Electromagnetism

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Electromagnetism Video Transcript In 1820, the Danish physicist and chemist Hans Christian Oersted was performing a lecture demonstration for some students and noticed that a compass needle was deflected by a nearby current-carrying wire. This was the first time the connection between electricity and magnetism was made, and it paved the way for a new branch of science: electromagnetism. To determine the direction of the magnetic field produced by a current-carrying wire, you can use the second right hand rule (RHR-2). Point your thumb in the direction of a current and your fingers will curl in the direction of the magnetic field loops created by it. You can see that the magnetic field produced by an electric current is always oriented perpendicular to the direction of the current flow. The magnitude of the field depends only on the distance from the wire, not on its position along the wire. Now, let’s look at the magnetic field set up by a circular current-carrying loop. Use RHR-2 to determine the magnetic field, which has north and south poles like a bar magnet. The larger the loop, the smaller the field at its center since the current is farther away. Adding another loop with the current in the same direction would create a stronger magnetic field. A coil of many such loops is called a solenoid. The magnetic field inside a solenoid is uniform and strong. Only near the ends does it begin to weaken and change direction, and the field just outside the coils is nearly zero. Essentially, a simple electromagnet is a solenoid wrapped around an iron core or other ferromagnetic material. The electromagnetic field of the solenoid magnetizes the iron atoms by aligning their magnetic domains to add to the magnetic field. Consequently, the combined magnetic force of the magnetized wire coil and the iron core makes an electromagnet very strong. Electromagnets are used in many electric devices such as doorbells, hair dryers, fans, loudspeakers, scrapyard cranes, and security doors. Perhaps the most obvious example of electromagnetism is the electric motor. Motors have loops of wire in a magnetic field. When a current is passed through the loop, the current- carrying coil in a magnetic field experiences a force on both sides of the coil. This creates a twisting force, called torque, on the loops which makes it turn. Electrical energy is converted to mechanical energy (motion). Can you think of any other devices that use electromagnets? About this transcript: • Transcript title: Electromagnetism • Corresponding Lesson: Electromagnetism • Author and curator: Jinxiu Yuan for The TEL Library. • Creative Commons License: CC BY NC SA.

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