Welcome to Chapter 5 — DC Electrics: Magnetism — Page 66, Lesson 68

Welcome to Chapter 5 — DC Electrics: Magnetism. This is a fresh topic, so let's start from the ground up. Magnetism is the fundamental force that makes electric motors, generators, and many aircraft instruments work. I want to walk you through the core ideas first. We begin with the concept of a temporary magnet. A temporary magnet is a material that acts like a magnet only while it is under the influence of an external magnetic field. Once that external field is removed, it loses its magnetism. Think of a soft iron bar placed near a permanent magnet — it becomes magnetic, but drops that property when the magnet is taken away. Next, we have permanent magnets. These are materials that retain their magnetism after the external magnetising field is removed. They are made from materials like steel or special alloys that hold their magnetic alignment. Now, a key property of any magnetic material is permeability. Permeability is a measure of how easily a material can be magnetised — in other words, how well it allows magnetic lines of force to pass through it. High permeability means the material is easy to magnetise; low permeability means it resists becoming magnetic. Let's look deeper into the molecular structure of magnets. Inside a magnetic material, the atoms are arranged in small regions called magnetic domains. In an unmagnetised piece of iron, these domains point in random directions, so their magnetic effects cancel out. When you apply an external magnetic field, the domains align themselves in the same direction, and the material becomes magnetised. In a permanent magnet, the domains stay aligned even after the external field is removed. Now, a very important principle: the magnetic effect of a current. Whenever an electric current flows through a conductor, it creates a magnetic field around that conductor. This is the foundation of electromagnetism. To determine the direction of that magnetic field, we use the corkscrew rule. Imagine a corkscrew being turned in the direction of the current flow — the direction the corkscrew moves forward is the direction of the current, and the direction you turn it is the direction of the magnetic field lines circling the conductor. More simply, if you point your right-hand thumb in the direction of conventional current flow (positive to negative), your curled fingers show the direction of the magnetic field lines around the wire. Next, consider a solenoid. A solenoid is simply a coil of wire, wound in a helix. When current flows through a solenoid, the magnetic field of each turn adds together, producing a field very similar to that of a bar magnet — with a north pole at one end and a south pole at the other. To find the polarity of a solenoid, we use the right hand grasp rule. Here's how it works: grasp the solenoid with your right hand so that your fingers curl in the direction of the current flow through the coils. Your thumb then points toward the north pole of the solenoid. Now, the strength of the field of a solenoid depends on several factors: the amount of current flowing through the wire, the number of turns in the coil, and the type of core material inside the coil. A higher current, more turns, or a core with high permeability (like soft iron) all increase the magnetic field strength. Finally, let's talk about practical applications: the solenoid and relay. A solenoid can be used as an electromagnetic switch. When current flows through the coil, it creates a magnetic field that pulls a movable iron core or armature. This movement can open or close a set of contacts. A relay works on the same principle but uses a small current in the solenoid coil to control a much larger current in a separate circuit — very common in aircraft electrical systems for switching heavy loads safely. That diagram on page 65 illustrates the solenoid and relay arrangement. You can see the coil, the armature, and the contacts clearly. And on page 66, there's a figure showing the magnetic field around a current-carrying conductor and the corkscrew rule. So to summarise: we've covered temporary and permanent magnets, permeability, the molecular domain structure, the magnetic field around a current, the corkscrew rule, the solenoid field, the right hand grasp rule, factors affecting solenoid field strength, and the solenoid/relay as a switching device. That's the foundation of magnetism in DC electrics.