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Boatowner's Illustrated Electrical Handbook
By Charlie Wing
The McGraw-Hill Companies, Inc.Copyright © 2006Charlie Wing
All rights reserved.
Basic DC Circuits
Basic to the ability to work with electrical wiring—be it residential or marine—is an understanding of what electricity is. You'll find that the discovery of electricity, as we now understand it, was fairly recent.
The key that unlocks the wiring puzzle is the concept of the electrical circuit. With this simple concept and a single formula—Ohm's Law—you can understand and predict the behavior of 99% of the wiring on your boat. You will be able to deal with circuits containing loads in series, loads in parallel, even loads in series/parallel combinations. Similarly, you will be able to predict the behavior of voltage sources in series and voltage sources in parallel. You will also discover the differences between voltage, current, energy, and power.
Finally I have provided a set of 18 practice problems on which you can cut your electrical teeth.
What Electricity Is
We live in the age of electricity. Without electricity we couldn't watch television, drive automobiles, make frozen margaritas, microwave popcorn, read at night, or talk to our friends on the phone.
Many people think electricity is difficult to understand. They are wrong. Because you are surrounded by and unconsciously use electrical devices every day, little lights will go on in your head as you discover the concepts. You'll probably say, "Oooh—so that's why my boat battery is dead every morning!"
I believe you will find electricity to be fun. I am absolutely sure that, having grasped the very simple concepts behind boat wiring, you will feel more confident both in your boat and in yourself.
Electricity consists of electrons. An electron is the smallest quantity of electricity that exists. It is such a small quantity, however, that we use the unit coulomb (1 coulomb = 6.24 × 1018 electrons) in calculations.
The flow of electrons is often compared to the flow of water, so it is natural that we call electron flow "electric current." The basic unit of electric current is the ampere (1 ampere = 1 coulomb per second of electrons moving past a point).
What we usually refer to as electricity is the control of electrons for useful purposes. Our understanding of electron behavior allows us to predict the flow of electrons through electrical circuits. The instruments on your boat contain circuits. Indeed, a boat's wiring is no more than a collection of circuits. When we understand circuits, we will understand the behavior of electricity on a boat.
Electrons can be neither created nor destroyed, but can move through conductive materials. An electric current requires a continuous path of electrically conductive material, through which the electrons can return to their source.
If this were not so, electrons would dribble from the end of a wire like water from a leaky faucet, and batteries would soon sit like empty water glasses with all their electrons lying around them in a pool.
We call a continuous electrical path a circuit. If a circuit is unbroken, we call it a closed circuit. If it is interrupted, preventing the flow of electricity, we say the circuit is open.
All materials present a degree of resistance to electron flow, but the variation is so great that some materials are termed conductors and others insulators.
The best conductors are gold, silver, mercury, copper, and aluminum. Copper is most often the best compromise between cost and conductivity. The best insulators are glass, ceramics, mica, and plastics. Plastic is the material most often used due to its low cost, durability, and ease of manufacture.
Unfortunately for boaters, salt solutions, such as seawater, are also good conductors.
Electrical current is expressed as a rate of electron flow. In a circuit, two factors control the current (I): the electrical driving force, or voltage (V), and the resistance (R) to flow of the circuit materials.
To see just how simple electricity is, we are going to consider the basic equation of current flow in an electrical circuit. With this equation you will be able to understand, predict, and troubleshoot more than 90% of all the electrical problems on a boat.
Question 1. Does current, I, increase or decrease as we increase the driving force (voltage, V)?
Answer 1. Current most likely increases with increasing voltage.
Question 2. Does current, I, increase or decrease as we increase the resistance to electron flow (resistance, R)?
Answer 2. Current most likely decreases with increasing resistance.
Question 3. Considering the answers to questions 1 and 2, what would be the simplest and, therefore, most likely, relationship between current (I), voltage (V), and resistance (R)?
Answer 3. There are four possible "simplest" equations:
I = V + R, I = V – R, I = V × R, and I = V/R
If you play with the values for a minute, you'll agree that the first three equations are unlikely. For example, if we make resistance, R, infinite, current, I, becomes ∞, -∞, ∞, and 0. Only the last value is reasonable, so the relationship is most likely
I = V/R
Congratulations! You have just discovered Ohm's Law. If Georg Ohm hadn't beaten you to it in 1827, you might be up for a Nobel Prize.
Using Ohm's Law
This is such an important relationship, we must be precise in its definition and the ways in which it can be used. First, if we wish to calculate electrical quantities, we must define the units in which these quantities are measured.
I = V/R
I = amperes, abbreviated as A
V = volts, abbreviated as V
R = ohms, abbreviated as Ω
Let's see how Ohm's Law is used. Ohm's Law applies to all situations, but it is useful only in circuits where electricity is flowing.
The voltage source in Figure 1.1 is a device that produces a voltage difference. Examples are batteries and power supplies. Unless otherwise noted, assume voltage sources are batteries. The load is any device or component that consumes electrical energy and, in so doing, results in a voltage drop. Examples are resistors, lamps, and motors. Unless otherwise stated, assume loads are resistances (the zigzag symbol). For a table of electrical symbols used in this book, see Figure 6-1.
Example: If the load is a resistance of 2 ohms, and the voltage source is a 12-volt battery, then by Ohm's Law
I = V/R = 12 V/2 Ω = 6 A
We can also rearrange Ohm's Law so that we can calculate either V or I, given the other two values. The alternate forms of Ohm's Law are:
V = I × R
R = V/I
There is more good news. Ohm's Law applies to more complex circuits as well. We can combine loads and sources in series (end-to-end), parallel (side-by-side), and series/parallel, and the equations remain the simplest possible, as you will see in the figures and examples that follow.
Loads in Series
Resistive loads in series act like one continuous load of a total resistance equal to the sum of the individual resistances:
R = R1 + R2 + R3, etc.
I = V/(R1 + R2 + R3, etc.)
R1 = 2 Ω, R2 = 3 Ω, R3 = 5 Ω, V = 12 V
R = 2 + 3 + 5 = 10 ]
Excerpted from Boatowner's Illustrated Electrical Handbook by Charlie Wing. Copyright © 2006 by Charlie Wing. Excerpted by permission of The McGraw-Hill Companies, Inc..
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