Resistance: what slows electricity down

Plug in a phone charger and the cable stays cool. Switch on a toaster and its wires glow red-hot. Both are just metal wires carrying current. Why the difference?

The answer is resistance.

Resistance is the squeeze

In the pipe picture, resistance is how hard the pipe is to get through. A wide, smooth pipe lets water flow easily (low resistance). A narrow, bumpy one holds it back (high resistance).

We measure resistance in ohms, and write it with the Greek letter Ω (omega). It's named after the German scientist Georg Ohm.

Why does it exist? Electrons moving through a material bump into its atoms. Each bump slows them and passes some energy to the atoms, making them jiggle more. That jiggling is heat.

What changes resistance?

Four things:

• The material. Copper has very low resistance, so wires are made of it. Rubber has enormous resistance, so it stops current.
• The length. A longer wire is a longer squeeze, so more resistance.
• The thickness. A thicker wire is a wider pipe, so less resistance.
• The temperature. For most metals, hotter means more resistance.

That is why a long, thin extension lead gets warmer than a short, thick one, and why a thin filament inside a bulb gets hot enough to glow.

Why a resistor is useful

A resistor is a small part made to have a known resistance. Its job is to limit the current. Choose the right resistor and you decide exactly how much current flows. You'll meet this in the Don't pop the bulb and LED guard missions of Current Control.

Try it

This time change only the resistance and leave the voltage alone.

1. Set the resistance very low. What happens to the current, and to the lamp?
2. Set it very high. What happens now?
3. Find a resistance that gives exactly 0.5 amps at 6 volts. (Hint: it is the same as dividing 6 by 0.5.)