- Physics
- Ready-made model
- Class 10 to 12
Pencil as a Resistor and Resistance Physics Working Model Educational Investigatory Project
Code B0BN2FLH89
- Paid for and delivered by Amazon. The current price and the delivery date for your PIN code are shown there.
- Sold by Gurukul Technicals Institute, Delhi.
- Need a different topic, a custom version or a school order? Ask on WhatsApp.
In this project guide
Ready-made model
About this model
This physics model uses the graphite in an ordinary pencil as a resistor to show how resistance depends on length and material. It is a low-cost way to demonstrate a variable resistor and the formula R = ρL/A in a school project.
What the listing says
- Pencil as a Resistor and Resistance Resistance Physics Working Model Educational Investigatory Project
How it works
The science behind it
The idea the model demonstrates, in words you can use in your report and when you explain it.
Pencil lead is not lead at all; it is graphite mixed with clay. Graphite is a form of carbon in which each atom bonds to three others in flat layers, leaving one electron per atom free to move, so it conducts electricity, though far less well than copper. That makes a pencil core, or a dark pencil line on paper, a handy resistor. Resistance follows R = ρL/A: a longer piece of graphite has more resistance and a thicker one has less. Sliding a contact along the pencil core changes the length in the circuit, so the current and the brightness of a bulb or LED change; this is how a rheostat or a volume knob works. Soft dark pencils such as 6B contain more graphite and give lower resistance than hard H pencils, which contain more clay. Diamond, which is also pure carbon, does not conduct because all its electrons are held in bonds.
Make it yourself
Make a pencil resistor model yourself
Build your own version from scratch, or use the steps to understand how the ready-made model is put together before you present it.
What you need 8
Step by step 8 steps
Ask an adult to split a soft pencil lengthwise so the core is exposed from end to end, or use a loose thick lead.
Tape the lead along a strip of card on the base, fixing only its ends so the middle stays bare.
- Connect the circuit
Cell holder positive to the bulb, bulb to the fixed crocodile clip at one end of the lead, and the slider clip back to the cell holder negative.
Slide the second clip along the lead: when the graphite in the circuit is short the bulb glows brighter, and when it is long the bulb dims.
Disconnect the cells and measure the resistance of 2, 4, 6, 8 and 10 cm of lead with the multimeter, recording each value.
On paper, shade thick graphite strips of the same length but different widths using the softest pencil, pressing hard in many layers. Measure each strip: the wider strip has less resistance.
- Compare leads of different grades
Softer leads contain more graphite and less clay, so they usually have lower resistance.
Plot resistance against length and show that it is close to a straight line, as R = ρL ÷ A predicts.
Read this first
Safety first
Go through these points before you build or switch anything on. Younger students should do this with an adult.
- An adult must split the pencil with a cutter; the blade can slip on the round wood.
- The lead can warm up when the two clips are very close together, so keep the circuit on only while you watch it.
- Always keep the 100 Ω resistor in series if you use the LED.
- Wash your hands after handling graphite, as it leaves black marks.
Troubleshooting
If something does not work
The usual problems with this model and how to fix them. Open a problem to see the fix.
The bulb's brightness hardly changes
The lead's resistance is small compared with the bulb's. Use a longer, thinner or harder lead, or switch to the LED version, which shows changes more clearly.
The pencil line on paper does not conduct
The line is too light or patchy. Shade it again many times with the softest pencil until it looks shiny and solid.
The multimeter readings jump about
The clips are not gripping firmly. Press them on well, clean the lead where they touch, and measure at marked points.
The lead keeps breaking
Graphite is brittle. Support it along its whole length on card and fix it with tape at the ends.
Exhibition day
How to present it
A running order for the exhibition table or the practical, step by step.
Slide a wire contact slowly along the graphite and let the judges watch the brightness of the bulb or LED change.
Draw lines of different lengths and thicknesses on paper with a soft pencil and compare them.
Compare HB and 6B pencils to show that the material affects resistance too.
Write R = ρL/A on your chart and link each change you show to one term in the formula.
Practice
Viva questions
Practice questions on this model, with short answers. Try each one aloud before you open the answer.
Why does graphite conduct electricity?
In graphite each carbon atom bonds to only three others, so one electron per atom is free to move and carry current.
How does the length of a conductor affect its resistance?
Resistance is directly proportional to length, so doubling the length doubles the resistance.
Which has lower resistance, a 6B pencil line or an HB pencil line of the same size?
The 6B line, because soft dark pencils contain more graphite and less clay.
What is a rheostat?
A variable resistor whose resistance is changed by sliding a contact, used to control the current in a circuit.
Project file
Project file outline
The headings of the project file, in order. Give each heading its own section and write it in your own words.
Free ebook: Writing a Science Project FileContents
- Aim: to show that pencil graphite acts as a resistor and that its resistance depends on length and thickness
- Introduction: conductors, insulators and resistance
- Why graphite conducts electricity
- Factors affecting resistance: R = ρL ÷ A
- Materials and construction
- Observations: resistance for different lengths, widths and pencil grades
- Graph of resistance against length
- Applications: rheostats, potentiometers and volume controls
- Conclusion
Buying checklist
Before you buy
What to check before you order the model, and what to do when it arrives.
1Before you order
- Check that resistance is in your syllabus and that your teacher accepts this as an investigatory project.
- Read the listing to see what is mentioned with the model and whether cells are needed.
- Keep a spare pencil lead, since graphite snaps easily.
- Keep the box so the graphite part is not broken in transit.
- Check the delivery date for your PIN code on the marketplace, and order well before the exhibition or practical.
- Practise explaining it. Judges ask what the model shows and why; knowing the science matters as much as the model. The viva questions above are a good place to start.
2When it arrives
- Try the sliding contact soon after it arrives and again a few days before the exhibition.
Next steps
Take it further
Ways to turn this model into a bigger investigation or a stronger exhibition entry.
Measure the diameter of the lead with a screw gauge and calculate the resistivity of graphite from your readings.
Make a table comparing the resistance of HB, 2B, 4B and 6B leads of the same length.
Draw a working paper circuit with graphite tracks that light an LED.
Audience
Who it suits
Suits Class 8 to 12 physics exhibitions and investigatory projects on resistance and resistivity.
- ClassClass 10 to 12
- SubjectPhysics
- TypeReady-made model
Free to read
Free ebooks for this topic
Free PDF ebooks from Gurukul Technicals Institute on the science, the making and the presenting of projects like this one.
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Your Science Exhibition Guide
Choose a project, build it, present it and answer the judges with confidence
