DC Electrics - Magnetism — Page 76, Lesson 70

I want to walk you through the fundamentals of magnetism as they apply to DC electrics. This is where a lot of the behaviour of generators, motors, and relays starts to make sense, so let's get the basics solid. A magnet has three defining properties. First, it will attract and pick up bits of iron and steel — not all metals, specifically iron and steel. Second, if you suspend it freely so it can rotate without friction, it will come to rest pointing in a north-south direction. Third, it produces a magnetic field — that's the region surrounding the magnet in which its magnetic effects can be detected. Now, if you take a sheet of paper, place it over a magnet, and sprinkle iron filings on top, the filings arrange themselves into a distinctive pattern. They trace out invisible lines of influence in the magnetic field. These lines are called lines of flux or lines of force. Both terms mean the same thing. We can give direction to these lines of flux by putting arrowheads on them. The arrow points in the direction a compass needle would point if you placed it in that part of the magnetic field. So the lines of flux of a magnet emerge from the N pole and re-enter at the S pole. That's the conventional direction — out of north, into south. In diagrams, you sometimes see lines of flux drawn as if they stop in space, but that's just a drawing convenience. In reality, lines of flux are always continuous — they form complete loops through the magnet and the space around it. Also, lines of flux never cross each other. That's a fundamental rule. When you bring two magnets close together, their combined field is modified because the lines of flux cannot cross. Where lines from the two magnets are pointing in the same direction, they reinforce each other and the flux density — the concentration of lines in a given area — increases. Where lines from the two magnets oppose each other, they tend to cancel each other out. Magnetic effects are most powerful at two points, usually near the ends of the magnet. These points are called the poles of the magnet. When a magnet is freely suspended and comes to rest, the end that points toward the Earth's magnetic north pole is called the north-seeking pole, or simply the North (N) pole. The other end is the South (S) pole. Here's the key interaction rule. If you bring the N pole of one magnet near the N pole of another magnet, the two poles repel each other. Similarly, two S poles repel each other. Attraction occurs only between a N and a S pole. So the rule is simple: like poles repel, unlike poles attract. That figure shows the flux distribution pattern I've been describing — the lines emerging from N, re-entering at S, and never crossing. It's worth studying that pattern because it's the foundation for understanding how magnetic fields interact in generators and motors later in the syllabus.