Basic Electrical Principles (current, voltage, resistance, Ohm's law, power and energy)
What the exam tests
This domain tests the meaning of current, voltage and resistance, the units ampere, volt and ohm, Ohm's law, series and parallel circuits, and the difference between power and energy. Expect simple calculations in 230 V, 50 Hz single-phase domestic circuits.
Core rules
- Current is rate of charge flow, I = Q/t. Voltage is energy per unit charge; resistance opposes current.
- Ohm's law: V = IR at constant temperature. Power: P = VI = I²R = V²/R. Energy: E = Pt; domestic energy is billed in kWh.
- Series: same current, resistances add, voltages divide. Parallel: same voltage, currents add, reciprocal resistance.
Worked example
A 2000 W heater runs at 230 V for 2 hours. I = P/V = 2000/230 = 8.70 A. R = V/I = 230/8.70 = 26.4 Ω, or V²/P = 26.45 Ω. Energy = 2 kW × 2 h = 4 kWh. At ₹8 per kWh the cost is ₹32.
Traps
Do not write 4 kW for 2 hours; the unit is kWh. Do not add parallel resistances directly. Do not use 230 V for a component drop in a series circuit. Resistance of a hot element differs from its cold value, so calculated and measured values may differ.