This group covers the two ways components store energy, how impedance describes the opposition to alternating current, how unit abbreviations are written, and the formula for power in a DC circuit.
Capacitance and inductance
Capacitance is the ability to store energy in an electric field, as in two conductive plates separated by an insulator. Its unit is the farad (F). In practice, capacitors are rated in microfarads or picofarads.
Inductance is the ability to store energy in a magnetic field, typically in a coil of wire. Its unit is the henry (H).
Keep them apart: capacitance goes with the electric field and the farad; inductance goes with the magnetic field and the henry. The coulomb is a unit of charge, and the ohm is for resistance or impedance.
Impedance
Impedance is the opposition to AC current flow. It combines resistance with reactance (the opposition from capacitors and inductors). Its unit is the ohm, the same as resistance. It is not the inverse of resistance or of reactance, and it is not a power rating.
Writing frequency units correctly
Capitalization matters because a lowercase m means milli and an uppercase M means mega.
- Kilohertz: kHz
- Megahertz: MHz
- Hertz: Hz
- Gigahertz: GHz
Spellings like khz, KHz, KHZ, mHz, mHZ and Mhz are all wrong for kilohertz or megahertz (mHz would actually mean millihertz).
Calculating power
In a DC circuit, power equals voltage times current: P = E x I (the same as P = I x E). Rearranged:
- Current: I = P / E
- Voltage: E = P / I
Worked examples:
- 13.8 V x 10 A = 138 W
- 12 V x 2.5 A = 30 W
- Current needed for 120 W at 12 V: 120 / 12 = 10 A
Choices that divide voltage by current give resistance rather than power. Multiplying I squared by E is also wrong: squared current goes with resistance (P = I squared x R), not with voltage.
Key facts to memorize
- Capacitance: electric field, farad (F)
- Inductance: magnetic field, henry (H)
- Impedance: opposition to AC current flow, in ohms
- Abbreviations: kHz, MHz (note capital M)
- P = E x I; I = P / E
- 13.8 V x 10 A = 138 W
Memory hooks
- Capacitor: C for electric field plates; Inductor: L for the coil’s magnetic field.
- The “power wheel” idea: watts = volts x amps.