Voltage, current and resistance
What you’ll learn
- What voltage, current and resistance are
- Their units: volts (V), amps (A) and ohms (Ω)
- What makes a complete circuit
The water analogy
Electricity is invisible, so it helps to imagine water in pipes:
| Electricity | Symbol | Unit | Water picture |
|---|---|---|---|
| Voltage | V | volt (V) | Water pressure, the push |
| Current | I | amp (A) | How much water flows per second |
| Resistance | R | ohm (Ω) | How narrow the pipe is |
- More pressure (voltage) pushes more water (current) through.
- A narrower pipe (more resistance) lets less water (current) through.
It’s not a perfect analogy, but it gets you surprisingly far.
Voltage: the push
Voltage is the difference in electrical “pressure” between two points. A battery creates this difference: one end (+) is at higher voltage than the other (−).
Common voltages in robotics:
| Source | Voltage |
|---|---|
| AA battery | 1.5 V (alkaline), 1.2 V (rechargeable NiMH) |
| USB | 5 V |
| Arduino UNO logic | 5 V |
| ESP32, Raspberry Pi Pico logic | 3.3 V |
| Single lithium cell | 3.7 V (4.2 V when full) |
| Small robot motor packs | 6–12 V |
Current: the flow
Current is how much electric charge flows past a point each second. Small electronics use milliamps (mA): 1 A = 1000 mA.
- An LED: about 10–20 mA
- An Arduino board: about 50 mA
- A small hobby motor: 100 mA to over 1 A when stalled
Resistance: the squeeze
Resistance limits current. Resistors are components made to have a specific resistance, marked with coloured stripes. Common values: 220 Ω, 1 kΩ (1,000 Ω), 10 kΩ (10,000 Ω).
A complete circuit
Current only flows if there is a closed loop from + back to −:
(+) battery ──── switch ──── resistor ──── LED ──┐ │ │ (−) ─────────────────────────────────────────────┘Open the switch and the loop breaks. Current stops everywhere in the loop at the same instant.
Example: common voltages side by side
| Source | Voltage | Good for |
|---|---|---|
| One AA cell | 1.5 V | Clocks, remotes |
| 4 × AA in series | 6 V | Small robot motors |
| USB port | 5 V | Arduino, phone charging |
| Arduino 3.3V pin | 3.3 V | Small 3.3 V sensors |
Example: open or closed?
| Situation | Current flows? |
|---|---|
| Battery, switch closed, LED, back to battery | Yes, the loop is complete |
| Same, but the switch is open | No, the loop is broken |
| LED wired to + only, nothing to − | No, there’s no return path |
| A wire straight from + to − | Yes, far too much: a short circuit |
Try it
This circuit has a 5 V battery, a switch, a 220 Ω resistor and an LED. Press Start simulator, then:
- Watch the moving yellow dots: they show current flowing around the loop.
- Click the switch at the top to open it. What happens to the dots and the LED, everywhere in the loop?
- Close it again. Double-click the resistor and change it to 2000. Is the LED brighter or dimmer?
- Hover over any part to see its voltage and current in the bottom-right corner.
Runs on Falstad Circuit Simulator, free, no account needed.
Break the loopEasy
In the simulator, open the switch. What happens to the current everywhere in the loop? It stops everywhere at once: there’s no complete path any more.
Dim the LEDMedium
Make the LED dimmer without changing the battery. Increase the resistor (double-click it and try 1000 Ω). More resistance means less current, so a dimmer LED.
Where is the push used up?Hard
Hover over the resistor and the LED. Which one has more voltage across it, and do the two add up to 5 V? The LED takes about 2 V (its forward voltage) and the resistor takes the remaining ~3 V. Together they add up to the 5 V supply.
Quick check
Finished this lesson?