Basic Electrical Principles and Units
What the domain tests
Basic Electrical Principles and Units tests the relationships used in every later topic: charge, current, voltage, resistance, power, energy and AC power factor. You must use correct units and distinguish power (kW) from energy (kWh).
Core rules
- Current I is the rate of charge flow, in amperes; voltage V is potential difference, in volts; resistance R is opposition, in ohms.
- Ohm’s law: V = IR. Power in DC or resistive AC: P = VI = I²R = V²/R.
- Energy = power × time; 1 kWh is one “unit” of electricity.
- In series, resistances add and current is common. In parallel, voltage is common and 1/R = 1/R₁ + 1/R₂.
- For AC, P = VI cos φ, where cos φ is power factor; kVA is apparent power, kW is real power.
Worked example
A 230 V heater draws 10 A. Its power is P = 230 × 10 = 2,300 W = 2.3 kW. Run for 8 hours, energy = 2.3 × 8 = 18.4 kWh. At ₹6 per kWh, cost = 18.4 × 6 = ₹110.40. If an AC motor instead takes 10 A at 230 V with power factor 0.8, real power is 230 × 10 × 0.8 = 1,840 W, not 2,300 W.
Traps
Do not add parallel resistors as if they were series. Do not write kW for a bill quantity-billing uses kWh. Do not ignore power factor in motor or inductive-load calculations. Do not mix milliamperes and amperes in the same formula.