DC Electrics - Basic Principles — Page 10, Lesson 13

I want to walk you through the basic principles of DC Electrics. We're starting right at the foundation — what electricity actually is and how we make it flow. First, let's talk about the circuit. For current to be maintained, the material that allows electrons to move must be capable of absorbing electrons from the positive terminal and transferring them through itself back to the negative terminal. That's the key idea: as long as there is a complete circuit, the current can be sustained. Now, here's an important historical point. Electricity was in use before electrons were even discovered. People had assumed that electricity was the flow of something from positive to negative, and all the early laws of electricity were based on that idea. That's what we call conventional flow. The actual physical flow of electrons from negative to positive is called electron flow. So when you hear 'conventional flow', think positive to negative; when you hear 'electron flow', think negative to positive. Both are used in different contexts, but you need to know the difference. Let's move to how we actually get electrons moving. There are six basic means to provide the force that causes electrons to flow. I'll list them: - Friction — this gives us static electricity. - Chemical Action — this gives us cells and batteries, both primary and secondary cells. - Magnetism — this gives us generators and alternators. - Heat — this gives us thermocouples, which are junctions of two dissimilar metals. - Light — this gives us the photo electric cell. - Pressure — this gives us piezoelectric crystals. Now, of those six methods, only two produce electrical power in sufficient quantities for our normal daily needs: Chemical Action, which is batteries, and Magnetism, which is generators. The others exist and have their uses, but for the kind of power we need on an aircraft, those two are the workhorses. Next, let's talk about what actually drives the flow. For electric current to flow, there must be a force behind it. Think of water: water needs pressure to make it flow. Electricity needs pressure too, and that pressure is called Electromotive Force, or EMF. If you have a water tank and the pressure decreases, the flow decreases. Same in electrics: if the EMF decreases, the flow of electrons decreases. EMF is measured in units of Voltage. The number of volts is a measure of the EMF, or what we also call Potential Difference — often written as pd. That's the difference in electrical potential between the positive terminal and the negative terminal. Voltage is given the symbol V or E. Here's the relationship: by increasing the voltage, the flow of electrons increases past any point in a circuit. Decreasing the voltage decreases the flow. So to maintain the correct flow, it is normal to keep a constant voltage in a circuit. That's a fundamental operating principle — you design your system to hold voltage steady so that current behaves predictably.