DC Electrics - Magnetism — Page 76, Lesson 72

I want to walk you through the basics of magnetism as it applies to DC electrics. We'll start with the two main types of magnets you'll encounter: temporary and permanent. Temporary magnets are made from soft iron. Soft iron is a material that is easily magnetized — meaning you can give it magnetic properties quickly — but it also readily loses those magnetic properties. So it's a temporary state. Permanent magnets, on the other hand, are made from hard alloy steels. These materials are difficult to magnetize in the first place, but once they are magnetized, they retain their magnetism very well. That's why we call them permanent. Now, let's talk about a key property called permeability. Imagine you take an unmagnetized piece of soft iron and place it inside an existing magnetic field. What happens? The lines of flux — those invisible lines of magnetic influence — concentrate and flow through the iron. The iron itself becomes magnetized and actually produces additional lines of flux of its own. This property of increasing the flux density is what we call permeability. If you then remove that soft iron from the magnetic field, it loses most of its magnetism. We say that soft iron has low magnetic retentivity — it doesn't hold onto its magnetism well. The tiny amount of magnetism that remains is called its residual magnetism. Magnetism itself can be destroyed in three ways: by heating the material, by hammering the material, or by placing the material inside a solenoid that is supplied with an alternating current. Let's look at the molecular structure of magnets. In an unmagnetized piece of soft iron, the molecules tend to form closed chains — they're randomly oriented. When the iron becomes magnetized, those molecules tend to line up with the invisible lines of influence in the magnetic field — the lines of flux. When all the molecules are lined up, the magnet is said to be saturated, and it cannot be magnetized any further. You can see this in the molecular distribution diagram . Now, a very important concept: the magnetic effect of a current. When a conductor carries an electric current, a magnetic field is set up around that conductor in the form of concentric lines of flux — circles around the wire. To determine the direction of those lines of flux, we use the Corkscrew Rule. Imagine a right-handed corkscrew. If you turn the corkscrew so that it moves in the direction of the conventional current flowing through the conductor, then the direction the corkscrew rotates is the direction of the lines of flux. So if current is flowing away from you — into the paper — the corkscrew moves away from you, and its rotation tells you the direction of the magnetic field around the wire. If current is flowing toward you — out of the paper — the rotation reverses accordingly. These principles combine when you have multiple conductors or a coil of wire, producing combined magnetic fields, and that's how we get the magnetic field in a coil, which is fundamental to how many electrical components work.