Electromagnet

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What is an electromagnet?

An electromagnet is an electrically operated magnet. It must be powered by electricity, and the strength of the electromagnet can be regulated by the current. If the current is switched off, the magnetic field also disappears. This is why electromagnets, and not permanent magnets, are often used in technology since controllable magnetic fields offer advantages. In the simplest case, a wire coil through which a current runs acts like an electromagnet.

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Operating principle of electromagnets

An electromagnet is a device that generates a magnetic field when connected to a power source. It is usually a current-carrying conductor coiled around a ferromagnetic coil core. The magnetic field is caused by the moving charges of the current in the conductor.

According to current knowledge, the movement of charge carriers is the only way to generate a magnetic field. This is also described by Maxwell's equations, the basic equations of electrodynamics, established by the physicist James Clerk Maxwell. Maxwell's equations precisely describe the magnitude of magnetic and electric fields depending on currents and charges. You can read more about the history of magnets in our guide.

Basically, only magnetic fields exist that are generated by the movement of charges. This always creates a magnetic field with a north pole and a south pole. Sources of the magnetic field, similar to the charges that are the sources of the electric field, do not exist.

The first physicist to recognise the magnetic forces of a current-carrying conductor, interpret them correctly and record his discovery was Hans Christian Oersted. In 1820, Oersted observed the deflection of a compass needle in the vicinity of a current-carrying wire.

The magnetic forces of permanent magnets are also caused by microscopic charge movement in matter. The electrons in the atoms move at high speed. The electrons also have a characteristic electron spin. Both cause a magnetic moment and thus magnetic forces.

To date, the largest magnetic fields ever to have been generated were created with large coils through which strong currents were passed. The magnetic field H in the centre of a coil with length l and radius R is proportional to the current in the coil I, it is proportional to the number of turns in the coil n and it is indirectly proportional to the length of the coil l for very long coils, or indirectly proportional to the radius of the coil R for very short coils. The formula for the magnetic field H on the axis of a current-carrying cylindrical coil is as follows:

\(H = \frac{n\cdot{I}}{\sqrt{l^2+4\cdot{R^2}}}\)
The magnetic field of a coil with a small diameter and a very large number of turns is, therefore, particularly strong when a very large current is sent through.

Superconducting coils with many turns are therefore used today to generate particularly large magnetic fields. Superconductors are materials that have no electrical resistance and thus conduct electricity without friction. Hence, a very large current can flow through the superconducting material. To reduce the cross-sectional area and thus the diameter of the coil, superconducting coils have been explosion-welded in basic research experiments on high magnetic fields using an explosive charge placed around the coil. In the experiment, the diameter of the superconducting coil suddenly decreases, and the magnetic field increases sharply for a short time, even if it then immediately collapses again because the coil has been destroyed. The explosions essentially compressed the magnetic field lines.

This process has already produced magnetic fields with a magnetic field strength of several 10 000 tesla. Even larger magnetic fields exist in space on the surface of neutron stars.

Illustration of the structure of an electromagnet

The illustration on the left shows a section of the magnetic field H of a conductor through which current I flows. In the middle, the course of the field lines is shown when the conductor is bent into a loop. If many conductor loops (turns) are wound around a ferromagnetic core (right side) and a voltage U is applied to the conductor, a current I flows and a magnetic field H is created, which is many times stronger than the magnetic field of the individual conductor loop due to the ferromagnetic core and the large number of turns. This structure is consistent with a classic electromagnet. However, the shape of the magnetic field is similar to that of a conductor loop and identical to that of a rod-shaped permanent magnet. For the sake of clarity, the field lines of the coil on the right have only been indicated. They are much denser than in the single conductor loop and run from the north pole (here the bottom of the coil) to the south pole (here the top of the coil) to close again in the interior of the electromagnet. The north and south poles can be reversed by swapping the poles of the voltage source and thus reversing the direction of the current.

In a ferromagnetic material, elementary magnetic polarisations exist that can align themselves in an external magnetic field and amplify it up to a thousandfold. This is why ferromagnetic materials are used as coil cores in electromagnets. In the simplest case, a wire is basically wound around a cylinder of ferromagnetic material (e.g. iron).

Anyone can carry out a simple experiment at home. All you need to do is wrap a copper wire around a pencil. When the ends of the copper wire are connected to the positive and negative terminals of a battery, a current flows through the wire and an electromagnet is created. This can be used to deflect a compass needle, for example.

If the copper wire is wound around an iron cylinder instead of a pencil, e.g. a nail, the magnetic field is significantly stronger. It is amplified by the ferromagnetic core by a factor of μ, the magnetic permeability. For iron, μ can have values greater than 1 000.

Technical applications for electromagnets

Today, electromagnets are used in generators and electric motors, can be found in relays and are a prerequisite for numerous electronic components in the radio and television sector. Conventional transformers, for example, consist of opposing coils with different numbers of turns.
In a transformer with two coils, the magnetic field of one coil induces a voltage in the opposite coil. The magnitude of this voltage depends on the ratio of the number of turns in the two coils. It is therefore possible to increase or decrease voltages without losing much power (apart from heat loss).

If you would like to build a simple electric motor yourself, you will find instructions in the following customer project:

The World's Simplest Electric Motor

On the other hand, the following customer projects show how you can generate electricity from motion (generator):

Author:
Dr Franz-Josef Schmitt

Dr Franz-Josef Schmitt is a physicist and academic director of the advanced practicum in physics at Martin Luther University Halle-Wittenberg. He worked at the Technical University from 2011-2019, heading various teaching projects and the chemistry project laboratory. His research focus is time-resolved fluorescence spectroscopy in biologically active macromolecules. He is also the Managing Director of Sensoik Technologies GmbH.

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