What it shows
When one end of equal rods is heated, the heat travels along each at a different speed. Dabs of butter melt in order along the rod, showing which metal is the better conductor.
The science
Heat is carried in metals by free electrons and by vibrating ions. The rate of heat flow along a rod is Q/t = k A ΔT / L, where k is the thermal conductivity. Copper (about 400 W/m K) has much higher k than aluminium (about 240), brass (about 110) or iron (about 80), so the butter melts further and sooner along it. Over a fixed time the melted distance grows roughly as the square root of the conductivity (for the same material properties) and in the order of the conductivities.
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.
- Rods or strips of copper, aluminium, brass and iron of the same size (about 20 cm long, from a hardware shop)
- Small dabs of butter or wax at 3 cm spacing along each rod
- A wide steel tray with hot water at about 90 degrees C (an adult heats it)
- A wooden stand or a stack of books to hold the rods level
- A stopwatch
- A ruler and a notebook
Safety
For this project: An adult must heat and handle the water and use tongs, never fingers, to move the rods; do not use an open flame.
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.
Stick small dabs of butter at 3, 6, 9, 12 and 15 cm from one end of each rod, using a matchstick to place them evenly.
Lay the four rods on a stand like spokes of a wheel or side by side so their free ends overhang the tray.
Ask the adult to pour the hot water over the ends or place the ends in the hot water for the same length (3 cm) at the same moment, and start the stopwatch.
Note the time at which each dab on each rod melts and falls or slides.
Draw a table of time against distance for each metal.
Plot distance melted against time for the four rods on one graph.
Rank the metals from best to worst conductor and compare with the known values.
What you should see
The butter on the copper rod melts first and furthest, then aluminium, then brass, with iron slowest, matching the order of their thermal conductivities.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which of the four is the best conductor of heat?
Copper.
2Why do all the rods start at the same hot temperature?
The end is held in the same hot water, so only the metal’s conductivity decides the speed.
3How is heat carried through a metal?
Mainly by free electrons, and also by lattice vibrations.
4Why do cooking pots have wooden or plastic handles?
These are bad conductors, so the handle stays cool.
Take it further
Add a glass rod and a wooden skewer of the same size and see how little their butter dabs melt.
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.
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