By Eleftheria Safarika
Electromagnetism is undoubtedly one of the most intriguing and fascinating fields of physics. Though the two fields it consists of - electricity and magnetism - were at first thought of as completely separate and distinct realms of physics, during the 18th century, some physicists started believing that, in some way, these two were connected to each other, while others remained steady in their view that they were in no conceivable way related.
While some had tried before him, the Danish physicist Hans Christian Oersted was the first one to actually prove that, in reality, electricity and magnetism were the “two faces of the same coin”. His discovery, which took place in 1819, was of immense importance for physics, because it gave an answer to one of its most fundamental questions. It was made through an experiment which took place in front of university students. Some say that it was completely incidental, while others believe that Oersted had tried it before on his own. Oersted discovered that, when electricity flowed through a wire connected to a battery, it influenced a magnetic needle nearby, moving it from its standard north-south direction, to a direction perpendicular to the wire. When the wire was disconnected from the battery, the needle returned to its original position. Oersted, seeing these results, came to the conclusion that the electricity flowing through the cable created a magnetic field around it, which influenced the magnetic needle, causing it to change its direction. Otherwise, a steady electric current produces a steady magnetic field. This discovery triggered a lot of research in the field of electromagnetism during the rest of the 18th and the 19th century.
The direction of the current, however, plays a role in the direction of the magnetic field that is created. In image 2, we see that by changing the direction of the electric current, the direction of the magnetic field changes as well, causing the magnetic needle to rotate in the opposite direction.
Since electricity is electrical charges moving in an oriented motion, Oersted had actually proved that charged particles, apart from creating an electric field, also generated around them a magnetic field. But did this relationship go the other way? Could a steady magnetic field produce a steady electric current? In 1831, another physicist, Michael Faraday, working on the fields of electricity and magnetism, tried to prove this last claim. Initially, he created a circuit consisting of a battery, a switch, and a solenoid. When he turned the switch on, electric current flowed through the wires, thus creating a magnetic field around it, according to Oersted’s experiment. Then, he wrapped the solenoid in a loop. However, whenever he created a current in his initial solenoid, a current was not created in the loop around it, as he would expect, since in the solenoid there would be a magnetic field. So, he realized that a steady magnetic field does not create a steady electric current. Nevertheless, he realized that a current was created in the loop whenever he opened or closed the switch of the circuit. Therefore, he came to the conclusion that a changing magnetic field creates an electrical current, not a steady one. In another version of his experiments on the same topic, Faraday used a magnet and a coil. He discovered that when moving a magnet inside an electromagnetic coil, then in the coil electricity started flowing. Therefore, a changing magnetic field generated electricity. This phenomenon is called electromagnetic induction. The same thing happens (current is created in the coil), when, instead of the magnet, the coil is the one moving, and the magnet is held stationary. The current produced by the relative motion of the coil or the magnet is called an induced current, and is said to be set up by an induced electromotive force.
These discoveries may not individually seem that important, but they are fundamental factors that make our life what it is today. They are behind all electrical devices we use today, while also moving the economy. They are used in the way electricity is generated in power stations, where electric generators are used. These basically move copper coils inside magnetic fields, the opposite of what Faraday did in his experiment.
Lewin, W.& Goldstein, W. (2011). For the Love of Physics. N.Y.: Free Press
Physics - Understanding Electromagnetic induction (EMI) and electromagnetic force (EMF) Retrieved from: https://www.youtube.com/watch?v=tC6E9J925pY
Lectures by Walter Lewin. They will make you ♥ Physics. 8.02x - Lect 16 - Electromagnetic Induction, Faraday's Law, Lenz Law. Retrieved from: https://www.youtube.com/watch?v=nGQbA2jwkWI