What it shows
A bulb’s filament gets hotter as the current rises, and a hotter metal resists more. The graph of voltage against current therefore bends and the bulb does not obey Ohm’s law.
The science
Ohm’s law holds for a constant-temperature conductor. In a metal the resistance rises with temperature because the ions vibrate more and scatter the moving electrons more often. A tungsten filament runs near 2500 degrees C, so its resistance when glowing is about ten times its cold value. The V-I curve therefore bends toward the voltage axis, and V / I grows with current.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 2.5 V torch bulb with a holder
- Four AA cells in a holder (6 V; use one to four cells as steps)
- A multimeter used as a voltmeter and another for current, or a single one used twice
- A 10 ohm rheostat or a 22 ohm pencil-lead slider
- A switch, wires and crocodile clips
- Graph paper
Safety
For this project: An adult must check the wiring. Do not push a 2.5 V bulb above 3 V: it will burn out and the glass could pop. Use cells only.
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
7 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.
Measure the cold resistance of the bulb with the multimeter on its ohm range; it is usually 1 to 2 ohm.
Connect the bulb, the milliammeter or ammeter in series, and the rheostat as a slider to give variable voltage from the cells.
Put the voltmeter directly across the bulb, and start at zero volts, raising the voltage in 0.25 V steps up to 2.5 V.
At each step write the voltage and the current and watch the colour of the filament, noting when it first glows.
Compute the resistance V / I for each row of the table.
Plot voltage on the x axis and current on the y axis, joining the points smoothly.
Plot resistance against voltage or current in a second graph, and compare the hot resistance with the cold one.
What you should see
The V-I graph curves instead of being a straight line, and the calculated resistance rises from about 2 ohm at low voltage to about 8 ohm at 2.5 V. The bulb glows only at the higher readings.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Does a bulb obey Ohm’s law?
Not over the full range; Ohm’s law needs a constant temperature, and the filament heats up.
2Why does the filament’s resistance rise with temperature?
The ions in the metal vibrate more and collide with the electrons more often.
3Why is the cold resistance so low?
At room temperature the filament is cool, so there is little scattering of the electrons.
4Which material does show a falling resistance when it is heated?
A semiconductor such as a thermistor or carbon.
Take it further
Measure a pencil-lead resistor over the same range and show that its graph is much closer to a straight line.
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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