DC Electrics - Basic Principles — Page 10, Lesson 15

I want to walk you through the basic principles of DC electrics, starting with voltage and current. Let's begin with voltage. Think of voltage like water pressure in a pipe — the higher the pressure, the more force pushing the water along. In electricity, voltage is the electrical pressure that pushes electrons through a conductor. The source of that voltage can be either a battery or a generator. Batteries have a limitation: they become discharged as their voltage is used, so they're limited in how long they can supply power. Generators, on the other hand, are used to maintain a constant voltage — they keep producing voltage as long as they're driven. Now, voltage comes in different magnitudes, and we use standard prefixes to express them. One microvolt is one millionth of a volt, written as 1 µV. One millivolt is one thousandth of a volt, written as 1 mV. And one kilovolt is one thousand volts, written as 1 kV. To measure voltage, we use an instrument called a voltmeter. The key point about connecting a voltmeter is that you connect it across the two points between which you want to measure the voltage — in other words, in parallel with the component or circuit — and you do this without disconnecting the circuit. Now let's move to current. Current, which has the symbol I, is defined as the number of electrons passing any point in a conductor in one second. It's measured in amperes, or amps for short, with the symbol A. So when we say "one amp of current," we mean a certain number of electrons flowing past a point every second. To measure current, we use an instrument called an ammeter. Unlike the voltmeter, the ammeter is connected into the circuit — meaning you have to break the circuit and insert the ammeter so that the current in the circuit actually passes through the ammeter. That's a fundamental difference from voltage measurement. Just like with voltage, small values of current have their own prefixes. One microamp is one millionth of an ampere, written as 1 µA. One milliamp is one thousandth of an ampere, written as 1 mA. Now, an electric current produces three important effects that we use in aircraft systems and equipment. First is the heating effect: when a current flows through a conductor, it always causes the conductor to become hot. This is the principle behind electric fires, irons, light bulbs, and fuses — including the fuses that protect aircraft electrical circuits. Second is the magnetic effect: a magnetic field is always produced around a conductor when a current flows through it. This is the foundation for motors, generators, and transformers — all critical components on an aircraft. Third is the chemical effect: when a current flows through certain liquids called electrolytes, a chemical change occurs in the liquid and in any metals immersed in it. This is what happens during battery charging and electroplating. So to summarise what we've covered: voltage is electrical pressure from a battery or generator, measured with a voltmeter connected across a circuit. Current is electron flow measured in amperes with an ammeter connected in series. And current produces three effects — heating, magnetic, and chemical — that we harness throughout aircraft electrical systems.