Circuit & Ohm’s Law Lab
Build a circuit, slide the voltage and resistance, and watch Ohm’s law V = I·R light it up — with series vs parallel side by side.
This is an exact DC circuit solver — no faked numbers. Pick a preset or add resistors, switch between series and parallel, and tune the battery voltage and each resistance. Watch the current I = V/R flow as charge carriers, the bulb brighten with its power, and the table show how series circuits share one current while parallel branches share one voltage.
Presets
Per-resistor breakdown
| R | Voltage drop | Current | Power |
|---|
Selected resistor
Tap a resistor on the canvas, a chip, or a row in the table to read its voltage drop, current and power — and adjust its resistance live.
Formulas & sources
V = I·R
P = V·I = I²R = V²/R
Series: R = Σ Rᵢ
Parallel: 1/R = Σ 1/Rᵢ
The units and constants behind this circuit
Ohm's law connects three quantities, and one of them — the ampere — is defined by a constant you can count: the charge on a single electron.
The constant that defines the ampere
- e = 1.602176634 × 10⁻¹⁹ C Elementary charge · exact since 2019 — the ampere is defined by fixing this number
What one ampere actually is: electrons per second
| Current draw | Current | Electrons per second |
|---|---|---|
| An indicator LED | 0.02 A | 1.25 × 10^17 / s |
| Phone charger | 2 A | 1.25 × 10^19 / s |
| Microwave oven | 10 A | 6.24 × 10^19 / s |
Calculated live as I / e, using the CODATA value of the elementary charge from our constants page. One ampere is 1 coulomb per second, and one coulomb is 1/e ≈ 6.24 × 10¹⁸ electrons.
The ampere used to be defined by two wires
Until 2019 the ampere was defined by the force between two infinitely long parallel wires one metre apart — a definition nobody could actually build. Since May 2019 it is defined by fixing the elementary charge at exactly 1.602176634 × 10⁻¹⁹ coulombs. The current in this simulator is, quite literally, a count of electrons.
See all seven defining constants →