Projectile Motion: Formulas, Worked Example and Calculator
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Ohm’s Law: Calculator and Examples
Ohm’s law explained with a free calculator: V = IR, how to find voltage, current or resistance, power in circuits, worked examples and common mistakes.
Key takeaways
- Ohm’s law: voltage = current × resistance, V = IR.
- Rearrange to find current (I = V/R) or resistance (R = V/I).
- Electrical power is P = VI, or P = I²R.
On this page
Ohm’s law is the most important rule in basic electronics. It links three quantities in a circuit: voltage, current and resistance, and it’s one of the key equations on our physics formula sheet.
The formula
V = I × R
- V = voltage, in volts (V)
- I = current, in amperes (A)
- R = resistance, in ohms (Ω)
Rearranged:
- I = V ÷ R
- R = V ÷ I
Ohm’s law calculator
Enter any two values; the third is calculated.
Ohm’s law calculator
Enter any two values.
Worked examples
- Find current: a 9 V battery across a 450 Ω resistor: I = 9 ÷ 450 = 0.02 A (20 mA).
- Find resistance: 12 V drives 0.5 A through a lamp: R = 12 ÷ 0.5 = 24 Ω.
- Find voltage: 0.1 A through 100 Ω: V = 0.1 × 100 = 10 V.
Power
Electrical power, in watts: P = V × I, or P = I² × R, or P = V² ÷ R. See work, energy and power for the mechanical version.
When Ohm’s law applies
It works well for ohmic conductors like resistors at steady temperature. Components such as LEDs and diodes don’t follow it simply.
Common mistakes
- Mixing milliamps and amps: 20 mA = 0.02 A.
- Using kilohms without converting: 4.7 kΩ = 4,700 Ω.
Frequently asked questions
What does Ohm’s law state?
The current through a conductor is proportional to the voltage across it, for a constant resistance.
What is the unit of resistance?
The ohm (Ω).
Does Ohm’s law work for LEDs?
Not directly. LEDs are non-linear; use a series resistor calculated with the LED’s forward voltage.
Sources
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