Electricity for the Absolute Beginner Dr. Klge Words 1369 In Home Power articles, volts and amps are everyday words. Circuit schematics are nearly as common as photos. If these are mysteries to you, here are some conceptual tools to see how all this electricity stuff fits together. So what is electricity anyway? If you've ever been shocked by a live wire or static electricity you've had about the most direct experience of electricity of anyone. Beyond this, no one really knows what electricity is. On the other hand we do know a lot of ways to make electricity, and even more ways to use it. All of these involve circuits. Electrical Current ~ Water Current A good way to think about electricity flowing in a circuit is to think of water in plumbing. Water flows through pipes. Electrical current flows in wires. A water current is a flow of water molecules; an electrical current is a flow of electrons. Pipes and wires both can be connected together into networks. Below is a schematic of an electrical circuit that uses a battery to power a record player. To the left is Dr. Klge's plumbing "circuit", which uses water to do the same thing. INSERT KlugeSeriesPICT Voltage ~ Pressure Electricity for this circuit comes from a battery. In the plumbing network, water under pressure is supplied by the storage tank. The higher the storage tank is placed above the rest of the plumbing 'circuit' the greater the water pressure in the pipes. In the electrical circuit the analog for pressure is the voltage. Voltage is raised by adding more cells to the battery. Resistance In the plumbing case the water flows out of the storage tank and though a very narrow section of pipe (small inner diameter). A narrow pipe presents more resistance to water flowing than a larger pipe. Thick copper wires carry lots of electricity easily, while thinner wires resist the flow of electricity. The property of a object to resist the flow of electrical current is called resistance. A resistor is an electrical part which has a known (usually large) resistance. Water loses pressure if it flows though a pipe, and loses more if the pipe is narrow. Water loses pressure in the process of powering a turbine. The same thing happens with electricity. If a current flows though something with electrical resistance then the voltage is reduced. Mathematically: V = IR; the voltage drop (V) is equal to the current (I) times the resistance (R). This is called Ohm's law, and is used everywhere. Frequently resistance is not desired. With this in mind, electrical people take great care to ensure they use big enough wires and make good connections between wires. Bad connections and corroded connections have high resistance. The record player itself has resistance, just as Dr. Klge's turbine powering the old phonograph resists the flow of water more than if it weren't there. Even the battery has a resistance, but don't worry about it if it hurts your head. Let's assume the wiring and the battery have negligible resistance. Kirchhoff's Laws: How much current flows in the circuit? Two laws discovered by Gustav Kirchhoff in the mid 1800s are the only tools we need. The first seems so silly it's surprising that it gets a name. Kirchhoff's Law #1: At any point the current going in must equal the current going out. Think about this in plumbing terms: If water flows in the top part of the pipe, it will flow out the bottom part too. In plumbing there might be a delay as water fills up an empty pipe. There is no delay (or really a delay that moves at the speed of light) for electricity. The wire is always "full" of electrons. The important part here is that current is not lost in the circuit. Voltage is lost in a circuit, but never current. Every electron that starts the circuit finishes it. This circuit is a series circuit, meaning the components follow each other in a series, one after another, like the links on a chain. In this circuit there are only three components: the battery, the resistor and the record player (the storage tank, the thin pipe, and the record player). For a series circuit, Kirchhoff's law tells us even more: that the current at any point in the circuit must be equal to the current at every other point. Digest this point for a second, find out why this is true. Kirchhoff's Law #2: For any closed circuit, the total voltage drop across all the components is equal to the voltage of the power source. Remember Ohm's Law: when current flows though a component, there's a voltage drop across that component equal to the current times the resistance of the component. Kirchhoff's #2 says that the battery will supply more and more current until the sum of the voltage drops across the components is equal to the battery voltage; and it will sustain current at this level until the resistance of the circuit or the voltage of the battery changes. A bit of nomenclature here: the voltage of the power source is often called the electromotive force (emf) designated by the letter "E". A Parallel Circuit In order to enjoy a cup of fresh ground coffee while he listens to his phonograph, Dr. Klge adds a coffee grinder to the circuit. The grinder takes more current than the phonograph. If Klge put the grinder in series, its current would be restricted by the phonograph turbine and the bit of narrow pipe. The doctor decides to install a parallel circuit to bypass these components. INSERT KLUGEPAR INSERT Series/Parallel The Electrons Aren't Confused! The current in the circuit now has a choice: go left to the coffee grinder, or right to the old series circuit. In the electric circuit, Kirchhoff's first law has a slightly deeper meaning if we choose the intersection as our point to inspect. Current in equals current out, and here current out is current in the coffee loop plus current in the phonograph loop. Ibattery = Icoffee + Iphonograph. Kirchhoff's second law says that the voltage drop in each conductor equals the battery voltage. In the phonograph loop the same amount of current will flow as before. The resistance of this part of the circuit hasn't changed, and it is resistance that governs how much current flows. Electrons who choose this path will have it just as easy as before. But there will also be an alternate route Ä through the coffee grinder loop. In the coffee grinder loop the battery pushes current to maintain a voltage drop across the coffee grinder equal to the battery voltage. It's the same scenario as in the phonograph loop, only here the resistance is probably different (lets say it's less Ä Klge has a powerful coffee grinder). Lower resistance means more current will flow so that Icoffee times Rcoffee equals Vbattery. In this way, figure out the current in each loop separately, then add them up to get the total current, just like you would do for water flowing in parallel pipes. Limitations of the Plumbing Analogy You'll find the plumbing analogy for circuits is far from perfect. For one thing, water sometimes leaks, electricity doesn't. In the plumbing Kluge, water flows from storage tank down to a kidee pool. Electrical current only flows if the circuit is closed - if the "bottom" of the circuit is hooked to the battery negative. Bag O' Tools There you have it. These tools will allow you to find the current in all parts of any direct current circuit. There's a lot of information packed into this article, and it will take a while to get comfortable with it if you've never seen this stuff before. To summarize: 1) every electron that starts the circuit finishes it; and 2) the charging source supplies more and more current until the voltage drop across the circuit equals the EMF. The best way to get a feeling for voltage in circuits is to get a volt meter and start measuring. As long as you stay away from household currents (110 ac or higher) and you keep the meter in voltmeter mode you've got little to fear.