Circuit Builder

What you're looking at

A breadboard of connection points. Drop a battery to push current, join things with wires, and add resistors and bulbs to control and use the flow. An ammeter measures the current passing through it; a voltmeter measures the voltage across whatever it sits beside. The moving dots show conventional current, brighter and faster where more current flows.

Ohm's law

The current through a resistor is set by the voltage across it and its resistance:

Raise a resistor's value and the current drops; raise the battery's EMF and it climbs. The whole board is solved with Kirchhoff's laws — current is conserved at every junction, and voltages add up to zero around every loop.

Series and parallel

• Series (one path): the same current flows through everything, and resistances add up, R = R₁ + R₂. More resistors → less current.
• Parallel (branches): each branch sees the full battery voltage, and the total resistance is less than either branch, 1/R = 1/R₁ + 1/R₂. More branches → more total current.

Meters, done right

An ideal ammeter has almost no resistance, so it's wired in series — break the circuit and insert it. An ideal voltmeter has almost infinite resistance and is wired in parallel, tapping the two points without drawing current. Try the quick-circuit buttons to see correct readings, then tinker.

Things to try

Load Ohm's law, then drag the resistance slider's value (re-drop the resistor) and watch the ammeter follow V/R. Build a parallel pair and confirm the total current is the sum of the branches. Add a switch and click it to break the circuit.