Electricity
Core concepts
Electrostatics (Coulomb's law, electric field, potential, capacitance), direct current circuits (Ohm's law, Kirchhoff's laws, series and parallel combinations, internal resistance, power), and basic circuit analysis. You should be comfortable computing equivalent resistance, using potential dividers, and understanding energy stored in a capacitor (½CV²).
Traps
- Applying Ohm's law to non-ohmic devices such as filament lamps or diodes.
- Forgetting internal resistance when computing terminal voltage.
- Mixing up series and parallel rules for resistors versus capacitors (they are reversed).
- Confusing electric field (vector, N/C) with potential (scalar, V).
- Ignoring the direction of conventional current versus electron flow.
Rule of thumb
Resistors in parallel: 1/R = 1/R₁ + 1/R₂. Capacitors in parallel add directly; in series, 1/C = 1/C₁ + 1/C₂.
Worked example
A 12 V battery with 1 Ω internal resistance drives a 5 Ω external resistor. Current I = 12/(5 + 1) = 2 A. Terminal voltage = 12 − I×r = 12 − 2 = 10 V. Power dissipated in the external resistor = I²R = 4×5 = 20 W.