Electromagnetism is the phenomenon where electric current and magnetic fields are linked. Electric currents, for example, induce magnetic fields. The strength of the magnetic field is proportional to the electric field strength. In other words, electromagnetic induction occurs when there is an interaction between electric and magnetic fields, such as those created by moving electrons in a wire or when two wires are placed next to each other. Electricity and magnetism have been known since antiquity but only recently has their connection been understood as Electromagnetism The scope of this article is limited to electromagnetic induction in non-conducting materials (so-called “dielectrics”) like glass or other insulators; it does not address electrostatics or electrodynamics.
Electromagnetism was discovered in 1819 by Hans Christian Oersted and Joseph Henry both of whom noticed a connection between electricity and magnetism. Michael Faraday, a chemist, is credited with the discovery of electromagnetic induction in 1831. He noticed that a changing magnetic field (i.e. one that was moving or changing) caused a current to flow in a wire placed near the magnet. This was his first clue that a changing magnetic field could generate electricity. Before Faraday’s discovery, electricity had been thought of as a flow of electrons within a conductor. Faraday’s work, however, led others to believe that electricity could result from a change in the direction of the flow of electrons alone, regardless of the material in which they were moving. This change in viewpoint led to the development of electrostatic generators that do not rely on the flow of electrons through a conductor. These generators produce electricity, but not electromagnetism.
Electromagnetic theory in the 19th century
In the 1800s, two competing theories were proposed to explain how electromagnetism worked. The first was the line of “pressure” theory, which stated that moving electric charges, such as those in an electric current, pushed against an adjacent magnetic field and thus caused a magnetic field to be induced in the adjacent conductor. The second theory, the “E-field” theory, stated that the moving charges themselves exerted a force on the adjacent field and thus caused a magnetic field to be created. Today we know that the E-field theory is correct but there were many experiments done in the 19th century to try and prove which theory was correct. In 1832, Faraday performed a series of experiments with a moving magnet (i.e. a rotating bar magnet) and a nearby wire. He observed that a current was induced in the wire even though the wire was not in the path of the moving magnet. This was evidence that the “pressure” theory was incorrect because the wire was not in the direct path of the moving charges. Instead, the magnetic field of the moving magnet caused the current to be induced in the wire, supporting the E-field theory.
Induction of magnetic fields by moving electric charges
Currents in a conductor create a magnetic field in the direction of flow. A moving (i.e. changing) current also creates a magnetic field, but in the direction opposite to the flow. If the current is changing in strength, the direction of the magnet field is changing. When a current is moving in a regular, predictable manner, the resulting field is strongest at right angles to the direction of flow. However, if the current is changing in strength, the direction of the resulting field changes as the strength of the current changes. If a wire is placed near an electrically conductive material and the current through the wire is varying, the resultant field will be at right angles to both the wire and the material.
Magnetic field strength and orientation
The strength of the magnetic field created by a conductor depends on its current and the distance between the wire and the nearby material. If the distance between the conductor and the material is small, the strength of the field is proportional to the current in the conductor. If the conductor is moved farther away, the strength of the field diminishes with the distance. The field is strongest at right angles to both the wire and the material. It is weakest at right angles to both the wire and the material. This is true for both a varying current and a steady (i.e. unchanging) current. The strength of the field varies inversely to the cube of the distance. This means that as the distance is doubled, the strength of the field is reduced to one-eighth of what it was.
Coils and electromagnets
If a loop of wire is placed near a conductor, a magnetic field is induced in the loop. The strength of the field is proportional to the current in the adjacent conductor. The direction of the field is at right angles to both the loop and the adjacent conductor. When the wire is switched so that it is moving a current through the loop, an electromagnet is created. The strength of the electromagnet depends on the current in the adjacent conductor. When the current is steady the strength is approximately proportional to the square of the current. When the current is varying, the strength is approximately proportional to the current.
Applications of electromagnetic induction
Electromagnetic induction has many important applications. The most familiar is an electric generator. When a conductor (rotor) is rotated near a stationary wire, a current is induced in the wire. This current is used to power lights, computers, and other electrical devices. Another important application is the electromagnet. Electromagnets are used in a wide range of applications, including medical imaging, the manufacturing of microchips, and the launching of spacecraft. Electromagnets are used to lift heavy objects. They are also used to control and direct the flow of materials in industries such as mining, food processing, and steelmaking. Electromagnets are used as current sensors in electric power systems to determine if there is a current flowing in the system.
Electromagnetism is an interesting phenomenon that has important applications in many parts of the world. The connection between electricity and magnetism was not well understood until the late 19th century. Michael Faraday laid the foundation for the discovery of electromagnetic induction in 1831. He discovered that moving a magnet near a wire would induce a current in the wire. This discovery opened up a new area of research and new technologies were born as a result. Electromagnetic induction is responsible for the production of electricity in many parts of the world.