Electronics Information Study Guide for McGraw-Hill's ASVAB

Technical Terms

To understand electricity, you also need to know a few technical terms. Study the following list.

• Electric currentis the amount of electrons flowing through a conducting material.
• Electric poweris the amount of power consumed by an electrical device.
• Voltageis a force that affects the rate at which electricity flows through a conductor. It is sometimes called electrical pressure. The higher the voltage, the more likely electricity is to "leak" across an insulator or an air gap. That's one reason higher voltages are more dangerous. Voltage droptells how much electrical pressure is used in a part of the circuit.
• Frequencyis the number of complete alternations— from one direction to the other and then back again—that alternating current makes per second. Each complete alternation is called a cycle.
• Resistanceis the opposition of a material to the flow of electricity through it. All circuits must have a resistance. If they don't, they are called short circuits, and wires can overheat.

Units of Measure and Measuring Devices

Different aspects of electricity are measured using different units of measure. Special measuring devices are used. The following table shows the different units and devices.

Ohm's Law

Ohm's law describes the relationship among electrical pressure (voltage), current strength (amperage), and resistance (ohms) in any circuit:

If you know two of these three quantities, you can always calculate the third.

Here's an easy way to remember how to use Ohm's law to find the third quantity if you know two already. On the circle (below), place your finger over the quantity you want to find. Look at the remaining two quantities to see how to calculate the third quantity.

Memorize the equations on the circle; you'll need them on the ASVAB. Let's look at a couple of examples of how to use the Ohm's law circle.

Find the amperes if a 120-volt current runs through 6 ohms.

Amperes = volts/ohms

Amperes = 120/6 = 20 amperes

Or cover ohms in the circle. Notice that what remains is volts divided by ohms (the same result you would get by remembering the three formulas above).

Amperes = 120/6 = 20 amperes

In a 6-volt circuit with 24 amperes flowing, what is the resistance?

Ohms = volts/amperes

Ohms = 6/24 = 1/4 ohms

The Law of Electric Power

The amount of power consumed by an electrical or electronic device can be calculated using the following formula:

Watts = volts = amperes

You can use this equation to find any one of the three factors as long as you know the other two. Here are two examples.

How much power is consumed by a lamp that draws 10 amperes of current at 120 volts?

Watts = volts = amperes

Watts = 120 = 10 = 1,200 watts

A clothes dryer is rated at 2,400 watts. At 120 volts, how much current does it draw?

Amperes = watts/volts

Amperes = 2400/120 = 20 amperes

ELECTRICITY AND MAGNETISM

Electricity and magnetism are tightly connected: It's easy to change from an electric current to a magnetic field and back again. This close relationship explains electromagnets, transformers, motors, and generators. Let's start with electromagnets.

A current passing through a conductor creates a magnetic field around it. In most electromagnets, the conductor (wire) is wrapped around an iron core.

A transformer is like two electromagnets placed next to each other. If the transformer has more turns of wire on the output side, it is a step-uptransformer, and the output voltage will be greater than the input voltage. If there are more turns on the input side, it's a step-downtransformer, and the output voltage is smaller than the input voltage.

Motors and Generators

Electromagnetism also explains motors and generators. In fact, motors and generators are really the same machines, operating backwards.

• A generatorchanges rotating (kinetic) energy into electric energy.
• A motorchanges electric energy into kinetic energy.

Here's how a motor works. Each magnet has two poles: north and south. Opposite poles attract, and like poles repel: North attracts south, but repels north. A motor has two magnets: a rotorthat spins inside a stator, a fancy name for a stationary magnet. One of these magnets, usually the rotor, is an electromagnet. It is wired so that the magnetic field changes twice per rotation. When the rotor starts rotating, the rotor and the stator repel each other, forcing the rotor to start turning. At just about the point where the magnets would stop repelling, the rotor changes polarity, and it again repels the stator. This change in polarity is what drives the motor. We call them electric motors, but motors are all about magnetism.

ELECTRICAL DEVICES AND SYMBOLS

Electrical devices are the guts of many electrical and electronic systems. You'll need a basic acquaintance with these devices. You should also know the symbols used for them in electrical diagrams.

A capacitoris a device that can briefly store electricity.

A resistorcreates resistance to the flow of electrons. If a circuit does not have any resistance, it's called a short circuit. Excess current will cause such a circuit to heat up, which may cause a fire.

A transformerchanges the voltage and amperage of a current. Transformers work by changing electricity into magnetism, then back into electricity. As you can see from the diagram, transformers have two coils. One gets current (usually AC) from a source; the other supplies the output. The first coil creates a magnetic field, which rises and falls as the current alternates. The second coil is inside a changing electric field, and any wire in a changing magnetic field will pick up current from that magnetism.

A batterystores electricity as chemical energy that can be readily converted into electric current. Batteries may be wet (like the lead-acid storage batteries used in cars) or dry (like the nickel-cadmium or metal hydride batteries used in flashlights, computers, and the like). Batteries always make direct current. Depending on their chemistry, some batteries can be recharged.

In the diagram, the electrochemical reactions between the cathode and the carbon electrode at the center result in free electrons, which can travel through a circuit to make an electric current.

SERIES, PARALLEL, AND SERIES-PARALLEL CIRCUITS

Electric current exists only when electrons can flow through a circuit. There are two basic types of circuits, and a third type that blends the two. Let's start with the basics: the series and parallel circuits.

In a series circuit, all moving electrons pass through every part of the circuit, including all the loads and switches. Current (the quantity of electrons) is the same at all points of the circuit, but voltage drops as the current goes through each device. The total voltage of the loads must equal the voltage of the circuit: In a series circuit supplied by a 12-volt car battery, a single light bulb must have a 12-volt drop. If the circuit has two lights, their combined voltage drop is 12 volts.

When a series circuit is used in a string of Christmas tree bulbs, the whole string goes dark if any bulb burns out. Thus, although they are simple, series circuits are less common than the next basic type, the parallel circuit.

In a parallel circuit, the loads are placed between the two supply wires, so that they all get the same voltage. A second advantage of the parallel circuit is this: Current can flow through any of the loads, even if one is switched off. (In a series circuit, a switch controls the current in the entire circuit.)

Remember this rule: Current is the same at all points in a series circuit. Voltage is the same at all points in a parallel circuit.

To find the total resistance in a series circuit, add the resistance of each load. For example, in a series circuit with one 12-Ω and one 8-Ω resistor, total resistance = 12 + 8 = 20 Ω.

It's more complicated to calculate total resistance in a parallel circuit since you must add the inverse of the resistances. What is the total resistance in a parallel circuit with one 12- Ω and one 8- Ω resistor?

1/R_total = 1/12 + 1/8 = 2/24 + 3/24 = 5/24 = 1/R_total

Solve for R_total:

Multiply both sides by R_total: 5 × R_total/24 = 1

Multiply both sides by 24: 5 × R_total = 24

Divide both sides by 5: R_total = 24/5 ohms

Note that R_total, 24/5, is simply the inverse of 5/24, the fraction equaling 1/R_total.

The third type of circuit is the series-parallelcircuit, which combines features of series and parallel circuits. The series-parallel circuit is a hybrid with many advantages of each. You'll find seriesparallel circuits throughout your house. The branch circuits that bring electric power to the lights and outlets are series-parallel: All the current goes through a fuse or circuit breaker, but then it is distributed in parallel. Why is this circuit needed? Because voltage must be the same for all outlets and lights, and because the circuit must work whether any particular light is switched on or not.

SEMICONDUCTORS

So far, we've talked about insulators and conductors. But there is an important category of materials between these two categories.

A semiconductor can act as a conductor or as an insulator. Silicon, the main semiconductor, is the basis for computer memory and logic boards. Chemicals called dopantsare applied to the silicon to determine whether it will act as an insulator or as a conductor. They do this by making electrons available or not available to flow. (When electrons can flow, a material becomes a conductor.)

The basis for computer applications is a group of components, particularly transistors and diodes.

Transistorsare devices that can switch a current, regulate its flow, or amplify a current, all based on the presence of a smaller current. Millions of tiny transistors are built on small pieces of semiconductor, which are the basis of computer logic and memory.

Diodesare devices that allow a current to flow in one direction only. In addition to electronics, diodes are also used in devices called rectifiers, which convert AC into DC.

PRACTICAL ELECTRICITY

The ASVAB will also ask you about more practical matters, including simple electric circuits. You'll have an advantage here if you've ever worked on your home wiring (which is usually much easier than most people think).

In a simple electric circuit like the one in your home, electricity is distributed at a fuse box or circuit breaker box. The box has two functions:

• Breaking up the load in the building into a number of circuits
• Preventing excess current from flowing into the circuits

Many simple circuits have two separate conductors: hot and grounded. The hot conductor is usually black, but it can be red or another color. The grounded (sometimes called neutral) conductor is white. Together, the black and white wires are called the supplywires, because they form the circuit that electric current needs to travel.

To work safely on any electric circuit, you need to shut off the electricity, and check that it is off. The fuses and circuit breakers can shut off the circuit. Before starting work, use an electrical tester to test whether the hot wires are energized.

Bigger (heavier) wires can carry more current. Home wiring systems are usually rated for 15 or 20 amperes. 15-ampere circuits require 14-gauge wires. Larger, 12-gauge wires are needed for 20-ampere circuits.

Grounding Systems

Electricity always "wants" to complete a circuit, to get back to where it came from. The wiring in a building always includes two options for returning current to the circuit breaker or fuse box. The white (grounded) wire is the normal return path for electricity. The bare (equipment grounding) wire is the alternative path, available if something goes wrong with the grounded wire. In many systems, the alternative return path is provided by a steel sheath on the wires, called a conduit.

Because the two supply wires are different (black brings the current, and white drains it away), modern electric plugs are polarized.This means that they can fit into the socket in only one way, because one lug on the plug is bigger than the other.

Switches and Rheostats

Switches turn electric circuits on and off. One single-pole switch controls a circuit. Two three-wayswitches can work together to control a circuit from either of two locations, such as at the top and bottom of a stairway. A rheostat continuously controls the voltage, and is used to dim lights. In home wiring, switches always control the hot side of the circuit—the one that supplies the current.

Electrical Connections

Electrical connections must be secure and tight. Otherwise, electric arcs will develop, causing heat and ruining the connection. One of the best connections comes from solder, a metal that is melted over a connection. Solder joints must have flux, a chemical that prevents oxidation from happening when the wires get heated. Solder can be used to join wires to other wires or wires to terminals.

Circuit Breakers and Fuses

Because electric circuits get hot when something goes wrong, circuit breakers and fuses are used in most circuits. Both have the same job: breaking the circuit (cutting off the power) when something goes wrong. A circuit breaker usually has an electromagnet that opens a circuit when the current gets too high. A fuse has an element that melts when the current is too high. Fuses are easier to make, but circuit breakers can just be reset after they trip, whereas a fuse must be replaced. If a fuse continually blows, do not replace it with a larger-capacity fuse; this invites fire. Find the problem and fix it.

Coaxial Cables

Coaxial means "having the same axis." Coaxial cables, usually used in cable TV systems, have an inner and an outer conductor.

Practice problems for this study guide can be found at:

Electronics Information Practice Problems for McGraw-Hill's ASVAB