Electronics Simplified

Electronics Concepts, lesson 8 of 13

What is current? Voltage, resistance and current with water pipes

beginner6 min read

You can't see electricity, which makes it hard to imagine. Water is much easier. So for this lesson, let's pretend every wire is a pipe and every electron is a drop of water.

Try it yourself

The circuit below has a battery, a resistor and an LED. An LED is fussy: it needs about 2 V just to switch on, it glows nicely with 10–20 mA, and it burns out with much more than that.

5V220 Ω

13.6 mA of current flowing. Just right. A healthy flow and a bright LED.

I = (V − 2 V for the LED) ÷ R = (5 − 2) ÷ 220 = 0.0136 A

Here are three experiments to try:

  1. Set the resistor to 220 Ω and slide the voltage from 0 V up to 5 V. When does the LED first light up?
  2. Keep the battery at 9 V and make the resistor smaller, one step at a time. When does the LED start running hot?
  3. Find two different combinations of voltage and resistor that both give you about 15 mA.

The rule behind it

All three experiments follow one simple rule, called Ohm's law:

Current = Voltage ÷ Resistance        I = V ÷ R

More pressure means more flow. A narrower pipe means less flow. For our LED circuit, we first subtract the 2 V the LED uses, so the current is (V − 2) ÷ R.

For example, with a 5 V battery and a 220 Ω resistor: (5 − 2) ÷ 220 = 0.0136 A, which is about 14 mA. That is just right for an LED. This is why 220 Ω is the resistor you will see in almost every Arduino LED project on this site.

Check what you learnt

  1. In the water analogy, what does the resistor stand for?

  2. You double the voltage and keep the same resistor. What happens to the current through a plain resistor?

  3. Why does an LED need a resistor in series?

What next?

Now that you know why the resistor is there, build the real thing: Simplest Circuit to Learn Arduino.