Electricity Basics: Current, Voltage and Resistance

Atomic Structure and Electrical Charges

All physical matter is made of basic building blocks called atoms. Inside an atom, you will find positively charged protons and negatively charged electrons, while most atoms also contain uncharged neutrons. The protons and uncharged neutrons stay tightly packed inside the central atomic nucleus. Electrons do not sit in the nucleus; instead, they reside in shells outside the atomic nucleus. Electrons in an atom's outermost shell are called valence electrons, and they play a central role in electrical behavior; electrons do not follow fixed orbits around the nucleus.

Electrical behavior depends on how these subatomic charges interact. A fundamental rule of electrical physics is that like electrical charges repel each other, while opposite electrical charges attract each other. This means two negative electrons will push away from one another, whereas a positive proton and a negative electron will pull toward each other. Electric charge is measured in units called coulombs. Approximately 6.24 quintillion electrons carry a total charge of −1 coulomb, whose magnitude is 1 coulomb.

Materials behave differently depending on how firmly their atoms hold onto their valence electrons. Conductors are materials that allow electric charge to flow easily because their valence electrons can break free and move; in metal conductors, mobile electrons carry the current. Insulators, by contrast, are materials that offer very high electrical resistance because they tightly bind their valence electrons, stopping charge from moving freely. Sitting between these two groups are semiconductors, which are materials with conductivity levels between conductors and insulators.

Electric Current and Circuit Pathways

Electric current is the flow of electric charge; it can be carried by electrons in a metal wire or by ions in a liquid, for example. In electrical formulas, current is represented by the variable letter I. The basic unit of electric current is the ampere, which is commonly called the amp and abbreviated as A. By definition, one ampere represents one coulomb of electrical charge passing a given point in one second.

Electric current moves in two distinct forms: direct current and alternating current. Direct current, abbreviated as DC, is an electric current that flows in only one direction. Familiar power sources such as chemical batteries and solar cells produce direct current. Alternating current, abbreviated as AC, is an electric current that periodically reverses its direction of flow. Standard residential wall outlets supply alternating current. In the external metal wires of a DC circuit, conventional current is shown flowing from the positive terminal toward the negative terminal, while electrons move in the opposite direction.

To allow current to flow, an electric circuit must form an unbroken, continuous closed loop. If there is a break anywhere along that path, it becomes an open circuit, and an open circuit has a break that stops current from flowing through that circuit path. Conversely, a short circuit provides an unintended low-resistance path that bypasses a load. Because this unintended path lacks normal resistance, a short circuit can cause excessive current.

Voltage and Sources of Electromotive Force

Current cannot move through a circuit on its own; it requires a driving influence. Voltage is the electric potential difference between two points, and it is often described as the electrical 'pressure' that drives current through a circuit. A related concept is electromotive force (EMF), which is the energy a source supplies per unit of charge. Electromotive force is measured in volts and is often represented by the letter E in formulas.

The basic unit of voltage is the volt, abbreviated as V. In terms of energy, one volt equals one joule of electric potential energy per coulomb of charge (1 V = 1 J/C). A crucial characteristic of voltage is that it always exists between two distinct points. Because voltage represents a difference in electrical potential rather than a physical movement of charge, voltage can be present even when no current is flowing, such as across the terminals of a disconnected battery.

Devices generate this electrical potential difference in different ways. A battery produces voltage through chemical reactions involving electrodes and an electrolyte. A generator produces voltage by changing the magnetic field through a coil, such as by rotating a magnet or a coil. Both devices create the necessary electrical pressure to drive current once a closed circuit is established.

Resistance and Conductor Characteristics

As charge moves through a circuit, it encounters opposition. Resistance is the opposition a material offers to the flow of electric current. In electrical formulas, resistance is represented by the variable letter R. The basic unit of electrical resistance is the ohm, which is symbolized by the Greek uppercase letter omega. By definition, one ohm of resistance allows one ampere of current to flow when one volt of potential is applied across it.

In practical circuits, components are often added specifically to control current levels. A resistor is a passive electrical component specifically manufactured to provide a set amount of resistance. In addition to dedicated resistors, the connecting wires themselves introduce resistance based on four physical factors: material, length, cross-sectional area, and temperature.

The physical dimensions of a wire directly determine how much it opposes current. A wire's resistance increases in direct proportion to its length when its material, cross-sectional area, and temperature stay the same; doubling the length doubles the resistance. In contrast, the resistance of a conductor decreases as its cross-sectional area or thickness increases. A thicker wire has less resistance than a thinner wire of the same material and length, and it can generally carry more current safely when other conditions are the same. Finally, temperature plays an important role: for most metal conductors, electrical resistance increases as the temperature of the conductor rises.

Ohm's Law, Metric Prefixes, and Circuit Meters

The foundational principle linking these concepts is Ohm's Law. Ohm's Law defines the relationship between voltage, current, and resistance using the formula voltage equals current multiplied by resistance. Under Ohm's Law, current is directly proportional to voltage when resistance remains constant; increasing the voltage causes current to increase. Conversely, under Ohm's Law, current is inversely proportional to resistance when voltage remains constant; increasing the resistance causes current to decrease.

Because calculators are not permitted on the ASVAB, practice Ohm's law using mental math or scratch-paper arithmetic. For example, if a circuit has a 12-volt battery connected across a 4-ohm resistor, you can find the current by rearranging the formula: since voltage equals current multiplied by resistance, current equals voltage divided by resistance. Dividing 12 volts by 4 ohms gives 3 amperes of current. If another circuit has 2 amperes flowing through a 5-ohm resistor, multiplying 2 amperes by 5 ohms reveals a voltage of 10 volts.

Electronics calculations also rely on standard metric prefixes to represent very large or very small quantities. The prefix micro represents one-millionth of a base unit and is symbolized by the Greek letter mu. The prefix milli represents one-thousandth of a base unit and is symbolized by a lowercase m; for instance, one milliampere equals one-thousandth of an ampere, or 0.001 A. For larger values, the prefix kilo represents one thousand base units and is symbolized by a lowercase k; for example, one kilovolt equals one thousand volts, or 1,000 V. The prefix mega represents one million base units and is symbolized by an uppercase M; thus, one megohm equals one million ohms, or 1,000,000 ohms.

Technicians use specialized meters to measure these quantities, and connecting them properly is critical. An ammeter measures current in a circuit path and is connected in series with the component whose current is being measured. A voltmeter measures the voltage between two points and is connected in parallel across the component being measured. An ohmmeter measures resistance in a de-energized circuit or isolated component; testing a live circuit can give an incorrect reading and damage the meter. Finally, a multimeter is a single instrument capable of measuring voltage, current, and resistance.

AR, MK, WK, and PC make up the AFQT; Electronics Information is separate from the AFQT but may contribute to branch-defined composite scores used for some occupational qualifications, while student ASVAB results may also be used for career exploration. Minimum score requirements and job qualifications are set by each military branch and can change, so mastering these testing fundamentals is an essential step in your preparation.