UNIT 1.3 — FUNDAMENTALS

Ohm's Law and the power wheel

CERT 1 · DC/AC Theory & Safety · ~12 min

Community content — not verified by a licensed electrician. Lessons and questions here are written for study practice and pinned to NEC 2023. Code editions and local amendments vary by jurisdiction. Always confirm against your adopted code book and your licensing board's published exam outline before test day.

Ohm's Law states that current is directly proportional to voltage and inversely proportional to resistance: I = V ÷ R. Rearranged, V = I × R and R = V ÷ I. Cover the unknown in the classic triangle (V on top, I and R below) and the remaining two show you the operation.

Worked example: a 120 V circuit feeding a heater element of 15 Ω draws I = 120 ÷ 15 = 8 A. If the element degrades to 20 Ω, current drops to 6 A — resistance up, current down.

Power joins the family through P = V × I (and by substitution, P = I²R and P = V²/R). That 8 A heater dissipates 120 × 8 = 960 W. Exam questions constantly chain Ohm's Law into power calculations — the twelve rearrangements form the 'power wheel' worth memorizing whole.

Exam focus: two-step problems are the norm ('a 240 V circuit dissipates 5 760 W; what is its resistance?'). Drill both directions in the Formula Lab until rearranging is automatic.

Ohm's Law

I = V / R

Power

P = V × I

Chained

P = I²R = V²/R

Knowledge check

Quiz — 3 questions

1. A 240 V circuit with a 12 Ω load draws:

2. If resistance doubles and voltage stays constant, current:

3. A 240 V circuit dissipates 5 760 W. Its resistance is:

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