Electrical Terminology
Electrical terminology questions test unit fluency and correct use of symbols and formulas. You must handle SI prefixes, distinguish resistance from resistivity and conductance, and apply Ohm's law without mixing quantity names.
Core rules
- Base electrical units: volt (V), ampere (A), ohm (Ω), coulomb (C), watt (W), joule (J), second (s).
- Prefixes scale by powers of ten: k = 10^3, M = 10^6, m = 10^-3, µ = 10^-6.
- Ohm's law: V = IR, where V is potential difference across a component, I is current through it, and R is resistance.
- Resistance depends on material and geometry: R = ρL/A, with ρ the resistivity, L length, A cross-sectional area.
- Conductance G = 1/R, measured in siemens (S).
- Temperature coefficient describes how resistance changes with temperature; metals usually rise, thermistors may fall.
Worked example
A 4.7 kΩ resistor has 12 V across it. Find the current. Convert: 4.7 kΩ = 4,700 Ω. I = V/R = 12 / 4,700 = 0.00255 A = 2.55 mA. Reasoning: Ohm's law needs consistent units, so convert kilohms to ohms before dividing.
Traps
- Confusing mΩ (milliohm) with MΩ (megohm).
- Treating resistivity as resistance; resistivity is a material property, resistance also depends on length and area.
- Using wire diameter instead of cross-sectional area in R = ρL/A.
- Forgetting that conductance is the reciprocal of resistance, not a synonym for resistance.