T5C: Capacitance, Inductance, and Power

Capacitors and inductors are two of the most fundamental components in radio circuits. They store energy — capacitors in electric fields, inductors in magnetic fields — and they interact with AC and RF signals in ways that resistors do not. Understanding their basic properties and units, along with the concept of impedance and the DC power formula, completes the core electrical vocabulary you need for the Technician exam.

T5C covers the definitions and units of capacitance and inductance, the definition and unit of impedance, the meaning of the abbreviation RF, the correct abbreviations for megahertz and kilohertz, the power formula for DC circuits (P = I × E), and three power calculation examples using that formula.

Capacitance: Energy in Electric Fields

Capacitance describes the ability to store energy in an electric field. A capacitor consists of two conductive surfaces (plates) separated by an insulating material (the dielectric). When voltage is applied, opposite charges build up on the two plates, creating an electric field between them. This stored charge — and the energy associated with it — can be released later when the circuit demands it.

The unit of capacitance is the farad (F), named after Michael Faraday. One farad is an enormous amount of capacitance; practical capacitors are typically measured in microfarads (µF), nanofarads (nF), or picofarads (pF). Capacitance is not measured in ohms, volts, or henrys — those units belong to resistance/impedance, voltage, and inductance respectively.

The ability to store energy in an electric field is capacitance — not inductance (magnetic field), not resistance (which dissipates energy rather than storing it), and not tolerance (which describes a component's accuracy specification).

Inductance: Energy in Magnetic Fields

Inductance describes the ability to store energy in a magnetic field. When current flows through a coil of wire, it creates a magnetic field around and through the coil. The energy stored in this magnetic field can sustain the current flow even briefly after the driving voltage is removed — a property exploited in power supplies, filters, and RF circuits throughout amateur radio equipment.

The unit of inductance is the henry (H), named after Joseph Henry. Practical inductors are measured in millihenrys (mH) or microhenrys (µH). Inductance is not measured in coulombs (charge), farads (capacitance), or ohms (resistance or impedance).

The ability to store energy in a magnetic field is inductance — not admittance, not capacitance (electric field), and not resistance.

Impedance: Opposition to AC Current

Impedance is the total opposition to AC current flow in a circuit. It combines resistance (which opposes all current regardless of frequency) with reactance (the frequency-dependent opposition provided by capacitors and inductors). Like resistance, impedance is measured in ohms.

Impedance is specifically the opposition to AC current flow — not the inverse of resistance (that is conductance), not the quality factor Q of a component, and not a component's power handling capability. When you read the impedance of a coaxial cable (commonly 50 ohms), you are reading a measure of how that transmission line impedes the flow of RF current.

The fact that impedance uses the same unit (ohm) as resistance can be confusing, but the distinction matters: resistance applies equally at all frequencies, while impedance changes with frequency as the reactive components (capacitance and inductance) have frequency-dependent effects.

RF Terminology and Abbreviations

The abbreviation RF stands for radio frequency signals of all types. It is a broad term covering any signal in the radio frequency spectrum — it does not specifically mean the resonant frequency of a tuned circuit, the real frequency of a transmitted signal, or reflective force in transmission lines. RF simply means radio frequency.

Two frequency unit abbreviations appear in the exam with specific capitalization requirements:

• MHz — megahertz. Capital M, capital H, lowercase z. Not MH, mh, or Mhz.
• kHz — kilohertz. Lowercase k, capital H, lowercase z. Not KHZ, khz, or khZ.

The capitalization matters because in the SI unit system, the case of prefix letters is significant — lowercase k is kilo (×1000), while uppercase K is Kelvin. Getting the capitalization wrong indicates a misunderstanding of the system, and the exam specifically tests this.

The DC Power Formula

The formula for calculating electrical power in a DC circuit is:

The correct formula is P = I × E. Not P = E / I (that would give a result in ohms, not watts), not P = E − I (subtraction makes no dimensional sense for power), and not P = I + E (addition also has no physical meaning for power). Power in watts equals current in amperes multiplied by voltage in volts.

This formula is also the basis for rearranged versions: if you need to find current when you know power and voltage, rearrange to I = P / E. If you need voltage, rearrange to E = P / I.

Power Calculation Examples

The T5C exam includes three straightforward power calculations. Each one uses P = I × E or its rearranged form I = P / E.

These calculations are direct substitutions into the formula. The 13.8 volt example is particularly worth recognizing — 13.8 V is the standard operating voltage for mobile amateur transceivers, and 10 A at 13.8 V is 138 W, a power level typical of high-duty-cycle operation. For the current calculation, recognize that dividing power by voltage gives current, not resistance — that would be R = E / I, a different formula.

T5C Practice Questions