What it shows
A metal rod lengthens by only a fraction of a millimetre when heated. A pointer fixed on a rolling needle magnifies the tiny change so that the coefficient of linear expansion can be measured.
The science
For a rod of length L0, a temperature rise ΔT gives an extension ΔL = α L0 ΔT, where α is the coefficient of linear expansion; for aluminium α is about 23 × 10⁻⁶ per K and for copper about 17 × 10⁻⁶. A 50 cm aluminium rod heated by 60 K lengthens by 0.69 mm. The free end pushes against a needle resting on two supports, so the needle rolls as the rod moves; a straw pointer of length l on a needle of radius r multiplies the movement by l / r.
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 50 cm aluminium or copper rod (a curtain rod or a pipe)
- A long tray or a length of PVC gutter to hold hot water
- Hot water from a kettle at about 90 degrees C (an adult pours it)
- A sewing needle and a long straw as the pointer
- Clay to hold one end of the rod fixed
- Two thermometers
- A ruler and a paper scale behind the pointer
Safety
For this project: An adult must heat and pour the water; hot water and metal can scald. Do not touch the rod during the test.
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.
Lay the rod in the trough, fix one end in clay so it cannot move, and put the other end against a needle resting on a flat support.
Tape the straw pointer to the needle so that it points upwards, and put a paper scale behind its tip.
Measure the room temperature, the rod length L0 and the needle radius r with a ruler or by measuring its diameter.
Read the pointer’s start position and pour hot water along the rod, keeping the water only around the rod.
When the pointer stops moving, record its new position and the water temperature.
Compute the extension ΔL = (pointer movement × r) / l.
Calculate α = ΔL / (L0 ΔT) and compare with the textbook value.
What you should see
The pointer moves by 1 to 2 cm, showing the rod’s tiny expansion, and the measured α is within about 25 percent of the accepted value for the metal.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Define the coefficient of linear expansion.
The fractional increase in length per unit original length per degree rise in temperature.
2Why are gaps left in railway tracks?
To let the rails expand in summer without buckling.
3Why is a pointer on a needle used?
It magnifies a tiny movement of a fraction of a millimetre into a visible one.
4Which expands more for the same temperature rise, aluminium or iron?
Aluminium, since its coefficient is about twice that of iron.
Take it further
Build a bimetallic strip from aluminium foil taped to thin steel and watch it curl when warmed by hot water.
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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