Stretching copper wire (qualitative)
The behaviour of a metal when it is subject to a stretching force reveals clues about the forces between its particles.
Apparatus and materials
For each student group
Bare copper wire, 32 SWG (0.28 mm), at least 1.5 m for each student
Valve rubber tubing, 2 pieces, each about 2 cm long
Pencils or wood dowels, 2
G-clamps, 2, 10 cm and 5 cm
Eye protection for each student
Health & Safety and Technical notes
Students must wear eye protection. Though copper wire does not fly as wildly as steel wire when it breaks, the loose ends nevertheless present an eye hazard.
Warn students not to lean over as they pull on wires, to avoid the possibility of falling when the wire breaks.
A fresh piece of wire with no bending or kinking in it is needed for each extension.
a Take your length of copper wire and thread each end through the rubber tubing. Hold the rubber tubing about 10 centimetres from the ends. With each short length of wire sticking from the rubber tubing, make two loops around a wooden rod. Wind the remaining ends of the wire around the outside of the rubber tubing.
b Clamp one of the rods firmly to the bench, and pull the other. Feel how the wire has only a small amount of springiness and then begins to ‘give’.
c Gradually increase the force until the wire breaks. Take care not to fall! Look at the end of the wire where it breaks. Can you see anything extra with the hand lens?
1 Copper wire reaches its limit of proportionality, and ceases to obey Hooke’s law, when relatively low forces are applied.
2 Each student should 'feel' the elastic stretching of the wire and then the 'cheesy' yielding.
3 Allow pupils to look at the broken ends with a magnifying glass. They will see evidence of the narrowing that took place. They could consider why, since all parts of the wire were subject to the same forces, breaking took place at one point and not at others. (The answer lies in the existence of weak points in the metal’s crystal structure.)
4 With more advanced students, this is an opportunity to talk about particle ideas. Particles of any material cannot be pushed closer and closer together into a tiny high density dot, because at close distances they repel each other. But we know that these particles must attract each other at intermediate distances, or they would not stay together in the first place. At larger distances, the attraction fades, so that there is no measurable force between two shavings of copper, for example.
Atoms in a copper wire are normally an ‘equilibrium’ distance apart. Attempts to compress a piece of copper are also attempts to push its atoms closer together, and they have little effect because of the strong repulsion. The act of stretching a copper wire, on the other hand, increases the separation of atoms above the equilibrium distance, so that you can feel the atom-to-atom attraction as the material resists the pull.
Yielding of the material takes place when layers of atoms in the metal crystals begin to slide over each other. The atoms cannot be pushed back again, and the material becomes permanently stretched.
This experiment was safety-checked in September 2004