Electricity 5. Magnetism and Electric Current
This article will deal with the relationship between magnetism and electric current. We will explore how mechanical energy can be converted to electrical energy, how electrical energy can be converted to mechanical energy and the many applications where this is used.
The Earth has a giant magnetic field with one of the poles located about 15 degrees from its true axis of rotation at the North and the South Pole. This is referred to as Magnetic North Pole. If we take a needle which is hard steel and stroke it repetitively across the pole of a magnet it will become magnetized. Now if we take a piece of string, and tie a knot around the needle so it hangs horizontally and the string is long and soft enough to let it turn freely enough, one end will point North. We could do the experiment with a small disc of cork in a glass of water and lay the needle on it. Using either method, we have created a "compass." The end that seeks the north would be called the North pole. The North seeking pole is always referred to as the North pole of a magnet. Magnetism got its name from the properties of the iron oxide material magnetite. Magnetic properties that refer specifically to the magnetic properties of iron are referred to as ferromagnetic. Materials made from iron, nickel and cobalt are particularly able to concentrate their magnetic fields at opposite ends. Soft iron does not retain its magnetic properties nearly as long as harder steel and various alloys. The ability to concentrate magnetic lines of force is called permeability. The measure of relative permeability is always stated as one magnet to another. When we investigate electromagnets, we will explore this a little further.
Figure 1-1. This figure shows a couple of typical magnets. Note that the lines of force are concentrated between the poles in the horseshoe magnet. They are dispersed in the air gap, or the space between the poles.
In Figure 1-2 we see a bar magnet. The lines of force are concentrated in the pole pieces and loop around to the opposite pole piece. If you took a nail and placed it close to either end there would be a strong pull, if you placed it in the center there would be almost no pull at all because the lines of force are not concentrated there. One can easily to see the lines of force by taking a sheet of paper and sprinkling iron filings on it; then put a magnet under it. You will be able to see it in 3-dimensions. If you need a source of iron filings, find someone that grinds saw chains and take a magnet you can get all you need off the chain grinder.
Figure 1-3. We have 2 bar magnets placed with opposite polarities together. Placed this way the magnets are attracted to together because opposite poles attract. Opposite poles attract. If the magnets are stored this way they will retain their magnetism for a much longer period of time. If you put iron filings around them you would see very little external field effect, as the field would be concentrated in the two magnets.
Figure 1-4. Two bar magnets are placed together with like poles together. The magnets will repel each other. Like poles repel. If these magnets were stored in this manner and were forced close together, they would de-magnetize each other.
Figure 2a. This sketch represents a wire with current flowing through it. When current flows through a wire magnetic fields are set up around the conductor. These lines of force are circular around the conductor. To observe this, take a wire that is going to a load, for example, a wire from a 12v battery to a headlamp unit. Poke a hole in a sheet of paper and run the wire through it vertically. While current is flowing to the light, pour some iron filings [thin layer] around the wire. Lightly tap the paper and observe the pattern of the Iron filings. You will be able to see the effects of the magnetic field.
Looking at Figure 2d observe the Left Hand Rule. If you moved the hand up to 2a, the thumb would point to the positive end of the wire also the North Magnetic pole. The fingers point in the direction of rotation of the magnetic lines of force that surround the conductor.
Figure 2b. We have taken the wire and wound it in a coil with adjacent turns. The individual force fields around each turn combine with the field of the adjacent wire and add to the field. The result is a field for the coil that comes out of the center and loops around like the bar magnet to the other end. This type of coil doesn't concentrate the lines of force very well. In Figure 2c we will add an iron core. If the core is free to move it will center itself in the coil and will concentrate the force field in each end, the behavior will be like the bar magnet. The difference is that when the voltage applied to X and Y is removed, the magnetic field will collapse and all that will remain is the residual magnetism in the core. If the core is soft iron very little magnetism will remain. This is an electromagnet and its magnetic power is relative to the amount of power dissipated in the coil [Back to Ohms Law and the Power Formula] plus some qualities and the quantity of the core material.
In Figure 2a when the voltage is applied to the coil, [let's assume that the resistance of the coil is such that with this source 1 amp would flow] the current doesn't reach the 1 amp instantly like it does in a resistive circuit. The first thing that has to happen is for the magnetic field to develop, so the full voltage is applied to the ends of the coil and as the core of the coil becomes magnetically saturated the current is at the level of the resistance of the wire. This is called Inductance and is measured in Henrys in quantities ranging as low as picohenrys or 1/1000 of 1/1,000,000th of a henry. Inductance tends to resist current changes. The relationship between voltage, current, time and resistance is called inductive reactance. This is a very involved subject, and I will not get into it any deeper right now.
One other effect that MUST be mentioned now is BEM, Back Electromotive Force. When we remove the voltage source from the coil, all of the magnetic lines of force instantly collapse and as they cut the wires they induce a voltage spike that is many times higher than the voltage that was applied to the coil. Later when we get to ignition systems we will explore how this works and how we use it to generate very high voltages for ignition. When a conductor is moved through magnetic lines of force, and cut the lines of force, or the lines of force cross the conductor and are cut by it in a collapsing field, the voltage produced is said to be induced. And this is called induction.
Figure 3a illustrates a horseshoe magnet with a wire in the force field. If the wire is moved up and down so it cuts the magnetic field will cause electrons to move from one atom to another. As long as the wire is not moving no activity will transpire. When the wire is moved, mechanical energy will be converted to electrical energy and a voltage potential will appear at A and B with one being positive and the other being negative.
Figure 3b. The situation is the same as 3a except we have added 3 more wires in the field by winding a coil. Since we have 3 wires cutting the lines of force, and they are in series the voltage developed at A and B will be 3 times what it would be with the 1 wire in 3a. This is the principle that allows electricity to be generated from mechanical energy. The faster the conductor is moved, cutting the magnetic lines of force the more voltage is generated. This process is called induction. Generators, alternators, ignition systems, transformers all work on induction, or induced currents. Later when we get deeper into generator and alternator principles we will see how this action is expanded into multi-polar devices and how alternating current works. The next article will address how these principles are used in motors, solenoids, generators and other items.