Stretching copper wire (measuring extension) - quantitative

Demonstration

With a little ingenuity, even a small change in length of a wire becomes measurable.

Apparatus and Materials

• Bare copper wire, 26 SWG (0.45), new
• Single pulleys on clamps, 2
• Mass hangers with slotted masses (100 g), 2
• Mass hanger with slotted masses (10 g)
• G-clamp
• Polythene for clamping wire, heavy, 2 small sheets
• Needle (e.g. darning needle or knitting needle
• Drinking straw
• Adhesive
• Thread
• Hoffman clip (optional)
• Eye protection

Health & Safety and Technical Notes

Take care when masses fall to the floor. Use a box or tray lined with bubble wrap (or similar) under heavy objects being lifted. This will prevent toes or fingers from being in the danger zone.

A copper wire does not break as violently as a steel wire. However, it is wise for those close to the demonstration (including the demonstrator) to wear eye protection.

Read our standard health & safety guidance

The apparatus should be assembled before the lesson, to save lesson time. In particular, the thread should be glued to the wire, long enough in advance to ensure that the glue has set.

A fresh piece of wire is needed for each extension with no bending or kinking in it.

Procedure

1. Use a G-clamp, with two small polythene pads, to fix the end of about 2 metres of copper wire rigidly to the end of a bench.

2. At the other end of the bench, clamp the two pulleys as close together as possible. Pass the end of the copper wire over one of the pulleys to the larger mass hanger. This should be about 0.5 m above the floor.

3. About 60 cm from this pulley, stick the end of a length of thread to the wire. (Alternatively, use a Hoffman clip with two small polythene jaws to hold the thread and wire together). Pass the thread over the second pulley and hang the smaller hanger from it, so that the base of this hanger is about 30 cm above the floor. Put four 10 g, weights on this mass hanger to make a total of about 50 g. This load, which is not changed, serves to keep the thread taut.
4. Pass a needle through the two holes in the pulley supports, as illustrated. Loop a single turn of the thread round the needle, and push the needle point through a drinking straw to form a pointer. You may want to add a scale, made of card, to allow measurements.
5. When all is taut, note the position of the tip of the drinking straw.
6. Add 100 g masses to the load on the wire. As the wire stretches, the thread will move, rotating the needle and turning the pointer. An additional 500 g added to the wire will shift the position of the tip of the pointer about 3 cm per metre-length of the wire.
7. Reduce the load in 100 g steps to show that the pointer returns to its original position, revealing the elastic nature of the stretching.
8. Reload the wire in 100 g steps to a total of about 1,500 g (using a second mass hanger as well as the first). The wire will yield visibly.
9. Continue loading until the wire breaks. At an extension of about 30 cm, the weight on the thread will reach the ground and disengage. The assembly is protected from damage, since the wire yields gently through a considerable distance. Excitement builds up as students anticipate the snapping of the wire.

Teaching Notes

• You could either use a scale to make measurements, or use the activity simply to illustrate a method of measuring the extension of a wire.
• A simpler method would be to attach a flag to the copper wire some 50 cm away from the pulley, instead of the thread and pointer. A small piece of cotton wool or card makes a suitable flag.
• A discussion of the elastic (Hooke's law) stretching and plastic yielding of the wire could follow the demonstration. Most of the visible stretching of copper wire takes place beyond the elastic limit. (Copper wire stretches in the Hooke's law region only about 1 mm per metre of wire.)
• Students should observe, with a magnifying lens, the broken sharp point of the wire. The wire necks before snapping at its narrowest point. Students might also comment on the silky look of the stretched wire rather than the shiny look of the unstretched wire.
• How Science Works Extension: This experiment shows how a simple but ingenious technique can allow a small quantity to be measured. The extension of the copper wire (in the Hooke’s law region, at least) is small, perhaps a millimetre or two. However, the needle and drinking straw technique magnifies the extension greatly. Discuss why this method is better than the simple ‘flag on the wire’ technique.
• Fix a (circular) scale next to the straw. You might be satisfied with an arbitrary numerical scale; however, you could ask the class how they would set about calibrating the scale. Here are two methods:
• Determine the circumference of the needle, perhaps by measuring its diameter. This distance corresponds to one complete turn of the straw against its scale.
• Alternatively, pull thread so that the straw completes, say, 20 revolutions. Measure the length of thread and divide by 20 to find the extension corresponding to one complete turn.
• Students may be surprised to see the copper wire undergoing plastic deformation. It extends gradually even though the load is unchanging. This would be a good topic for an open-ended investigation.

This experiment was safety-checked in September 2004