Electricity: the exam basics

The technical bedrock. Three quantities: voltage V (volts), current I (amperes), resistance R (ohms).

Ohm's law: V = R · I. Memorise all three forms (I = V/R, R = V/I).

Power: P = V · I (watts), which rewrites as P = R·I² or P = V²/R. Power connects to decibels whenever you compare two levels.

Combining resistors:

• In series, resistances add: R = R₁ + R₂ + …
• In parallel, the inverses add: 1/R = 1/R₁ + 1/R₂ + … (the result is always smaller than the smallest).

DC / AC: under direct current (DC) the voltage is constant; under alternating current (AC) it oscillates (radio is just very-high-frequency AC). For a sine wave, distinguish the peak value from the effective (RMS) value: V_rms = V_peak / √2.

Units and prefixes to handle without hesitation: pico (p, 10⁻¹²), nano (n, 10⁻⁹), micro (µ, 10⁻⁶), milli (m, 10⁻³), kilo (k, 10³), mega (M, 10⁶), giga (G, 10⁹).

Next: the Composants électroniques that put these laws to work.