Click here to purchase the entire book in PDF format. Electromagnetism Once upon a time, you did an experiment, probably around grade 3 or so, where you put a piece of paper on top of a bar magnet and sprinkled iron filings on the paper. The result was a pretty pattern that spread from pole to pole of the magnet. The iron filings were aligning themselves along what are called magnetic lines of force. These lines of force spread out around a magnet and have some effect on the things around them (like iron filings and compasses for example...) These lines of force have a direction - they go from the north pole of the magnet to the south pole as shown in Figures 2.23 and 2.24. Figure 2.23: Magnetic lines of force around a bar magnet. Figure 2.24: Magnetic lines of force around a horseshoe magnet. It turns out that there is a relationship between current in a wire and magnetic lines of force. If we send current through a wire, we generate magentic lines of force that rotate around the wire. The more current, the more the lines of force expand out from the wire. The direction of the magnetic lines of force can be calculated using what is probably the first calculator you ever used... your right hand... Look at Figure 2.25. As you can see, if your thumb points in the direction of the current and you wrap your fingers around the wire, the direction your fingers wrap is the direction of the magnetic field. (You may be asking yourself ``so what!?' - but we'll get there...) Figure 2.25: Right hand being used to show the direction of rotation of the magnetic lines of force when you know the direction of the current. Let's then, take this wire and make a spring with it so that the wire at one point in the section of spring that we've made is adjacent to another point on the same wire. The direction of the magnetic field in each section of the wire is then reinforced by the direction of the adjacent bits of wire and the whole thing acts as one big magnetic field generator. When this happens, as you can see below, the coil has a total magnetic field similar to the bar magnet in the diagram above. We can use our right hand again to figure out which end of the coil is north and which is south. If you wrap your fingers around the coil in the direction of the current, you will find that your thumb is pointing north, as is shown in Figure 2.26. Remember again, that, if we increase the current through the wire, then the magnetic lines of force move farther away from the coil. Figure 2.26: Right hand being used to find the polarity of the magnetic field around a coil of wire (the thumb is pointing towards the North pole) when you know the direction of the current around the coil (the fingers are wrapping around the coil in the same direction as the current). One more interesting relationship between magnetism and current is that if we move a wire in a magnetic field, the movement will create a current in the wire. Essentially, as we cut through the magnetic lines of force, we cause the electrons to move in the wire. The faster we move the wire, the more current we generate. Again, our right hand helps us determine which way the current is going to flow. If you hold your hand as is shown in Figure 2.27, point your index finger in the direction of the magnetic lines of force (N to S...) and your thumb in the direction of the movement of the wire relative to the lines of force, your middle finger will point in the direction of the current. Figure 2.27: Right hand being used to find the current though a wire when it is moving in a magnetic field. The index finger points in the direction of the lines of magnetic force, the thumb is pointing in the direction of movement of the wire and the middle finger indicates the direction of current in the wire.