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Roue de la loi d’Ohm et de la puissance

Solve for the one you want from any two you know
To findFrom V and IFrom I and RFrom V and R
Voltage V—I × R—
Current I——V ÷ R
Resistance RV ÷ I——
Power PV × II² × RV² ÷ R

The two you actually need to remember: V = I × R and P = V × I. Everything above is algebra on those. Voltage in volts, current in amps, resistance in ohms, power in watts.

Worked example: 120 V, 48 Ω → I = 120 ÷ 48 = 2.5 A, P = 120 × 2.5 = 300 W.

Which formula, from what you have — three more, worked
You have → you wantFormulaWorked (practice numbers)
P and V → II = P ÷ V1,500 W on 120 V → 1,500 ÷ 120 = 12.5 A
P and I → VV = P ÷ I3,000 W drawing 12.5 A → 3,000 ÷ 12.5 = 240 V
V and R → PP = V² ÷ R240 V across 24 Ω → 57,600 ÷ 24 = 2,400 W
I and R → V, then PV = I × R;
P = V × I		10 A through 12 Ω → V = 120 V; P = 120 × 10 = 1,200 W

The two slips: squaring the wrong thing (P = I² × R squares the current, not the resistance — 8 A through 15 Ω is 64 × 15, not 8 × 225), and forgetting the second multiply (8 × 15 = 120 is the volts; you still have to multiply by 8 again to get the watts). Cover the letter you want on the wheel and what is left is the formula.

Every number on this sheet is a practice number. Real loads have a nameplate; real conductors have a code book. This sheet is the arithmetic between them.

Drill these until the rearranging is automatic on the Ohm's law practice page.

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