Ohm's law & power wheel
| To find | From V and I | From I and R | From V and R |
|---|---|---|---|
| Voltage V | — | I × R | — |
| Current I | — | — | V ÷ R |
| Resistance R | V ÷ I | — | — |
| Power P | V × I | I² × R | V² ÷ 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.
| You have → you want | Formula | Worked (practice numbers) |
|---|---|---|
| P and V → I | I = P ÷ V | 1,500 W on 120 V → 1,500 ÷ 120 = 12.5 A |
| P and I → V | V = P ÷ I | 3,000 W drawing 12.5 A → 3,000 ÷ 12.5 = 240 V |
| V and R → P | P = V² ÷ R | 240 V across 24 Ω → 57,600 ÷ 24 = 2,400 W |
| I and R → V, then P | V = 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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