Fundamentals of Electricity and DC Circuits
What this domain tests
This domain checks whether you can move confidently between charge, current, voltage, resistance and power, and solve series–parallel DC networks using Ohm's law and Kirchhoff's laws. Expect unit conversions and questions that combine circuit analysis with energy cost.
Core rules
- Current is the rate of charge flow, I = Q/t; one ampere is one coulomb per second.
- Ohm's law: V = IR. Resistance depends on material and shape: R = ρl/A, so doubling length doubles resistance and doubling cross-section halves it.
- Series: same current, voltages add, R_total = R1 + R2 + ... Parallel: same voltage, currents add, 1/R_total = 1/R1 + 1/R2 + ...
- Power: P = VI = I²R = V²/R. Energy in kWh = power in kW × time in hours.
- Kirchhoff: current law at a node (sum in = sum out); voltage law around a loop (algebraic sum of EMFs equals sum of drops).
Worked example
A 12 V battery of internal resistance 0.5 Ω feeds a 5.5 Ω load. Total resistance = 6.0 Ω, so I = 12/6.0 = 2 A. Terminal voltage = EMF − internal drop = 12 − (2 × 0.5) = 11 V. Load power = I²R = 4 × 5.5 = 22 W; internal loss = 4 × 0.5 = 2 W. The 24 W generated splits as 22 W useful and 2 W heat, which is why terminal voltage falls as current rises.
Traps that cost marks
Confusing EMF with terminal voltage; adding parallel resistances directly instead of using reciprocals; converting cm² to m² wrongly (1 cm² = 10⁻⁴ m²); treating kWh as a unit of power rather than energy; and forgetting that an open circuit has full voltage but zero current.