Basic Electrical Fundamentals and Safety
Core concepts and units
The paper tests whether you can move confidently between the four basic quantities: current (ampere), voltage (volt), resistance (ohm) and power (watt). Ohm's law, V = I × R, holds for DC and for purely resistive AC circuits. Power is P = V × I = I²R = V²/R, and energy is power × time in kWh; one unit is 3.6 × 10⁶ joule. In AC, meters read RMS values; for a sine wave the form factor is 1.11 and the peak factor 1.414, and the supply frequency in India is 50 Hz.
Circuits
Series: same current, voltage divides, resistances add. Parallel: same voltage, current divides, 1/R = 1/R₁ + 1/R₂; two resistors simplify to product/sum. Copper has resistivity about 1.72 × 10⁻⁸ Ω·m; PVC insulation is many orders higher, which is what makes a conductor a conductor.
Worked example
A 230 V supply feeds a 2 kW water heater. Current I = P/V = 2000/230 = 8.70 A, so a 10 A MCB or fuse is correct - 6 A would nuisance-trip. Element resistance R = V²/P = 52900/2000 = 26.45 Ω. Energy in 3 hours = 2 × 3 = 6 kWh.
Safety fundamentals
About 30 mA through the chest can cause respiratory arrest and around 50 mA ventricular fibrillation. Body resistance collapses to a few hundred ohms when wet, which is why 230 V is lethal at a wet sink. Apply the five rules before work: isolate, lock and tag, test for dead, earth or discharge, and cover adjacent live parts.
Traps that cost marks
Mixing RMS and peak values; adding parallel resistances as if in series; forgetting the √3 in three-phase power P = √3 V_L I_L cos φ; and sizing a protective device from habit instead of from computed current.