We rely on electric circuits every day, in our homes, schools and places of work, as well as in our portable gadgets and kitchens. In this activity, a circuit drawn by a pencil conducts electricity to light an LED.

You’ll need to get hold of an LED, some wires and crocodile clips – these are all available cheaply online or from high street electronics shops.

1. Draw a picture on the paper or card. Make sure you draw one continuous line, and press quite hard to make the lines thick. Make sure that one part of the drawing touches the edge of the paper.
2. Identify the negative part of the battery (–) and connect one end of a wire to it. Connect the other end to the part of your drawing that touches the edge of the paper.
3. Identify the positive terminal (+) of the battery, and the positive leg of the LED (the longest leg). Connect these together with the second wire using the crocodile clips.
4. Bend the negative leg of the LED outwards, and touch it against any part of your drawing. You should see the LED light up.

• Why does the LED light up?
• What happens if you place the LED on different parts of your picture?
• What could a circuit like this be useful for?

### Investigate…

• What happens if you make the drawing with harder (eg 4H) or softer (eg 6B) pencils?
• Can you connect more than one LED in a circuit? Could you light up a whole picture?

### Did you know?

Softer pencils, such as a 4B, have a greater graphite-to-clay ratio than harder pencils, such as a 4H.

### What’s the science?

Pencil ‘lead’ is made of graphite mixed with clay. Graphite is a form of carbon, and is a conductor of electricity: the carbon atoms’ electrons can move within the material. The pencil line on the paper is a continuous trail of graphite. This trail is a route along which electrons flow – from the negative to the positive terminal of the battery – forming an electric current that lights the LED.

Graphite is only a fairly good conductor, and longer circuits have more resistance to current flow. That’s why touching the crocodile clip at different points along the line can make the LED brighter or dimmer.