T5A: Current and Voltage

Every piece of radio equipment runs on electricity, and understanding the basic quantities of electricity — what they are, how they are measured, and how they relate to each other — is the starting point for making sense of everything from power supplies to antenna feed lines. T5A covers the fundamental vocabulary that the rest of the electrical principles subelement builds on.

This group teaches the definitions and units for electrical current, voltage, resistance, power, and frequency. It also covers why metals conduct electricity well, what makes a material a good insulator, the difference between alternating and direct current, and the key fact that resistance opposes all types of current flow.

Current: The Flow of Electrons

In any electrical circuit, electrons move through conductors from areas of higher electrical potential to lower potential. This movement of electrons is called current. Current is measured in amperes (often shortened to amps). One ampere represents approximately 6.24 × 10¹⁸ electrons flowing past a point per second — an enormous number, but the unit itself is straightforward: amperes measure how much charge is moving per unit of time.

The term for the flow of electrons in a circuit is "current." Voltage, resistance, and capacitance are related but distinct quantities. Current is the name for the flow itself.

Voltage: The Force Behind Current

Electrons do not move on their own — something must push them. Voltage is the electrical term for the force that causes electron flow. It represents an electrical potential difference between two points: a higher-potential point pushes electrons toward a lower-potential point, just as a hill causes water to flow downhill. Voltage is measured in volts.

Voltage is not the flow itself (that is current), not the opposition to flow (that is resistance), and not a measure of storage capacity (that would be capacitance). It is the driving force — sometimes called electromotive force (EMF) — that makes current flow possible in a circuit.

Resistance and Its Units

Every material offers some opposition to the flow of electrons. This opposition is called resistance, and its unit is the ohm (symbol: Ω). A high resistance limits current flow; a low resistance allows more current for the same applied voltage. Resistors are components specifically designed to provide a known amount of resistance in a circuit.

The unit of resistance is the ohm — not siemens (which measures conductance, the inverse of resistance), not mhos (an older term also for conductance), and not coulombs (which measure electric charge).

Power: The Rate of Energy Use

Power describes how quickly electrical energy is converted to another form — heat, light, radio waves, mechanical motion. The term for the rate at which electrical energy is used is power, and its unit is the watt. A 100-watt light bulb uses energy at a rate of 100 joules per second. A 50-watt transmitter converts 50 joules of electrical energy per second into radio frequency energy (plus heat losses).

Power is measured in watts — not volts (which measure voltage), not amperes (which measure current), and not watt-hours (which measure energy over time, not rate of use). The distinction between power (watts) and energy (watt-hours) matters: a battery stores energy in watt-hours, but a transmitter consumes power in watts.

Frequency and Its Unit

Frequency describes how rapidly something repeats. In electronics, it most often refers to how many complete cycles per second an alternating current or a radio wave completes. The unit of frequency is the hertz (Hz). One hertz means one complete cycle per second. One kilohertz (kHz) is 1,000 cycles per second; one megahertz (MHz) is 1,000,000 cycles per second.

Frequency is not measured in henrys (which measure inductance), not farads (which measure capacitance), and not teslas (which measure magnetic field strength). The hertz is the unit of frequency, named after Heinrich Hertz who first demonstrated radio wave propagation.

The number of times per second that an alternating current makes a complete cycle is its frequency — not its pulse rate, speed, or wavelength.

Conductors: Why Metals Work

A conductor is a material through which electrical current flows easily. Metals are generally good conductors because they have many free electrons — electrons that are not tightly bound to individual atoms and can move freely through the material in response to an applied voltage. Copper, aluminum, and silver are excellent conductors because their atomic structures leave large numbers of electrons free to carry current.

Metals do not conduct because of high density (many dense materials are poor conductors) and not because of free protons (protons do not move through metal lattices). The key property is the abundance of free electrons available to carry charge.

Insulators

An insulator is a material that strongly resists the flow of electrical current. Insulators have very few free electrons — their electrons are tightly bound to atoms and are not available to carry current. Good insulators are used to support conductors without allowing current to leak away, to separate conductors that must not touch, and to prevent electric shock hazards.

Glass is a good electrical insulator. Copper and aluminum are excellent conductors, not insulators. Mercury is a liquid metal and also a conductor. Glass, rubber, ceramic, and plastic are the common insulating materials used throughout electronics and in antenna construction.

Alternating Current vs. Direct Current

Direct current (DC) flows in one direction continuously. A battery produces DC — the positive terminal is always positive and negative always negative, so current always flows the same way through a connected circuit. Transceivers and most radio equipment run on DC.

Alternating current (AC) reverses direction periodically. The precise description is that AC alternates between positive and negative directions — the current flows one way, then reverses and flows the other way, repeatedly. This is not the same as alternating between positive and zero (that would be pulsed DC), and not between negative and zero. The household power grid uses AC at 60 Hz in North America, meaning the current reverses direction 120 times per second (completing 60 full cycles).

Resistance Opposes All Current Types

Resistance is not selective — it opposes the flow of direct current, alternating current, and RF current equally. All types of current flow are opposed by resistance. This is worth stating explicitly because students sometimes assume that resistance only applies to DC circuits. In fact, resistors reduce current regardless of whether the current is a steady DC, a 60 Hz AC signal, or a 146 MHz radio frequency current. Other properties like inductance and capacitance also affect AC and RF differently than DC, but resistance affects all of them.

T5A Practice Questions