Page 2 Electronics Information Study Guide for the ASVAB

Household Electrical Systems

Questions in this area will concern the use of electricity in a home or business environment, including safety concepts.

AC vs. DC

DC represents direct current. AC represents alternating current.

Direct current is current that flows in only one direction. Alternating current, as its name indicates, alternates in the direction it flows.

Batteries produce DC, but rectifiers and commutators can also generate DC from AC. Alternators produce AC. In an alternator, a wire with a current running through it is rotated inside of a magnetic field. The position of the wire with respect to the direction of the magnetic field produces a current that alternates in direction.

Where DC circuits have resistance, AC circuits have impedance.

Grounding

Grounding is a method of neutralizing a charged object by connecting it to a much larger source of charge (most commonly the Earth). The immense size of the Earth enables it to act as a reservoir of charge as well as a reference point with which to compare voltages.

Ground wires provide a low resistance path to the earth and help prevent electric shock in the case of faulty circuits.

Resistors

Resistors are passive circuit components used to reduce current flow and voltage levels in a circuit. They are useful for modifying circuits to produce a desired amount of current.

Resistors come marked with four or five colored bands that signify the amount of resistance (and tolerance) they provide. The last band signifies the tolerance in the resistor, the second to last band represents the multiplier, or what the first two or three bands are multiplied by to find the resistance.

If the resistivity of the resistor’s material is known, the resistance can be found using the following equation:

, where represents the resistivity of the conducting material, L is the length, and A is the cross-sectional area.

Resistors can be used in a circuit in series, in parallel, and in series-parallel. Depending on the type of composition, the equivalent resistance, or resistance for the entire circuit, is calculated in different ways.

Fuse, Circuit Breaker, and Short Circuit

Fuses and circuit breakers are safety features that preserve the integrity of a circuit in the case of a power overload. Unwanted surges of power in a circuit can have detrimental effects. Consequently, features like fuses and circuit breakers are included as safety switches that will disrupt or break the circuit before any large scale damage is done.

A circuit is shorted when the intended pathway of the current in a circuit is “shorter” than it is designed to be. A short circuit is undesirable because of the surge in current running through the circuit and the possible dangers associated with this surge.

Capacitor and Capacitive Reactance

Capacitors, like resistors and inductors, are passive circuit components. They serve to temporarily store electrical energy in a circuit. When connected to a voltage source, a conductor becomes charged. The voltage source can then be disconnected, and the capacitor functions similarly to a battery in that it can discharge and provide energy to a circuit.

Capacitors are made of two conducting plates separated by a distance containing a dielectric (a material that can become polarized). Much like resistance can be calculated for resistors, capacitance can be calculated for capacitors:

, where C is capacitance, Q is charge, and V is voltage.

Capacitance is measured in Farads.

In response to the frequency produced in an AC circuit, capacitors have an impedance that is known as capacitive reactance. This reactance can be calculated using the formula:

, where w is the angular frequency and C is the capacitance.

Semiconductors and Doping

Semiconductors are components that can function as either an insulator or a conductor. Silicon and germanium are the most commonly used semiconducting materials. Unlike conductors, semiconductors increase in conductivity when their temperature increases.

Diodes and transistors are both examples of semiconductors.

Doping is the process of adding impurities (in the form of extra or absent electrons) to a semiconducting material as a means of altering its electrical properties. Through doping, a semiconductor can become more or less conductive.

Diodes

Diodes are electrical components that restrict the movement of current to only one direction and block the motion of current in the opposite direction. They are used in bridge rectifiers to convert AC to DC.

Transistors

Transistors are components used to regulate current and voltage in a circuit. They can amplify current and act as electrical switches.

Electricity and Magnetism

Electricity is the energy produced from the motion of charged particles. Moving charged particles in turn create magnetic fields. Electricity and magnetism are intrinsically linked as the electromagnetic force.

Magnetic Field

Magnetic fields arise from the motion of charged particles. This occurs at the atomic scale with electrons surrounding nuclei and at a macroscopic scale with currents passing through circuits.

Conducting wires wrapped around a core, called electromagnets, create magnetic fields when a current passes through.

Inductor

Inductors, like capacitors and resistors, are passive electrical components. Inductors are made of a core that is usually magnetic with a conductive wire wrapped around it. Due to their composition, inductors oppose currents through the generation of a back electromotive force (EMF).

When a large current reaches an inductor, the inductor creates a proportionally large, opposing EMF that diminishes the current. The capacity with which the inductor does this is known as inductance. In AC circuits, inductors and capacitors form the overall impedance that acts analogously to the resistance in a DC circuit.

Transformer

Transformers are used to convert high voltage to low voltage or low voltage to high voltage. They consist of two inductors in close proximity that differ in the number of wrapped coils.

The difference in the number of coils results in the creation of a current with a different voltage in the nearby inductor by way of the varying magnetic field.

Motor and Generator Basics

A motor is a machine that transmutes electric energy into mechanical energy and a generator is a machine that transmutes mechanical energy to electric energy. These machines, like electricity and magnetism, are intrinsically linked with each other.

Inside of a motor is a looped wire connected to an AC or DC source with a commutator. Surrounding this looped wire is a stationary magnet that provides a uniform magnetic field. As the current passes through the wire, the magnetic field exerts a force and rotates the wire.

The mechanical rotation of a current conducting wire inside of a magnetic field serves as an electrical energy generator.

Other Study Tips

Study Ohm’s Law

Ohm’s Law is an electrical law describing the relationship between voltage, amperage, and resistance. This law is written as such: amperage=voltage/resistance. Having two of the measurements allows students to determine the missing piece, much as they might solve a simple math equation. This equation is used to determine a wide range of electrical measurements and needs and is a pivotal aspect of performing electrical services. When studying Ohm’s Law, study its applications and uses, and practice solving several different uses of the equation.

Study the Everyday Application of Electronic Concepts

Research different conductors and how well they perform. For instance, is metal an effective conductor of electricity? If so, what type of metal is best used to safely and effectively conduct electrical currents? From here, study insulators, how they work and what qualifies as an insulating property. Study different types of currents (direct, alternating, etc.) and circuits. Finally, study the different ways electricity is used, including technology for entertainment (radio, television, etc.) and technology to improve quality of life (general electricity, refrigerators, etc.).

Become Familiar with Electronic Symbols

Electrical shorthand is a learned language, so to speak, and may appear on the ASVAB. To study for this part of the Electronic Information test, study different electrical symbols, such as the symbols for resistors and transformers. This allows you to read various diagrams and equations regarding the electric field, as well as formulate your own equations and notes regarding electric actions and needs. To study appropriately, acquaint yourself with the different symbols and signifiers associated with electrical terminology.