Basic Electrical Quantities and Units
This domain opens the Tamil Nadu HSE First Year syllabus. The exam tests whether you can name each quantity, give its SI unit and symbol, convert using decimal prefixes, and show how derived units are built from the ampere, metre, second and kilogram.
Core quantities and their units
- Charge (Q) - coulomb (C); one coulomb is the charge on about 6.24 × 10¹⁸ electrons.
- Current (I) - ampere (A); I = Q/t, so 1 A = 1 C/s.
- Potential difference (V) - volt (V); V = W/Q, one volt being one joule per coulomb. EMF is the voltage generated inside a source; p.d. is the drop across a component.
- Resistance (R) - ohm (Ω); conductance G = 1/R is measured in siemens (S).
- Power (P) - watt (W), one joule per second.
- Energy (W) - joule (J); the commercial unit is the kilowatt-hour, 1 kWh = 3.6 × 10⁶ J.
Magnetic and electrostatic quantities follow the same logic: flux Φ in webers, flux density B in tesla (Wb/m²), inductance in henry, capacitance in farad, frequency in hertz.
Prefixes you must convert fluently
kilo 10³, mega 10⁶, giga 10⁹; milli 10⁻³, micro 10⁻⁶, nano 10⁻⁹, pico 10⁻¹².
Worked example
A conductor carries 2.5 A for 4 minutes. Find the charge passed. Convert time first: t = 4 × 60 = 240 s. Then Q = I t = 2.5 × 240 = 600 C.
Traps that cost marks
Writing kWh as a unit of power; confusing EMF with terminal p.d.; forgetting the factor of 60 when time is given in minutes; calling the volt a base unit when it is derived from the joule and the coulomb; and omitting the unit symbol in the final answer line.