
The main disadvantage of electromagnets is on the energy-transfer side. While the permanent magnets are magnetized in advance and do not need energy input during application, the electromagnets require a constant current source. This property makes the electromagnets become much less efficient to create strong magnetic force at close distance compared to the permanent magnets. Due to different variables related to the current source disturbance and heating, the electromagnets require more care on the design and maintenance compared to the permanent magnets to keep a high precision field output.
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