Fundamentals

Electrical Theory & Ohm's Law

Voltage, current, resistance, power, and the formulas the exam builds everything else on.

~8 min read·3 sections·4 key terms

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The Four Quantities

• VOLTAGE (E or V) — electrical pressure, measured in volts. • CURRENT (I) — flow of electrons, measured in amperes (amps). • RESISTANCE (R) — opposition to current, measured in ohms (Ω). • POWER (P) — rate of doing work, measured in watts.

Think of a water analogy: voltage is pressure, current is flow rate, resistance is the pipe's restriction.

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Ohm's Law & the Power Wheel

Ohm's Law: E = I × R (Voltage = Current × Resistance). Rearranged: I = E / R and R = E / I.

Power formula: P = I × E (Watts = Amps × Volts). Combined forms you'll use: P = I² × R and P = E² / R.

Memorize the pie/wheel: cover the quantity you want, and the remaining two show the formula.

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Series vs. Parallel Circuits

SERIES — one path. Current is the SAME everywhere; voltage divides; resistances ADD (R_total = R1 + R2 + ...). Break one point and the whole circuit stops.

PARALLEL — multiple paths. Voltage is the SAME across each branch; current divides; total resistance is LESS than the smallest branch. Most building wiring (receptacles, lights) is parallel so one device can fail without killing the rest.

📖 Key Terms

Voltage (E): Electrical pressure that pushes current, measured in volts.
Ampere: The unit of electrical current (flow of charge).
Ohm (Ω): The unit of resistance to current flow.
Watt: The unit of electrical power: volts × amps.

💡 Exam Tips

▸E = I × R is the foundation — know all three rearrangements.
▸Power: P = I × E. Also P = I²R and P = E²/R.
▸Series: current is the same, resistances add. Parallel: voltage is the same, total R drops.
▸Building branch circuits are wired in parallel.