Circuit Analysis For Dummies

Chapter 1

Introducing Circuit Analysis

In This Chapter

Understanding current and voltage

Applying laws when you connect circuit devices

Analyzing circuits with algebra and calculus

Taking some mathematical shortcuts

Circuit analysis is like the psychoanalysis of the electrical engineering world because it’s all about studying the behavior of circuits. With any circuit, you have an input signal, such as a battery source or an audio signal. What you want to figure out is the circuit’s output — how the circuit responds to a given input.

A circuit’s output is either a voltage or a current. You have to analyze the voltages and currents traveling through each element or component in the circuit in order to determine the output, although many times you don’t have to find every voltage and every current within the circuit.

Circuit analysis is challenging because it integrates a variety of topics from your math and physics courses in addition to introducing techniques specific to determining circuit behavior. This chapter gives you an overview of circuit analysis and some of the key concepts you need to know before you can begin understanding circuits.

Getting Started with Current and Voltage

Being able to analyze circuits requires having a solid understanding of how voltage and current interact within a circuit. Chapter 2 gives you insight into how voltage and current behave in the types of devices normally found in circuits, such as resistors and batteries. That chapter also presents the basic features of circuit diagrams, or schematics.

The following sections introduce you to current and voltage as well as a direction-based convention that’s guaranteed to come in handy in circuit analysis.

Going with the flow with current

Current is a way of measuring the amount of electric charge passing through a given point within a certain amount of time. Current is a flow rate. The mathematical definition of a current is as follows:

The variable i stands for the current, q stands for the electrical charge, and t stands for time.

The charge of one electron is 1.609 × 10–19 coulombs (C).

Current measures the flow of charges with dimensions of coulombs per second (C/s), or amperes (A). In engineering, the current direction describes the net flow of positive charges. Think of current as a through variable, because the flow of electrical charge passes through a point in the circuit. The arrow in Figure 1-1 shows the current direction.

Illustration by Wiley, Composition Services Graphics

Figure 1-1: Current direction, voltage polarities, and the passive sign convention.

Measuring current through a device requires just one point of measurement. As an analogy, say you’re asked to count the number of cars flowing through your long stretch of residential street for 10 minutes. You can count the number of cars from your home or your friend’s home next door or the house across the street. You need just one location point to measure the flow of cars.

Two types of current exist: alternating current (AC) and direct current (DC). With AC, the charges flow in both directions. With DC, the charges flow in just one direction.

If you have trouble keeping AC and DC straight, try this mnemonic device: AC means “always changing,” and DC means “doesn’t change.”

Recognizing potential differences with voltage

From physics, you know that plus and minus charges attract each other and that like charges repel each other. You need energy to separate the opposite charges. As long as the charges are separated, they have electric potential energy.

Voltage measures the amount of energy w required to move a given amount of charge q as it passes through the circuit. You can think of voltage as electric potential difference. Mathematically, voltage is defined as

Voltage has units of volts (V), which is the same as joules per coulomb (J/C). In a 12-volt car battery, the opposite charges on the battery terminals have a separation of 12 units of energy per unit of charge (or 12 V = 12 J/C). When terminals are separated, there’s no current flow. If you provide a conducting path between the opposite charges, you now have charges flowing, resulting in an electric current.

It takes two points to measure voltage across a device, just like it takes two points to measure height or distance. That’s why you can think of voltage as an across variable.

Refer to Figure 1-1 to see the positive and negative voltage signs (called polarities) of a device, labeled at Terminals A and B.

Staying grounded with zero voltage

Because measuring voltage requires two points, you need a common reference point called a ground. You assign ground as 0 volts, where all other points in a circuit are measured with respect to ground. This is analogous to defining sea level as a reference point of 0 feet so you can measure the height of mountains. When the sea canyon is below sea level, you assign a negative algebraic sign. In circuits, the negative sign means the answer is less than the ground potential of 0 volts. Refer to Figure 1-1 to see an example of a typical ground symbol.

You may have seen a streetcar with one trolley pole and an electric bus with two trolley poles. Why the difference? The streetcar uses its wheels as its ground point, so current coming from the generator flows back to the generator through the earth via the wheels grounded at 0 volts. For the electric bus, you need two wires because current can’t flow through the rubber tires.

Getting some direction with the passive sign convention

Along with the algebraic signs of your calculated answers, the passive sign convention orients you to what’s happening in a circuit. Specifically, it tells you that current enters a passive device at its positive voltage terminal.

Here’s how passive sign convention works: You assign plus and minus signs to each device to serve as reference marks. After you arbitrarily assign the polarities of a device, you define the current direction so that it enters the positive side. (You can see what I mean by referring to Figure 1-1.) If your answers for voltage or current are positive, then the polarities line up with your assigned polarities or current direction. If your answers come up negative, they’re opposite to your assigned polarities or current direction. A negative answer isn’t wrong; it’s just reverse to your assigned reference marks.

The way you assign your polarities and current direction doesn’t control the circuit behavior. Rather, the algebraic signs of your answers tell you the actual directions of voltages and current in the circuit.

Beginning with the Basic Laws

A circuit is basically a collection of electrical devices, such as resistors, batteries, capacitors, and inductors, arranged to perform a certain function. Each component of a circuit has its own constraints. When you connect devices in any circuit, the devices follow certain laws:

Ohm’s law: This law describes a linear relationship between the voltage and current for a resistor. You can find details about resistors and Ohm’s law in Chapter 2.

Kirchhoff’s voltage law (KVL): KVL says the algebraic sum of the voltage drops and rises around a loop of a circuit is equal to zero. You can find an explanation of voltage drops, voltage rises, circuit loops, and KVL in Chapter 3.

Kirchhoff’s current law (KCL): KCL says the algebraic sum of incoming and outgoing currents at a node is equal to zero. Chapter 3 provides info on applying KCL and defines nodes in a circuit.

With these three laws, you can solve for the current or voltage in any...