DC Electrics - Magnetism — Page 76, Lesson 74

I want to walk you through the magnetic field of a solenoid, because this is one of the most important building blocks for understanding how many aircraft electrical circuits are controlled. Let's start with the definition. A solenoid — also called an electromagnet — is a coil made of a large number of turns of insulated wire. The key phrase there is "large number of turns" — we're not talking about a single loop; we're talking about many loops of wire wound closely together. Between the individual coils, the magnetic flux cancels out, so the overall field pattern that emerges from the solenoid looks very similar to the field pattern of a bar magnet. That's an important mental picture: a solenoid behaves like a bar magnet with a North pole and a South pole. To find out which end of the solenoid is North and which is South, we use the Right Hand Grasp Rule. Here's how it works: imagine you hold the solenoid in your right hand, curling your fingers around it so that your fingers point in the direction of conventional current flow — remember, conventional current flows from positive to negative. When your fingers are curled that way, your outstretched thumb points toward the North pole of the solenoid. That's a quick, reliable way to determine polarity without any extra equipment. Now, what determines how strong the magnetic field of a solenoid is? The excerpt gives us three ways to increase the field strength. First, you can increase the number of turns on the coil — more loops of wire mean more magnetic field contribution. Second, you can increase the current flowing through the coil. Third, you can use a soft iron core placed inside the coil. Soft iron is a material that concentrates and strengthens the magnetic field significantly. There's an important behaviour to note when the current is switched off: the magnetic field collapses. However, a small amount of magnetism remains in the soft iron core — that's called residual magnetism. It's not enough to do useful work, but it's there. Now let's talk about how solenoids and relays are used in practice. Both are essentially remotely controlled switches. The idea is simple: from the flight deck, the pilot switches a small current, and that small current operates the solenoid or relay, which in turn switches a much larger current. A classic example is the starter solenoid in the starting circuit for a piston engine — a small switch in the cockpit controls a large current to the starter motor. But there's a mechanical difference between a solenoid and a relay. A solenoid has a moving core — the core itself moves when the coil is energised. A relay, on the other hand, has a stationary core and an attracted armature — a moving piece of metal that is pulled toward the stationary core. Both achieve the same switching function, but the moving part is different. One critical point for safety and understanding: the wires that form the coil of the solenoid or relay are insulated, and they have no physical or electrical contact with the circuit that is being controlled by the contacts. The coil circuit and the load circuit are electrically separate — they only interact magnetically. That isolation is a key design feature. Finally, let's move to the forces on a conductor carrying a current in a magnetic field. If you place a current-carrying conductor between two magnets, the magnetic field of the conductor interacts with the magnetic field of the magnets. The result is that the magnetic field becomes stronger on one side of the conductor and weaker on the other side. The stronger field exerts a force that pushes the conductor toward the weaker field side — so the conductor moves. This is the basic motor principle. The direction of that movement can be found using Fleming's Left Hand Rule, which we'll cover properly when we get to motors. But the name for this physical effect — the motion caused by a current through a conductor suspended in a magnetic field — is the Lorentz force. That's the technical term you'll see in more advanced study. So to summarise what we've covered: a solenoid is a coil that acts like a bar magnet; its polarity is found by the Right Hand Grasp Rule; its strength increases with more turns, more current, or a soft iron core; solenoids and relays are remotely controlled switches with different moving parts; and a current-carrying conductor in a magnetic field experiences the Lorentz force, which is the foundation of electric motors.