Electric 101 Back to Basics Part Ill COLUMN BY BOB THOMAS This month we’ll talk about resistance, but first, a few questions to see how much you remember from the December issue. 1 — Voltage can best be described as (a) the flow of current (b) pressure on electrons (c) volume of electrons (d) total power. 2 – Amperage can best be described as (a) pressure on electrons (b) volume of electrons (c) total power (d) resist- ance to electron flow. 3 — Resistance can be described as (a) ohms (b) restriction of electron flow (c) neither a or b (d) both a and b. 4 — Voltage x Amperage = (a) Ohms (b) Watts (c) a complete circuit (d) the volume of electrons. 5 — Electrons are moved in a circuit by (a) amperage (b) watts (c) voltage (d) ohms. 6 — In any complete circuit, amper- age times resistance will equal (a) total current (b) source voltage (c) total ohms (d) total horsepower. The answers are at the end of this article. If you did not get at least five right, you may want to go back and read Part II again. Now we’ll discuss resist- ance in electrical circuits. Resistance is the opposition to the flow of electrons within an electrical circuit. Every electric circuit must con- tain some resistance. It comes in many forms, but I’d like to break them down into two groups and refer to them as “good resistance” and “bad resistance.” This may be a technical oversimplifica- tion, but it will make it easier to under- stand that some resistance is essential while too much of the wrong kind can be catastrophic. All electric devices (lamps, motors, relays, horns, gauges) use good resist- ance to perform a job. As work is being done in any circuit, this type of desired resistance always plays a part. Without good resistance we could not crank an engine, turn a cooling fan, roll up an electric window, or light a lamp. You can think of resistance as a type of friction within the conductor. Lamps, motors, and coils all get hot when oper- ated because some electrical energy is lost as thermal energy, just as energy is lost in a mechanical system to friction. Another type of good resistance comes in the form of components built to reduce or control voltage. These are called resistors, quite appropriately, and can be fixed or variable. The little poten- tiometers we use when calibrating adjustable regulators are variable resis- tors. So is the rheostat that is used to dim or brighten instrument lamps. Electronic circuits are full of resistors, used to manipulate and control voltage. Bad resistance, on the other hand, can be found in connections, wire, switches and relay contacts. This type of resistance is not desired, and robs any circuit of some of its capacity to do work. There is always some bad resist- ance in any circuit, but it must be kept to a minimum. When connections become loose or corroded, wire size is too small, or contacts are burned or worn out, bad resistance increases. The more amperage that the circuit must carry, the greater that effect will be. This is because of the relationship between amperage and resistance in Ohm’s Law mandates that when one goes up the other must go down. Too much bad resistance in a starter circuit will limit amperage flow and cause the starter to drag, or turn slow. Too much bad resistance in a charging circuit will result in reduced amperage and lower voltage at the battery. For those two rea- sons alone, every rebuilder needs to understand resistance. Resistance is cumulative in a series circuit. In other words, series circuit resistance is always determined by adding the total of all resistance in that circuit. That is not the case in parallel circuits, but we’ll cover that one next month. To better explain the effect of resist- ance, look at this simple series circuit with a battery (voltage source), a motor (device or load) and a switch (see Figure 1, page 28). First, let’s assume we have no bad resistance (not true in the real world) in this circuit and the motor is a window motor known to draw 12 amps. We can then use Ohm’s Law to determine the resistance of the motor (load) is 1 ohm. That is because E=IxR or 12=12×1. The power equation tells us the window motor develops 144 watts of power to lift the window. I used even numbers here to keep the calculations simple. Now, let’s enter the real world. Assume we have .1 ohm resistance in the battery connections which are slightly corroded. The normal wiring in the window circuit has .2 ohms, which includes several harness connections. The switch contacts are worn and have .3 ohms resistance. And the connections at the motor itself are old and have been overheated so we have .4 ohms resistance there. The total resistance in the circuit is now 2 ohms. Using Ohm’s Law, the current will be reduced to 6 amps. The end result is that power is now reduced to 72 watts and the motor probably will not lift the window. The culprit is bad resistance. Resistance in wire and connections is only measurable when current is flowing, so we must use an