What it shows
For a graphite pencil lead at steady temperature, the current through it is directly proportional to the voltage across it. The V against I graph is a straight line through the origin and its slope is the resistance (Ohm’s law, V = IR).
The science
A conductor obeys Ohm’s law when the ratio V/I stays constant at a fixed temperature. Graphite in a pencil is a conductor with a resistance of a few ohms, so a few cells push a measurable current through it. By connecting one, two, three and four cells in series you change V in steps, measure I each time, and plot V on the y-axis against I on the x-axis. A straight line through the origin confirms the law and its slope gives R in ohms.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 2B pencil sharpened at both ends, or a bare 4B lead
- Four AA cells and a battery holder (or four separate cells and tape)
- A digital multimeter (about Rs 250, or borrow from the school lab)
- Four jumper wires or wires with crocodile clips
- A 100 ohm resistor from a repair shop as a check
- Graph paper and a pencil
Safety
For this project: Use only dry cells, never a mains adaptor. Take readings quickly and let the lead cool between readings, because a warm lead changes its resistance and the graph will bend.
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 rulesStep 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.
Sharpen both ends of the pencil so bare graphite shows, and clip one crocodile clip to each end.
Connect one cell, the pencil lead and the multimeter set to DC milliamps in a single series loop.
Record the current, then switch the multimeter to DC volts and measure the voltage directly across the two clips on the lead.
Add a second cell in series (positive of one to negative of the next) and repeat both readings; do the same for three and four cells.
Make a results table with columns for number of cells, V in volts, I in milliamps, and V/I in ohms.
Plot V against I on graph paper and draw the best straight line through the points and the origin.
Find the slope of the line, which is the resistance of the lead, and compare it with the average of your V/I column.
Repeat the whole set with the 100 ohm resistor to show that the method gives a known answer, which is a good check for your viva.
What you should see
A straight-line V-I graph through the origin with a slope of a few ohms for a soft pencil lead, and close to 100 ohms for the check resistor. Points that bend upwards at high current show the lead warming up.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does the slope of a V-I graph give?
The resistance of the conductor, in ohms.
2Why must the temperature stay constant?
Ohm’s law holds only at constant temperature; heating raises the resistance of graphite and metals and bends the graph.
3How is the ammeter connected and how is the voltmeter connected?
The ammeter in series with the lead, the voltmeter in parallel across it.
4What would the graph look like for a filament bulb?
A curve that bends because the filament heats up and its resistance increases.
Take it further
Measure the resistance of a 2H and a 6B pencil and explain the difference by the amount of graphite in each.
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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