Voltage, current and resistance are the three numbers of every circuit. One simple rule ties them together.
Key idea
Ohm's law: $V = IR$. Voltage $V$ (volts) is the push; current $I$ (amps) is the flow; resistance $R$ (ohms, Ξ©) is how hard the part resists. Know any two and you can find the third: $I = V/R$, $R = V/I$.
Why it works
Every pick of a part in robotics, from an LED's resistor to a motor's wire size, starts with Ohm's law. The physics behind it is on the Physics Ladder.
Worked examples
Worked example: Find the current
A 220 Ξ© resistor is across 5 V. What current flows?
- $I = V/R = 5/220 \approx 0.0227\ \text{A}$.
- Multiply by 1,000: about $22.7\ \text{mA}$.
Answer: About 22.7 mA.
Worked example: Find the resistance
You measure 3.3 V across a part and 10 mA through it. What is its resistance?
- $10\ \text{mA} = 0.010\ \text{A}$.
- $R = 3.3 / 0.010 = 330\ \Omega$.
Answer: 330 Ξ©.
The breadboard. A solderless breadboard has rows of five holes joined underneath, and long rails along the sides for power and ground. Parts in the same short row are connected; the gap down the middle separates the two halves.
Common mistake
Mixing milliamps and amps. Change mA to A (divide by 1,000) before you use $V = IR$, or use the shortcut: volts divided by kilohms gives milliamps.
Hands-on: measure it
Starter bench, three price points (approximate U.S. prices, 2026)
- \$0: a free online circuit simulator in your browser, and our Science Lab circuits page.
- About \$25 to \$40: a solderless breadboard, jumper wires, a resistor assortment, LEDs, and a basic digital multimeter.
- About \$50 to \$70: the same plus a microcontroller starter kit (an Arduino Uno-compatible board or a Raspberry Pi Pico, with sensors).
Put a resistor across a 9 V battery's clip on the breadboard. Measure the voltage across it with the multimeter, then calculate the current. Swap resistors and predict before you measure.
Safety at the bench.
- Work only with low voltage: batteries and USB power, 3.3 to 12 volts. Never open, modify or wire anything that plugs into a wall outlet.
- Switch off or unplug the power before you move a wire.
- Do not let a battery's two terminals touch through a wire or a tool. A short circuit can make a large current, heat and even fire.