IN THIS CHAPTER

Understanding current, voltage, power, and more

Comprehending electrical flow

Deciphering circuit diagrams

Amplifying your test score

When I was around 12 years old, I impressed my parents by taking an old television set apart and putting it back together. I impressed them right up to the point where I plugged it in and blew up the garage. But the world of electronics is a bit more complex than simply plugging something in and seeing whether it works. I (and the garage) learned this lesson the hard way.

Six years later, when I took the ASVAB, I scored very well on the Electronics Information subtest. (Go figure!) This subtest is designed to measure your knowledge of the principles of electricity and how these principles are applied in the real world. You may see questions about transistors, magnets, engines and motors, and radio and television. (Curiously, there are no questions on this subtest concerning the impromptu demolition of garages.)

You don’t have to be an electronics whiz to score well on this subtest. If you’re not familiar with this information and you want to pursue a military career that requires you to do well on this subtest, this chapter is calling your name. You also need to have some familiarity with basic mathematical and algebraic principles (see Chapters 6 and 7 for more information).

Not every military career requires a good score on this subtest. (Turn to Appendix A to find out which military jobs require a score on this — and other — subtests.) If the military feels that the Electronics Information subtest is important to your desired career, study intensively for this test. You can even take a course or two at the local community college if you don’t have a strong enough background in this area.

You have 9 minutes to answer 20 questions on this subtest on the paper version of the ASVAB and 8 minutes to answer 16 questions on the computerized ASVAB. Although 8 or 9 minutes is sufficient time to answer the questions, it doesn’t provide much time for anything else — if you don’t know an answer, guess and go.

Uncovering the Secrets of Electricity

One day in 1752, Benjamin Franklin was minding his own business, flying a kite in a storm. A key was tied to the kite string and when lightning struck the metal key, Ben was struck by the notion that lightning must be electrified air (well, it happened something like that). Although electricity was just a hobby for Ben Franklin, he made many important contributions. As a result of his famous kite flight, he created many of the terms used today when folks talk about electricity: battery, conductor, condenser, charge, discharge, uncharged, negative, minus, plus, electric shock, and electrician.

Electricity is a general term for the variety of phenomena resulting from the presence and flow of electric current. You can’t see electricity running through a wire (but you can certainly feel it). You only know electricity is there when you flip on the light switch and the light turns on. Even though electricity appears to be pretty mysterious at first glance, scientists understand a great deal about its properties and how it works.

Electricity is measured in three ways:

• Volts: Volts measure the difference of potential between two points.
• Amperes (amps): Amps measure the number of electrons that move past a specific point in 1 second.
• Ohms: Ohms measure resistance, including anything that could limit the flow of electrons.

Here are some other electricity terms that are important for you to know for the ASVAB:

• Current: Electricity is like water — it flows. Electrical current occurs when electrons move from one place to another. The use of conductors, such as copper and water, allows the electrons to move freely. Insulators, such as rubber and wood, discourage the electric current.
• Watt: A watt measures power, the rate at which electrical energy is consumed or transformed into another type of energy, such as light or heat.
• Watt-hour: A watt-hour is the amount of energy used in 1 hour at a rate of 1 watt. Most electricity is measured in kilowatt-hours, which is how much energy you’d use if you ran a 1,000-watt (1-kilowatt) device for an hour. For example, 10 kilowatt-hours is enough energy to run a 10,000-watt speaker system for an hour-long outdoor concert, or it could run a 5,000-watt air conditioner for 2 hours or a 1,000-watt waffle iron for 10 hours. You find watt-hours by multiplying wattage by time (expressed in hours).

The following sections explain electricity in more detail.

Resistance: Slowing the electrical river

Current doesn’t just flow in any properly working circuit unimpeded. Resistance pops up along the way. If the flow of electricity needs to be regulated, resistance is deliberately set up in a circuit. If the flow weren’t regulated, the motors powering devices like can openers and microwave ovens would quickly overheat and melt. (But before that happens, hopefully a fuse would blow or a circuit breaker would trip, halting current flow and saving the equipment.) In a sense, even a wire, such as a filament in a light bulb, is a type of resistance and is a way to deliberately create circuit resistance.

Adding or removing resistance

Sometimes a circuit must be opened in order to add or remove resistance. In other words, the flow of the electricity must be interrupted in order to physically change the resistance. Using a circuit breaker, which is a device that automatically interrupts the electrical current, is an example of opening a circuit to control the current. When the circuit breaker trips, the electrical device can no longer operate.

Some devices use a rheostat, which can vary the resistance without opening the circuit — the device can continue to work even as the resistance is altered. If an application doesn’t use all the electricity, the rheostat absorbs it. A dimmer switch on a light is an example of a rheostat. You increase the amount of resistance to dim the light and decrease the resistance to brighten the light.

Ohm’s law: Relating resistance to current and voltage

The amount of resistance that interferes with the flow is measured in ohms (pronounced just like those yoga chants). The symbol for ohm is the Greek letter omega, which looks like an upside-down horseshoe: . Resistance can be measured by dividing the voltage measured at any given point (the voltmeter reading) by the amount of current at the same point in a circuit (the ammeter reading). Or you can measure the resistance directly by an ohmmeter.

If you have a current flowing through a wire, three influences are present:

• The amount of voltage, measured in volts
• The resistance to the current, measured in ohms
• The amount of current, measured in amps

These three units are always present in a specific relationship to each other. If you know the value of any two of the influences, you can find the value of the third. (Yes, this requires more math. Sorry.)

Ohm’s law, which was first stated by Georg Simon Ohm, reads, “The current in a circuit is directly proportional to the applied voltage and inversely proportional to the circuit resistance,” but it’s actually easier to understand in mathematical terms. When stating the relationship mathematically, abbreviations are used, where I is current, E is voltage, and R is resistance:

This essentially means that current in a basic circuit is always dependent on the voltage and resistance in the circuit. If you use a higher-voltage battery (increase E), the resistance doesn’t change, but current in the circuit increases. By the same token, if you leave the same battery in the circuit but increase the resistance (increase R), current decreases.

Here are two other ways to write the same formula, solved for voltage and resistance:

• 
• 

Ohm’s law works exactly the same, no matter which format you use.

Getting around to circuits

Although this section suggests that electricity flows like water, it actually flows more like NASCAR. Electricity must be sent along the path of a closed circle (a circuit), just like all those NASCAR speedsters roaring around the track. The drivers never actually get anywhere; they just keep driving in circles. Electrical charges are a lot like that.

However, electricity does flow like a river in one respect. In general, electricity follows the path of least resistance. The conventional way in thinking about the electrical flow of current is based on the vacancies left by electrical particles “moving” from the positive (+) terminal to the negative (–) terminal of a battery. This concept is called conventional current. However, the military teaches current flow based on the flow of the electrons, and electrons, no matter how you look at them, flow from the negative terminal to the positive terminal (see Figure 11-1).

If any of the wires leading from one terminal to the other is broken, the circuit is shot — no more current. Current can’t flow because under most circumstances, the electrons can’t bridge the open gap in a conductor (the open gap is basically air, and air is an insulator).

In some cases, current does flow through an insulator — if there’s enough difference of potential (voltage). When lightning bridges an expanse of air from a cloud to ground (or a tree or a golfer), it’s because there is a huge amount of voltage, on the order of 100 million volts, between the source of the lightning and (literally) ground.

Here’s another circuit problem that may come up: A short circuit occurs when any wire accidentally crosses over another wire, causing the electricity to bypass the rest of the circuit and not follow the intended path.

Switching Things Up with Alternating and Direct Current

A current doesn’t always flow in one direction. A direct current (DC) does — it only and always flows in one direction. An alternating current (AC), however, constantly changes direction in a regular pattern. Higher voltages are easier to obtain with alternating current, and transferring high voltage down a power line is ultimately cheaper than transferring low voltage, so most electricity comes in the form of AC. The following sections cover some important points about alternating and direct current.

Impedance: Join the resistance!

Resistance interferes with the flow of current in a circuit. But the flow of current is also impeded by two properties of alternating currents:

• Capacitive reactance (capacitance): Capacitance is the storage of energy that occurs in a nonconductor. This property resists any change in voltage in a circuit.
• Inductive reactance (inductance): Inductance is the property that causes an electromotive force (another way of saying voltage) to be induced in a circuit.

These two types of reactance combine to impede the flow of current. Impedance can be expressed as the ratio of electromotive force to the current:

Electronic devices often require a specific capacitive or inductive reactance to work. Capacitors and inductors are devices used in circuits to provide the type of reactance needed. Capacitors are rated in microfarads , and inductors are rated in millihenries (mH).

You can relate impedance to Ohm’s law in reference to AC circuits. Simply substitute resistance in Ohm’s law with impedance and voltage with electromotive force.

Picture It: Decoding Electrical Circuit Codes

Electronic circuits can be combined to create complex systems, such as those required to operate a stereo system. Block diagrams are used to show the various combined circuits that form a complex system.

Many of the questions on the Electronics Information subtest require you to identify an electronic component symbol and know what that component does in an electronic circuit. Figure 11-2 shows the most common component symbols. The figure’s items are grouped based on similarity of functions. For example, cells, batteries, DC power supplies, and AC power supplies all have similar functions (they supply power to the circuit).

So, what do all these electronic doodads do when connected in a circuit? I cover each item in the following list:

• Wires: Wires are used to pass current from one part of the component to another. Wires that are connected to each other are indicated by a dark circle and are called joined wires. Sometimes in complex circuit diagrams, it’s necessary to draw wires crossing even though they aren’t connected. In this case, the dark circle is omitted, or a hump symbol is drawn to make it clear the wires aren’t connected — this is called unjoined wires.
• Cell: A cell supplies electrical current. Some call this a battery, but technically a battery is more than one cell. The large terminal (on the left side of the cell image in Figure 11-2) is positive.
• Battery: A battery is two or more cells. The large terminal is positive.
• DC power supply: A DC power supply provides direct current. Direct current always flows in one direction.
• AC power supply: An AC power supply provides alternating current. Alternating current constantly changes direction at a specific frequency.
• Fuse: A fuse is a safety device that blows (melts) if the current flowing through it exceeds a specified value.
• Transformer: A transformer consists of two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC voltages. No electrical connection exists between the coils. Energy is transferred between the coils by the magnetic field in the core.
• Ground: A ground is a connection to the earth.
• Transducer: A transducer is a device that converts energy from one form to another. Here are various types of transducers:
  • Lighting lamp: Converts electrical energy to light, such as in a light bulb or automobile headlight
  • Indicator lamp: Converts electrical energy to light for such uses as a warning light on a car’s dashboard
  • Motor: Converts electrical energy to kinetic energy (motion)
  • Heater: Converts electrical energy to heat
  • Bells and buzzers: Convert electrical energy to sound
  • Microphone: Converts sound to electrical energy
  • Earphones and speakers: Convert electrical energy to sound
• Inductor: An inductor is a coil of wire that creates a magnetic field when current passes through it.
• Switch: Here are several types of switches:
  • Push switch: A push switch allows current to flow only when the button is pressed, such as in a doorbell.
  • Push-to-break switch: With this switch, the circuit is normally closed (the device is on); the circuit is open (device is off) only when the button is pressed.
  • On/off switch: An on/off switch allows current to flow only when it’s in the closed (on) position.
  • Two-way switch: A two-way switch directs the flow of current to one of two routes, according to its position.
  • Dual on/off switch: This type is often used to switch main electricity because it can isolate both the live and neutral connections.
  • Relay (relay switch): A relay is an electrically operated switch that may operate multiple switches at one time. Current flowing through a coil sets up a magnetic field, which causes the lever(s) to move, effectively changing the (relay) switch’s position(s).

• Resistor (nonvariable): There are two different versions of the basic resistor symbol. Resistors restrict the flow of electric current. Resistors are rated in ohms and have a color code on them to indicate their value, tolerance, and sometimes quality. The band code is as follows:

  • Black is 0.
  • Brown is 1.
  • Red is 2.
  • Orange is 3.
  • Yellow is 4.
  • Green is 5.
  • Blue is 6.
  • Violet is 7.
  • Gray is 8.
  • White is 9.

  The first and second bands on the resistor are the first two digits in the resistor’s value. The next band indicates the multiplier (number of zeros after the first two numbers). So if the first band is red, the second is yellow, and the third band is orange, the resistor’s value is 24,000 ohms. A gold or silver band after the first bands indicates tolerance, and a quality band may follow the tolerance band.

• Variable resistor: Variable resistors also restrict the flow of electric current. There are several symbols in use in circuit diagrams for standard variable and preset variable resistors. Types of variable resistors include the following:
  • Rheostat: A type of variable resistor with two contacts, usually used to control current; examples of controlling current would be adjusting lamp brightness or adjusting motor speed
  • Potentiometer: A type of variable resistor with three contacts that’s used to control voltage
  • Preset variable resistor: A device that operates with a small screwdriver or similar tool; it’s designed to be set when the circuit is made and then left without further adjustment
• Capacitor: Capacitors store electric charge. They’re used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They’re also used in filter circuits because capacitors easily pass AC (changing voltage) signals but they block DC (constant voltage) signals. Two types of capacitors include the following:
  • Polarized capacitors must be connected the correct way in circuit.
  • Variable capacitors are used most often in radio tuning circuits.
• Diode: Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve, and early diodes were actually called valves. Light-emitting diodes (LEDs) emit light when an electric current passes through them. Specialized diodes, called Zener diodes, do allow current in the opposite direction after a threshold is met.
• Transistor: Transistors amplify current. For example, they can be used to amplify the small output current from a logic chip so it can operate a lamp, relay, or other high-current device.
• Amplifier: An amplifier isn’t actually an electronic component but instead is a complex circuit. The block diagram symbol shows where an amplifier circuit would be connected. Amplifier circuits are used to magnify power, current, or voltage.
• Antenna: An antenna is a device designed to receive and/or transmit radio signals.

Circuit diagrams show how electronic components are connected together. These diagrams show the connections as clearly as possible with all wires drawn neatly as straight lines. The actual layout of the components is usually quite different from the circuit diagram, however. Circuit diagrams are useful when testing a circuit and for understanding how it works. Figure 11-3 shows a diagram of an adjustable timer circuit. See how many components you can identify.

Eyeing Some Electronic Information Test Tips

When it comes to the electronics test, don’t feel like you have to know as much as Ben Franklin to get a passing score. Just use your common sense. If a question asks, “What’s the safest way to run an extension cord to a reading light?” the answer “across the middle of the floor” is probably going to be wrong.

You can also figure out quite a few answers if you remember these units of measure:

• Current: Amperes (or amps)
• Voltage: Volts
• Resistance: Ohms
• Power: Watts
• Energy: Watt-hours

Electronics Information Practice Questions

The questions in this section measure your knowledge of basic electronics principles.

If you need a good score on this subtest to get your military dream job or you want to rebuild that old television set without sacrificing your garage, you may want to check out Electronics For Dummies by Gordon McComb and Cathleen Shamieh (Wiley) for additional help.

Answers and Explanations

Use this answer key to score the Electronics Information practice questions.

1. C. DC stands for direct current. I made up the other choices. The correct answer is Choice (C).
2. D. Remember, the upside-down horseshoe (the Greek letter omega) is the symbol for ohm, the measure of electrical resistance. The correct answer is Choice (D).
3. C. Copper is the best conductor of electricity of those listed here. Therefore, it offers the least resistance to an electric current. The correct answer is Choice (C).
4. B. When the switch is in contact with point A (as shown), the charges are being stored. When the switch moves to point B, the flashbulb turns on.
5. B. A terminal is a device that connects electrical circuits together, a trigger initiates a circuit action, a transmitter is a device used to achieve transmission, and a transformer is an inductor with two or more windings. Windings are magnetic wires that are coated with enamel and wrapped around the core of a transformer. The primary winding is driven by transistors, and the secondary winding is driven by the core’s magnetic field, produced by the primary winding. Choice (B) is the correct answer.
6. D. A diode is a semiconductor that conducts electricity in one direction only; a transformer is a device that changes voltage (either “transforming” low voltage to high voltage or high voltage to low voltage); a rectifier is a circuit that changes alternating current to direct current. Choice (D) is the correct answer.
7. B. Rectification occurs in certain electronic circuits that need to change incoming AC to DC in order to run properly.
8. C. A watt measures the amount of power, the rate at which energy is produced or used. The correct answer is Choice (C).
9. C. A variable resistor is a potentiometer with two connecting wires instead of three; it allows for finer control over the current by changing the amount of resistance.
10. A. Capacitors store electric charge. They’re used with resistors in timing circuits because it takes time for a capacitor to store voltage (to become charged). The correct answer is Choice (A).
11. A. In conventional flow notation, the motion of charge is shown according to the (technically incorrect) labels of + and –, with the electric charge moving from positive to negative. But in electron flow notation, the actual motion of electrons in the circuit is followed. Negative electrons are always searching for positive charges, so current flows from a negative pole to a positive pole.
12. B. Wires connected to each other are indicated by a darkened circle. The correct answer is Choice (B).
13. C. When a wire is wrapped around an iron core and a current is sent through the wire, the iron becomes magnetized.
14. C. In a wire, magnetic lines of force are perpendicular to the conductor and parallel to each other.
15. B.

    

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    This symbol represents a fuse; the circuit doesn’t contain a fuse.

16. A. Resistance is set up in a circuit to regulate the electricity so the device isn’t destroyed by electrical heat.
17. C. Red, black, and blue wires are always “hot” and should never be tampered with unless the power is off. The gray wire is a neutral, earth-connected wire.
18. C. The symbol represents an electrical outlet, which indicates where electronics can be plugged into a circuit.