V = IR Formula – Ohm’s Law Practice
V = IR — Ohm’s law for a resistor
Ohm’s law relates voltage, current, and resistance for an ohmic resistor: V = I R. Rearrange to I = V/R or R = V/I. Convert mA and kΩ before substituting.
Guide — how to use this formula
- 1. Convert mA → A (÷ 1000) and kΩ → Ω (× 1000).
- 2. Use V = IR, I = V/R, or R = V/I.
- 3. Series resistors: add R first, then I = V/R_total.
- 4. Power: P = VI or P = I²R.
Derivation
For many materials at fixed temperature, current is proportional to voltage:
- Defining resistance as the constant of proportionality:
V = IR - Electrical power with the same quantities:
P = VI = I^{2}R = \frac{V^{2}}{R}
Never multiply V × R when you need current — that is I = V/R.
Examples
Example: 2 A through 5 Ω
V = 10 V.
Example: mA trap
100 mA = 0.1 A through 50 Ω → V = 5 V.