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Ohm's Law in Practice: V, I, R Calculation Examples
4 min read · Last updated: 2026-05-08
What is Ohm's Law?
Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. It is the fundamental starting point for all electronics design.
The core formula
V = I × R
- V: voltage (Volts)
- I: current (Amperes)
- R: resistance (Ohms, Ω)
Rearranged:
I = V / R(find current)R = V / I(find resistance)
Power formula
P = V × I (power, in Watts)
Substituting Ohm's Law:
P = I² × RP = V² / R
Worked examples
| Scenario | Calculation | Result |
|---|---|---|
| 5V supply, 100Ω resistor | I = 5 / 100 | 50 mA |
| 12V supply, 200 mA current | R = 12 / 0.2 | 60 Ω |
| 3.3V, 10 mA current | P = 3.3 × 0.01 | 33 mW |
| 220V, 100Ω load | I = 220/100, P = 220²/100 | 2.2 A, 484 W |
| 9V battery, 470Ω | I = 9/470 ≈ 19.1 mA, P = 9 × 0.0191 | ≈172 mW |
Series and parallel resistors
Series: R_total = R1 + R2 + R3 + …
Parallel: 1/R_total = 1/R1 + 1/R2 + 1/R3 + …
Two resistors in parallel: R_total = (R1 × R2) / (R1 + R2)
Example: 10Ω ∥ 10Ω → R_total = (10×10)/(10+10) = 5Ω
Practical tips
- Watch unit conversions: 20 mA = 0.02 A
- Always check a component's power rating. Calculate P = V × I and ensure it is not exceeded.
- Higher voltage through the same resistance produces proportionally more current and significantly more power.
Key takeaways
- V = I × R (Ohm's Law)
- P = V × I = I²R = V²/R (power)
- Series: add resistances; Parallel: add reciprocals