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Science project idea

Pencil lead rheostat that dims an LED as the clip slides along the graphite

Graphite conducts, but not as well as copper. The longer the length of lead the current has to pass through, the more resistance it has, so the LED gets dimmer as you move the clip away.

Make it yourself8 numbered steps, the materials and the science, all free.Go to the stepsGet a ready-made kitReady-made models, kits and spare parts in the store under Physics.Browse PhysicsAsk on WhatsAppAsk the head office whether a model or kit for this idea is available.Send a message

What it shows

Graphite conducts, but not as well as copper. The longer the length of lead the current has to pass through, the more resistance it has, so the LED gets dimmer as you move the clip away.

The science

Resistance of a uniform conductor is proportional to its length: R is directly proportional to L. Graphite lead is a poor conductor, so a 10 cm length may be tens of ohms or more. As the sliding clip moves away, more of the lead is in the loop, the total resistance rises, the current falls and the LED dims. The 100 Ω resistor keeps the LED safe when the clip is close.

Materials

Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • A soft 6B or 4B pencil, sharpened at both ends with the wood shaved to expose the lead
  • Two crocodile clips or paper clips
  • One red LED and a 100 Ω resistor
  • A 2 x AA battery holder with two cells
  • A ruler
  • Insulated wires

Safety

For this project: Cells only. Sharpen with an adult nearby. The graphite dust is messy: wash your hands afterwards.

Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.

All the safety rules

Step by step

8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.

  1. Split the pencil lengthways or shave the wood off one side to expose at least 10 cm of lead along its length.

  2. Tape the pencil down on the table with the lead facing up and mark every centimetre along it.

  3. Clip one wire to the pencil lead at the zero mark and connect it to the battery red wire.

  4. Connect the battery black wire to the short leg of the LED.

  5. Connect the long leg of the LED to the 100 Ω resistor and the resistor to the second crocodile clip.

  6. Touch the second clip to the lead at 1 cm and note the LED brightness; then move it to 2, 4, 6 and 8 cm.

  7. If you have a multimeter, switch off and measure the resistance for each distance and plot a graph of R against length.

  8. Test with 2B and HB pencils and compare which has the steeper graph.

What you should see

The LED gets dimmer as the clip moves along the lead, and the graph of resistance against length is roughly a straight line.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1Why does resistance increase with length?

A longer path means more obstacles for the moving charges, so resistance is higher.

2Which pencil conducts better, 6B or HB?

The 6B, because it has more graphite and less clay.

3Is graphite a metal?

No, but it is a non-metal that conducts electricity.

4Why is a fixed resistor in the circuit?

To protect the LED when the pencil resistance is very small.

Free ebook: Viva Questions for Science Projects

Take it further

Draw a thick pencil line on paper and measure its resistance as a function of length and width.

Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.

Free ebooks for this project

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